Thermal control of the seismogenic zone of southern central Chile

We developed thermal models for the Chile subduction zone along two profiles at 38.2°S and 42°S within the rupture area of the 1960 M = 9.5 Valdivia earthquake and south of the 2010 M = 8.8 Maule earthquake. The age difference of the subducting Nazca Plate has a major impact on the thermal regime, being much younger and hotter in the south. Seafloor heat flow observations confirm this difference but also indicate that in the southern area, heat advection at the outer rise cools the incoming plate. Heat flow values derived from the depth of gas hydrate bottom-simulating reflectors are in general agreement with probe and borehole measurements. The positions where the plate interface reaches temperatures of 100–150°C and 350–450°C differ between the two profiles. If these temperatures control the updip and downdip limits of the interplate seismogenic zone, the seismogenic zone widens and shifts landward to greater depths from south to north. Observed microseismicity, however, seems to fade at temperatures much lower than 350–450°C. This discrepancy can be explained in three alternative ways: (1) deformation in a thick subduction channel controls the seismic/aseismic transition; (2) microseismicity recorded over a limited time period does not represent the rupture depth of large interface earthquakes; or (3) the serpentinized mantle wedge controls the downdip limit.

Related papers:

title: Loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones
abstract: Earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front. Rather, the shallowest part of that interface moves mainly through stable, aseismic slip. That part of the plate boundary, referred to here as the aseismic zone, occurs along the base of an accreted wedge of young sediments. The probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone. Subducted sediment is progressively dewatered and underplated to the base of the overriding plate. Through this process, more consolidated rocks eventually come into contact at depth across both sides of the plate boundary. From the point where sufficiently hard rock is found across the plate interface, that interface will change its slip behavior to unstable stick-slip sliding, which is characteristic of consolidated material under most conditions. This type of motion is accommodated seismically as episodic slip in large earthquakes. The location of this transition to seismic behavior, referred to as the seismic front, marks the deep end of the aseismic zone and the top of the seismogenic zone, i.e., that part of the plate interface that moves primarily in thrust earthquakes. Several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone. We find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks. The location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events. The size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width, W, of that area. Thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of W and hence to deduce the maximum size of future interplate earthquakes. The average repeat time of such events is also related to W and so better estimates of average repeat time are possible from improved knowledge of W. The hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics. We use a mechanical analysis, laboratory modeling, and multichannel seismic information to develop a simple model explaining the growth of forearcs. The outer-arc high or trench-slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge. We argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate. That stronger material is called the backstop. Our laboratory modeling experiments indicate that a backstop with a trenchward-dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs. In our modeling an accretionary wedge develops trenchward of the backstop, an outer-arc high develops above the trenchward toe of the backstop, and farther arcward a passive forearc basin forms above the stronger material of the backstop. This model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis. We hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge. At several margins the seismic front is approximately coincident with the outer-arc high, supporting this interpretation.

title: GREAT THRUST EARTHQUAKES AND ASEISMIC SLIP ALONG THE PLATE BOUNDARY OF THE MAKRAN SUBDUCTION ZONE
abstract: The Makran subduction zone of Iran and Pakistan exhibits strong variation in seismicity between its eastern and western segments and has one of the world's largest forearcs. We determine the source parameters for 14 earthquakes at Makran including the great (Mw 8.1) earthquake of 1945 (the only instrumentally recorded great earthquake at Makran); we determine the loci of seismic and aseismic slip along the plate boundary, and we assess the effects of the large forearc and accretionary wedge on the style of plate boundary slip. We apply body waveform inversions and, for small-magnitude events, use first motions of P waves to estimate earthquake source parameters. For the 1945 event we also employ dislocation modeling of uplift data. We find that the earthquake of 1945 in eastern Makran is an interplate thrust event that ruptured approximately one-fifth the length of the subduction zone. Nine smaller events in eastern Makran that are also located at or close to the plate interface have thrust mechanisms similar to that of the 1945 shock. Seaward of these thrust earthquakes lies the shallowest 70–80 km of the plate boundary; we find that this segment and the overlying accretionary wedge remain aseismic both during and between great earthquakes. This aseismic zone, as in other subduction zones, lies within that part of the accretionary wedge that consists of largely uconsolidated sediments (seismic velocities less than 4.0 km/s). The existence of thrust earthquakes indicates that either the sediments along the plate boundary in eastern Makran become sufficiently well consolidated and de watered about 70 km from the deformation front or older, lithified rocks are present within the forearc so that stick-slip sliding behavior becomes possible. This study shows that a large quantity of unconsolidated sediment does not necessarily indicate a low potential for great thrust earthquakes. In contrast to the east, the plate boundary in western Makran has no clear record of historic great events, nor has modem instrumentation detected any shallow thrust events for at least the past 25 years. Most earthquakes in western Makran occur within the downgoing plate at intermediate depths. The large change in seismicity between eastern and western Makran along with two shallow events that exhibit right-lateral strike-slip motion in central Makran suggest segmentation of the subduction zone. Two Paleozoic continental blocks dominate the overriding plate. The boundary between them is approximately coincident with the transition in seismicity. Although relative motion between these blocks may account for some of the differing seismic behavior, the continuity of the deformation front and of other tectonic features along the subduction zone suggests that the rate of subduction does not change appreciably from east to west. The absence of plate boundary events in western Makran indicates either that entirely aseismic subduction occurs or that the plate boundary is currently locked and experiences great earthquakes with long repeat times. Evidence is presently inconclusive concerning which of these two hypotheses is most correct. The presence of well-defined late Holocene marine terraces along portions of the coasts of eastern and western Makran could be interpreted as evidence that both sections of the arc are capable of generating large plate boundary earthquakes. If that hypothesis is correct, then western Makran could produce a great earthquake or it could rupture as a number of segments in somewhat smaller-magnitude events. Alternatively, it is possible that western Makran is significantly different from eastern Makran and experiences largely aseismic slip at all times. A knowledge of the velocity structure and nature of the state of consolidation or lithification of rocks at depth in the interior portion of the forearc of western Makran should help to ascertain whether that portion of the plate boundary moves aseismically or ruptures in large to great earthquakes. A resolution of this question has important implications for seismic hazard not only for western Makran but also for other margins, such as the Cascadia subduction zone of western North America, where historical thrust events have not occurred.

title: Nature of seismic coupling along simple plate boundaries of the subduction type
abstract: The downdip width of the seismogenic zone is defined for 19 subduction zones. This width is measured from the base of the accretionary prism to the maximum depth of nucleation of thrust events along the plate boundary. Those two points are taken to define the upper and lower depth transitions from stable to unstable frictional sliding. The lower depth transition is found to be between 35 and 70 km. The dip angle of the thrust zone is also reevaluated. We find a linear increase in the dip angle as a function of depth, the slope of which varies between 0.2° and 0.6° km−1. The downdip width obtained, which is generally narrower than previously determined by most other authors, varies from about 50 to 150 km. We also determine the ratio of the rate of slip that occurs in earthquakes to the rate of relative plate motion. This ratio is defined as the seismic coupling coefficient (a). We obtain two different estimates of the seismic coupling coefficient: an average value from 90 years of seismicity and a value obtained using the slip-predictable recurrence model for large earthquakes. We find a large variation in the computed values of a along and among subduction zones. For most of the subduction zones a is much less than 1.0; for several it is less than a few percent. Worldwide, we find no significant correlation between either the seismic coupling coefficient or the width of the seismogenic zone and subduction parameters such as the age of the oceanic lithosphere that is being subducted, plate convergence rates or absolute velocity of the upper plate in the hot spot reference frame. Such correlation exists only for a few individual subduction zones where other parameters do not vary as much. The observed variations in seismic coupling could be explained as differences in the frictional behavior of materials at the plate interface. Some of these differences may be attributed to the subduction of large bathymetric features, the roughness of topography, the presence of unstable triple junctions and active-spreading ridges, and sediment composition.

title: The updip and downdip limits to great subduction earthquakes: Thermal and structural models of Casca
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title: Depth of seismic coupling along subduction zones
abstract: Underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth. The maximum depth of the seismically coupled zone (or seismogenic zone) is a transition from unstable to stable sliding along the plate interface. We have determined the depth of this stability transition for the circurn-Pacific subduction zones of: Honshu, Kuriles, Kamchatka, Aleutians, Alaska, Mexico, and Chile. These subduction zones have experienced great interplate earthquakes and the aftershock regions are well-located. Depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling. For an average P wave velocity of 6.7 km s−1 above the plate interface, we find that for most subduction zones the stability transition occurs at 40 ± 5 km depth. There are, however, several exceptions. At the Hokkaido trench junction, where the Japan trench and the Kurile trench intersect, seismic coupling is deep and extends down to 52–55 km. Deep coupling was also found in the Coquimbo region in central Chile. The Mexico subduction zone has shallow coupling: the transition occurs at 20–30 km depth. Previous studies of micro-earthquakes in Honshu, Hokkaido, the Aleutians, and Alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find. Given our measurements of seismic coupling depth, we then explore the mechanism that may determine coupling depth. The concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments, thus we first test whether a critical temperature can explain our results. Temperatures at the plate interface are dependent on many variables; but two that are poorly determined are shear stress and radiogenic heat generation. Shear stress has been constrained by inversion of heat flow data. Assuming a crustal radiogenic heat production rate of 3.1 exp−z/8.5 μWm−3 and a constant coefficient of friction, we find two critical temperatures of about 400 ° C and 550 ° C. The lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust. On the other hand, one single critical temperature of about 250 ° C can explain the coupling depths if shear stresses are constant with depth.

title: Thermal constraints on the seismogenic portion of the southwestern Japan subduction thrust
abstract: For coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault. In this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature. We have developed a transient thermal model for the Nankai subduction zone of southwest Japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from (1) seismicity and tsunami data for two great subduction earthquakes, (2) the coseismic faulting extent of these events estimated from geodetic deformation data, and (3) the interseismic locked zone determined from interseismic geodetic data. The Nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures. It has earthquake, tsunami, and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain. On the Nankaido margin off Shikoku Island, the thermal model indicates that a temperature of 350°C (taken to be the limit for seismic initiation from laboratory and field data) is reached on the subduction thrust fault 150 km from the trench. A transition zone into which rupture may extend with decreasing offset (taken to be 450°C) extends an additional 45 km downdip. The thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 Nankaido Ms = 8.2 earthquake off Shikoku Island and with the downdip extent of the present locked zone. In the region of the 1944 Tonankai Ms = 8.2 earthquake to the northeast, the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent. The seismogenic-locked zone from earthquake and geodetic data is also narrower. Thus our analysis of the southwest Japan margin indicates that all three constraints on the downdip extent of the seismogenic zone, thermal, coseismic and interseismic geodetic data, are in general agreement. The study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature. The thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip. The subducting plate age and thus heat flow, and the thickness of the insulating sediments on the incoming plate, are also very important to the thermal regime and thus to the seismogenic width. The relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as Cascadia where there have been no great historical events.

title: Updip limit of the seismogenic zone beneath the accretionary prism of southwest Japan: An effect of diagenetic to low-grade metamorphic processes and increasing effective stress
abstract: Off southwest Japan the seaward limit of coseismic displacement (or updip limit of the seismogenic zone) of the 1946 Mw 8.3 thrust earthquake reaches to 4 km depth and ∼40 km landward of the trench. This limit coincides with the estimated location of the 150 °C isotherm, and has been linked to changes in physical properties associated with the smectite to illite clay-mineral transition. Here we show that this limit correlates with a suite of diagenetic to low-grade metamorphic processes characterized by (1) declining fluid production and decreasing fluid pressure ratio (λ*) and (2) active clay, carbonate, and zeolite cementation and the transition to pressure solution and quartz cementation. These diagenetic to low-grade metamorphic changes cause the onset of velocity weakening during thrust faulting, an increase in effective stress, and strengthening of the hanging wall, which together combine to produce recordable earthquakes.

title: On the mechanical role of smectite in subduction zones
abstract: Smectites, a group of hydrated phyllosilicate minerals, are often a main component of sediments entering a subduction zone. They are common in hemipelagic sediments and consist of an authigenetic component that arises from the alteration of volcanic ash intercalated in the sediment and a detrital component. Smectite-rich strata are structurally weak, retain water, and are highly porous. A review of sediment lithology in subduction zones around the world suggests that decollement zones may preferentially form in smectite-rich horizons. One consequence of this observation is that the assignment of high frictional resistance to materials in a decollement may be unwarranted. At deeper structural levels smectite is transformed to illite, a mechanically stronger mineral. Estimates of the depth of this transition coincide with the depth at which earthquakes are first detected along plate interfaces. I speculate that the smectite-illite transition in subduction zones may be an important component of sediment consolidation processes that trigger interplate seismicity.

title: The depth of seismic faulting and the upper transition from stable to unstable slip regimes
abstract: A number of observations indicate that an upper stability transition occurs along well-developed faults, such as the San Andreas, as a result of unconsolidated gouge within shallow regions of these faults. These observations include the depth distribution of seismicity along faults with and without well-developed gouge zones, correlations between seismicity and shallow crustal structure, and modeling of co- seismic and post-seismic slip. In addition, recent experimental friction studies indicate that thick layers of simulated gouge exhibit a positive slip-rate dependence of frictional resistance (velocity strengthening) and thus inherently stable slip, whereas bare rock surfaces and thin gouge layers exhibit potentially unstable velocity weakening behavior. Subduction zones with large accretionary wedges also exhibit an upper stability transition in that slip is aseismic within the accretionary wedge. A stability transition due to the presence of unconsolidated material can also be invoked in this case.

title: Comparison of smectite- and illite-rich gouge frictional properties: application to the updip limit of the seismogenic zone along subduction megathrusts
abstract: Along plate boundary subduction thrusts, the transformation of smectite to illite within fault gouge at temperatures of ∼150°C is one of the key mineralogical changes thought to control the updip limit of seismicity. If correct, this hypothesis requires illite-rich gouges to exhibit frictionally unstable (velocity-weakening) behavior. Here, we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay-rich gouges. We sheared 5-mm-thick layers of commercially obtained pure Ca-smectite, a suite of smectite–quartz mixtures, and natural illite shale (grain size ranging from 2 to 500 μm) in the double-direct shear geometry to shear strains of ∼7–30 at room humidity and temperature. XRD analyses show that the illite shale contains dominantly clay minerals and quartz; within the clay-sized fraction ( 40 MPa). Our data, specifically the velocity-strengthening behavior of illite shale under a wide range of conditions, do not support the hypothesis that the smectite–illite transition is responsible for the seismic–aseismic transition in subduction zones. We suggest that other depth- and temperature-dependent processes, such as cementation, consolidation, and slip localization with increased shearing, may play an important role in changing the frictional properties of subduction zone faults, and that these processes, in addition to clay mineralogy, should be the focus of future investigation.

title: Finding Fault in Fault Zones
abstract: The seismogenic behavior of subduction faults depends on the structural evolution of the fault zone. What controls the location and size of seismic rupture along the 43,500 km of the world's subduction faults? Two reports in this issue, based on seismological observations, emphasize the key role of the structural evolution of faults. On page 207, Dean et al. (1) propose that a structural flaw of the sedimentary formation brought into the Sumatra subduction zone may facilitate rupture propagation. On page 210, Kimura et al. (2) propose that fracturing of the downward-going plate beneath the Kanto area in central Japan causes the subduction interface to be diverted to a deeper position and change its seismogenic behavior.

title: Hydrous minerals in the mantle wedge and the maximum depth of subduction thrust earthquakes
abstract: In many subduction zones the downdip limit of thrust earthquakes approximately coincides with the intersection of the subduction thrust with the forearc mantle. This limit may be explained by aseismic hydrous minerals present in the mantle wedge. During subduction, fluids released from the subducting slab infiltrate the overlying forearc mantle forming serpentine + brucite, especially in cool subduction zones. At the slab interface itself, talc-rich rocks form in the mantle by the addition of silica transported by rising fluids and by mechanical mixing of mantle and siliceous rocks. In the laboratory, serpentine generally exhibits stable-sliding aseismic behavior. The behavior of talc, a layered hydrous silicate, and brucite, a layered hydroxide, has not been investigated, but their structures also suggest weak stable-sliding behavior. We suggest all three layered hydrous minerals promote aseismic behavior and that their presence controls the downdip limit of thrust earthquakes in many subduction zones.

title: Interplate seismic activity near the northern Japan Trench deduced from ocean bottom and land-based seismic observations
abstract: The off-Sanriku area, the landward slope area of the northern Japan Trench, is characterized by its active seismicity which is directly related to the subduction process of the Pacific plate. In July–August 1992, a seismic swarm activity was observed near the trench. Five earthquakes of M > 6 occurred successively within 2 weeks at the beginning of the activity. We deployed an ocean bottom seismograph (OBS) array in the epicentral region to make an in situ observation of the spatial distribution of these events and to study the characteristics of the swarm activity in detail. 
 
We relocated 357 events from 23 July to 26 August with focal depth errors less than 5 km by using both the OBS and the land network data. The swarm events are not distributed homogeneously over the plate boundary but form several clusters. A remarkable concentration of the earthquakes was found in the vicinity of the hypocenter of the largest event during the OBS observation (M 6.2, 29 July); this formed the most active cluster. Almost all the earthquakes, including the M 6.2 event, occurred at depths of about 15 km and were concentrated along a thin plane dipping to the west at 5°. The location of the plane coincides well with that of the plate boundary derived from previous seismic explorations. 
 
We also relocated epicenters of the off-Sanriku events by introducing station corrections to the land network data during the period when the OBS array was absent. The corrections were estimated from the travel time residuals of the hypocenter relocation using the OBS and the land network data. The result of the epicenter relocation strongly indicates the existence of a zone of extremely low seismic activity beneath the inner trench wall, where low seismic velocity, mechanically weak, unconsolidated sediments overly the subducting Pacific plate.

title: Thermal regime of the Costa Rican convergent margin: 2. Thermal models of the shallow Middle America subduction zone offshore Costa Rica
abstract: At the Costa Rica margin along the Middle America Trench along‐strike variations in heat flow are well mapped. These variations can be understood in terms of either ventilated fluid flow, where exposed basement allows fluids to freely advect heat between the crustal aquifer and ocean, or insulated fluid flow where continuous sediment cover restricts heat advection to within the crustal aquifer. We model fluid flow within the subducting aquifer using Nusselt number approximations coupled with finite element models of subduction and explore its effect on temperatures along the subduction thrust. The sensitivity of these models to the initial thermal state of the plate and styles of fluid flow, either ventilated or insulated, is explored. Heat flow measurements on cool crust accreted at the East Pacific Rise are consistent with ventilated hydrothermal cooling that continues with subduction. These models yield much cooler temperatures than predicted from simulations initialized with conductive predictions and without hydrothermal circulation. Heat flow transects on warm crust accreted at the Cocos‐Nazca spreading center are consistent with models of insulated hydrothermal circulation that advects heat updip within the subducting crustal aquifer. Near the trench these models are warmer than conductive predictions and cooler than conductive predictions downdip of the trench. Comparisons between microseismicity and modeled isotherms suggest that the updip limit of microseismicity occurs at temperatures warmer than 100°C and that the downdip extent of microseismicity is bounded by the intersection of the subduction thrust with the base of the overriding crust.

title: Thermal models of the Mexico subduction zone: Implications for the megathrust seismogenic zone
abstract: It has been proposed that the seismogenic zone of subduction thrust faults is primarily controlled by temperature or rock composition changes. We have developed numerical models of the thermal structure of the Mexico subduction zone to examine the factors that affect the temperature of the subduction thrust fault. Although the oceanic plates subducting beneath Mexico are young, the top of the oceanic plate at the trench is cool, because of the lack of a thick cover of insulating sediments. Marine heat flow observations suggest that hydrothermal circulation may further cool the oceanic plate. This results in a cool subduction thrust fault, where the brittle part of the fault extends to depths of over 40 km. At these depths, even slight frictional heating may have significant effects on temperature along the thrust fault, particularly for regions with a high convergence rate and shallow plate dip. With the addition of a small amount of frictional heating, the temperatures of the deep (30–40 km) thrust fault are increased by over 200°C. As the observed downdip limit of rupture in recent well-constrained megathrust earthquakes is confined to depths above the intersection of the thrust fault and the continental Moho, a temperature of 350°C may control the downdip extent of the seismogenic zone. Thus, in order to be consistent with the observed shallow rupture areas, it is necessary to include a small amount of frictional heating, corresponding to an average shear stress of 15 MPa.

title: Seismic coupling along the Chilean Subduction Zone
abstract: Underthrusting at subduction zones can cause large earthquakes at shallow depths but is accommodated by aseismic creep below a certain depth. The maximum depth of the seismically coupled zone (or seismogenic zone) is a transition from unstable to stable sliding. We have determined the maximum depth of the coupled zone and its variability along the Chilean subduction zone. The maximum depth of seismic coupling is defined by the depth of large (M > 6) underthrusting earthquakes that have occurred at the downdip edge of the coupled plate interface. Earthquake depth is determined with omnilinear waveform inversion of long-period P waves. The statistical uncertainty in the depth is estimated using bootstrapping. Omnilinear inversion formally accounts for the scaling incompatibility between the P waveforms and decreases the uncertainty in the depth estimate. We have found the depths of 27 earthquakes in the time period from 1961 to 1987. Seismic coupling in Chile extends down to 48–53 km. There is a resolvable change in the maximum depth of coupling around 28°S. The region immediately north of this latitude has a coupled zone that extends no deeper than 36–41 km, while south of this latitude coupling extends down to 48–53 km. This transition is not simply related to the oceanic lithospheric age and plate convergence rate but coincides with a change in several geophysical and geological phenomena. With respect to shallower (north) and deeper (south) coupling there is a change from active volcanos to no volcanos, the dip of the deep slab changes from steep to shallow, the thickness of trench sediments changes from thin to thick, and the depth of the oceanic basement changes from deeper to shallower at 28°S. Due to the multiplicity of changes at 28°S there is no clear unique interpretation of what physical mechanism controls the variability in the maximum depth of the seismically coupled zone in Chile.

title: Long-Term Signals in the Present-Day Deformation Field of the Central and Southern Andes and Constraints on the Viscosity of the Earth’s Upper Mantle
abstract: As part of the South American Geodynamic Activities project we observed the present day deformation field in the territories of Chile and Argentina using the Global Positioning System. The results clearly show that the earthquake cycle dominates the contemporary surface deformation of the central and southern Andes. Compared to geological timescales, the transient elastic deformation related to subduction earthquakes presents a short-term signal which can be explained by interseismic, coseismic, and postseismic phases of interplate thrust earthquakes. We constructed the Andean Elastic Dislocation Model (AEDM) in order to subtract the interseismic loading from the observed velocities. The estimated parameters of the AEDM, and the amount and depth of coupling between the subducting Nazca and overriding South American Plates, represent long-term features and show that the seismogenic interface between both plates is fully locked and that the depth of coupling increases from north to south.

title: The Southern Andes between 36° and 40°S latitude: seismicity and average seismic velocities
abstract: The project ISSA 2000 (Integrated Seismological experiment in the Southern Andes) consists of a temporary seismological network and a seismic refraction profile. A network of 62 seismological stations was deployed across the Southern Andes at ∼38°S. Three hundred thirty-three local seismic events were observed in a 3-month period. P and S arrival times of a subset of high quality data were inverted simultaneously for 1-D velocity structure, hypocentral coordinates and station delays. Seismic refraction data along a transect at 39°S provide further constraints on the crustal structure. Low crustal velocities beneath the forearc may be either due to subducted trench sediments or serpentinized mantle material of the continental lithosphere. The continental Moho is not clearly observed in this region. Average velocities of the crust beneath the arc are higher than those beneath the forearc. Crustal thickness is about 40 km. Crustal seismicity concentrates in the forearc region along the Bio-Bio and Gastre fault zones. The area between these two prominent fault zones seems to be nearly devoid of crustal seismicity but shows highest uplift and topography in the forearc region. Benioff seismicity is observed down to 150 km depth resulting in the first accurate image of the Benioff zone in the Southern Andes. A maximum of seismicity at 60 km depth may be caused by dehydration embrittlement.

title: Interaction between forearc and oceanic plate at the south‐central Chilean margin as seen in local seismic data
abstract: We installed a dense, amphibious, temporary seismological network to study the seismicity and structure of the seismogenic zone in southern Chile between 37° and 39°S, the nucleation area of the great 1960 Chile earthquake. 213 local earthquakes with 14.754 onset times were used for a simultaneous inversion for the 1-D velocity model and precise earthquake locations. Relocated artificial shots suggest an accuracy of the earthquake hypocenter of about 1 km (horizontally) and 500 m (vertically). Crustal events along trench-parallel and transverse, deep-reaching faults reflect the interseismic transpressional deformation of the forearc crust due to the subduction of the Nazca plate. The transverse faults seems to accomplish differential lateral stresses between subduction zone segments. Many events situated in an internally structured, planar seismicity patch at 20 to 40 km depth near the coast indicate a stress concentration at the plate's interface at 38°S which might in part be induced by the fragmented forearc structure.

title: Seismicity and geometry of the south Chilean subduction zone (41.5°S–43.5°S): Implications for controlling parameters
abstract: In 2005 an amphibious seismic network was deployed on the Chilean forearc between 41.75°S and 43.25°S. 364 local events were observed in a 11-month period. A subset of the P and S arrival times were inverted for hypocentral coordinates, 1-D velocity structure and station delays. Main seismic activity occurred predominantly in a belt parallel to the coast of Chiloe Island in a depth range of 12–30 km presumably related to the plate interface. The 30° inclination of the shallow part of the Wadati-Benioff zone is similar to observations further north indicating that oceanic plate age is not controlling the subduction angle of the shallower part for the Chilean subduction zone. The down-dip termination of abundant intermediate depth seismicity at approximately 70 km depth seems to be related to the young age (and high temperature) of the oceanic plate. Crustal seismicity is associated with the Liquine-Ofqui fault zone and active volcanoes.

title: Alteration of the subducting oceanic lithosphere at the southern central Chile trench-outer rise
abstract: Hydrothermal circulation and brittle faulting processes affecting the oceanic lithosphere are usually confined to the upper crust for oceanic lithosphere created at intermediate to fast spreading rates. Lower crust and mantle rocks are therefore relatively dry and undeformed. However, recent studies at subduction zones suggest that hydration of the oceanic plate is most vigorous at the trench–outer rise, where extensional bending-related faulting affects the hydrogeology of the oceanic crust and mantle. To understand the degree of hydration, we studied the seismic velocity structure of the incoming Nazca plate offshore of southern central Chile (∼43°S); here the deep-sea trench is heavily filled with up to 2 km of sediments. Seismic refraction and wide-angle data, complemented by seismic reflection imaging of sediments, are used to derive a two-dimensional velocity model using joint refraction and reflection traveltime tomography. The seismic profile runs perpendicular to the spreading ridge and trench axes. The velocity model derived from the tomography inversion consists of a ∼5.3-km-thick oceanic crust and shows P wave velocities typical for mature fast spreading crust in the seaward section of the profile, with uppermost mantle velocities as fast as ∼8.3 km/s. Approaching the Chile trench, seismic velocities are significantly reduced, however, suggesting that the structures of both the oceanic crust and uppermost mantle have been altered, possibly due to a certain degree of fracturing and hydration. The decrease of the velocities roughly starts at the outer rise, ∼120 km from the deformation front, and continues into the trench. Even though the trench is filled with sediment, basement outcrops in the outer rise frequently pierce the sedimentary blanket. Anomalously low heat flow values near outcropping basement highs indicate an efficient inflow of cold seawater into the oceanic crust. Hydration and crustal cracks activated by extensional bending-related faulting are suggested to govern the reduced velocities in the vicinity of the trench. Considering typical flow distances of 50 km, water might be redistributed over most of the trench–outer rise area. Where trapped in faults, seawater may migrate down to mantle depth, causing up to ∼9% of serpentinization in at least the uppermost ∼2 km of the mantle between the outer rise and the trench axis.

title: The Chile ridge: A tectonic framework
abstract: A new Chile ridge tectonic framework is developed based on satellite altimetry data, shipboard geophysical data and, primarily, 38,500 km of magnetic data gathered on a joint U.S.-Chilean aeromagnetic survey. Eighteen active transforms with fossil fracture zones (FZs), including two complex systems (the Chile FZ and Valdivia FZ systems), have been mapped between the northern end of the Antarctic-Nazca plate boundary (Chile ridge) at 35°S and the Chile margin triple junction at 47°S. Chile ridge spreading rates from 23 Ma to Present have been determined and show slowdowns in spreading rates that correspond to times of ridge-trench collisions. The Valdivia FZ system, previously mapped as two FZs with an uncharted seismically active region between them, is now recognized to be a multiple-offset FZ system composed of six FZs separated by short ridge segments 22 to 27 km long. At chron 5A (∼12 Ma), the Chile ridge propagated from the Valdivia FZ system northward into the Nazca plate through crust formed 5 Myr earlier at the Pacific-Nazca ridge. Evidence for this propagation event includes the Friday and Crusoe troughs, located at discontinuities in the magnetic anomaly sequence and interpreted as pseudofaults. This propagation event led to the formation of the Friday microplate, which resulted in the transferal of crust from the Nazca plate to the Antarctic plate, and in a 500-km northward stepwise migration of the Pacific-Antarctic-Nazca triple junction. Rift propagation, microplate formation, microplate extinction, and stepwise triple junction migration are found to occur during large-scale plate motion changes and plate boundary changes in the southeast Pacific.

title: Space-geodetic estimation of the nazca-south america euler vector
abstract: GPS data from four sites in the Nazca Plate (Easter Island, Galapagos, Robinson Crusoe and San Felix Islands) and from five sites in the stable core of the South American Plate enabled us to estimate the Euler vector of the Nazca Plate with respect to South America. The observed velocities of Easter Island (6.6 cm/yr at 102.3°), Galapagos (5.1 cm/yr at 90.0°), Robinson Crusoe (6.6 cm/yr at 80.1°) and San Felix (6.0 cm/yr at 82.1°) are significantly slower than the global plate model NUVEL-1A velocites for those four sites. Our estimated Euler pole is located at 48.8°N, 91.7°W with a rate of 0.59°/m.y. The NUVEL-1A and earlier space-geodetic studies give rotation rates that are 20% faster.

title: The Nazca -South America Euler vector and its rate of change
abstract: We present velocities relative to the South American plate for five GPS stations on the Nazca plate and use these measurements to estimate the modern Euler vector. We find a pole at 55.88N, 92.58W with a rotation rate of 0.60 8/Myr. Because the GPS station at Easter Island appears to be moving at approximately 6.6 mm/yr relative to the other Nazca stations, we repeat our analysis with this station excluded from the inversion to obtain a second and preferred result (called CAP10) with a pole at 61.08N, 94.48W and a rate of 0.57 8/Myr. We compare these results with published finite rotation vectors and infer that during the past 10 – 20 Myrs, the Nazca – South America rotation rate has decelerated by 0.048 – 0.06 8/Myr 2 .

title: Latitudinal Variation in Sedimentary Processes in the Peru-Chile Trench off Central Chile
abstract: Four cruises of the German research vessel RV SONNE (cruises SO101, SO103, SO104 and SO161) surveyed the Chilean continental margin and oceanic plate using seismic measurements across the Peru-Chile Trench, swath-mapping bathymetry, sediment echosounding, dredges and gravity-core sampling. In this paper, we present data from cruise SO161 derived from the sediment-filled sector of the trench between 35 and 44° S. South of 33°10′ S, sediment fill in the trench ranges from 2200 to 3500 m thickness. The sediment volume decreases northwards, as the trench width narrows from 80 km at 41° S to 25 km at 33° S. Turbidity currents enter the trench mainly via nine canyon systems that are deeply incised into the continental slope. Reflection patterns from the trench fill exhibit a cyclicity that can be linked to Milankovic cycles. Turbiditic deposits at elevated positions within the trench indicate Pleistocene mass-wasting events that were able to overcome a height difference of some hundred meters. Within the trench, a fraction of the turbidity currents is channelled by a northward-dipping, axial channel. This axial channel has eroded up to 200 m into the trench fill and from 42° S, it extends northwards some 1000 km, terminating at the foot of the Juan Fernandez Ridge. The channel has no continuous precursor and might have evolved its present-day form during the last glaciation.

title: Tectonic Processes along the Chile Convergent Margin
abstract: The Chile subduction zone, spanning more than 3500 km, provides a unique setting for studying, along a single plate boundary, the factors that govern tectonic processes at convergent margins. At large scale, the Chile trench is segmented by the subduction of the Chile Rise, an active spreading center, and by the Juan Fernandez hot spot ridge. In addition, the extreme climatic change from the Atacama Desert in the north to the glacially influenced southern latitudes produces a dramatic variability in the volume of sediment supplied to the trench. The distribution of sediment along the trench is further influenced by the high relief gradients of the segmented oceanic lithosphere.

title: Deep lithospheric structures along the southern central Chile margin from wide-angle P-wave modelling
abstract: P>Crustal- and upper-mantle structures of the subduction zone in south central Chile, between 42 degrees S and 46 degrees S, are determined from seismic wide-angle reflection and refraction data, using the seismic ray tracing method to calculate minimum parameter models. Three profiles along differently aged segments of the subducting Nazca Plate were analysed in order to study subduction zone structure dependencies related to the age, that is, thermal state, of the incoming plate. The age of the oceanic crust at the trench ranges from 3 Ma on the southernmost profile, immediately north of the Chile triple junction, to 6.5 Ma old about 100 km to the north, and to 14.5 Ma old another 200 km further north, off the Island of Chiloe. Remarkable similarities appear in the structures of both the incoming as well as the overriding plate. The oceanic Nazca Plate is around 5 km thick, with a slightly increasing thickness northward, reflecting temperature changes at the time of crustal generation. The trench basin is about 2 km thick except in the south where the Chile Ridge is close to the deformation front and only a small, 800-m-thick trench infill could develop. In south central Chile, typically three quarters (1.5 km) of the trench sediments subduct below the decollement in the subduction channel. To the north and south of the study area, only about one quarter to one third of the sediments subducts, the rest is accreted above. Similarities in the overriding plate are the width of the active accretionary prism, 35-50 km, and a strong lateral crustal velocity gradient zone about 75-80 km landward from the deformation front, where landward upper-crustal velocities of over 5.0-5.4 km s<SU-1</SU decrease seaward to around 4.5 km s<SU-1</SU within about 10 km, which possibly represents a palaeo-backstop. This zone is also accompanied by strong intraplate seismicity. Differences in the subduction zone structures exist in the outer rise region, where the northern profile exhibits a clear bulge of uplifted oceanic lithosphere prior to subduction whereas the younger structures have a less developed outer rise. This plate bending is accompanied by strongly reduced rock velocities on the northern profile due to fracturing and possible hydration of the crust and upper mantle. The southern profiles do not exhibit such a strong alteration of the lithosphere, although this effect may be counteracted by plate cooling effects, which are reflected in increasing rock velocities away from the spreading centre. Overall there appears little influence of incoming plate age on the subduction zone structure which may explain why the M-w = 9.5 great Chile earthquake from 1960 ruptured through all these differing age segments. The rupture area, however, appears to coincide with a relatively thick subduction channel.

title: Three‐dimensional density model of the Nazca plate and the Andean continental margin
abstract: We forward modeled the Bouguer anomaly in a region encompassing the Pacific Ocean (85°W) and the Andean margin (60°W) between northern Peru (5°S) and Patagonia (45°S). The three-dimensional density model that reproduces the gravity field is a continental-scale representation of density structure to 410 km depth that characterizes the mantle and crust of the oceanic Nazca plate, subducted slab and continental margin with a minimum number of bodies. We predefined the density of each body after studying the dependency of density on composition of crustal and mantle materials and pressure-temperature conditions appropriate for the Andean setting. A database of independent geophysical information constrains the geometry of the top of the subducted slab, locally the Moho of the oceanic and continental crusts and, indirectly, the lithosphere-asthenosphere boundary underneath the continental plate. Other boundaries, notably the intracrustal density discontinuity separating upper from lower crust below the continent, were not constrained and their geometry is the result of fitting the observed and calculated Bouguer anomaly during forward modeling. This contribution presents the model to the Andean geoscientific community and contains some tools, like a sensitivity analysis, that helps potential users of the model to interpret its results. We describe and discuss some of these results in order to illustrate the application of the model to the study of a wide range of phenomena (e.g., modification of oceanic plate structure by hot spots, shape of the subducted slab, thermal structure of the continental lithosphere, compensation mechanism and formation of orogenic relieve, causes of Andean segmentation).

title: Holocene sediments from the Southern Chile Trench: a record of active margin magmatism, tectonics and palaeoseismicity
abstract: Sedimentology, petrography and the provenance of Holocene sediments from the Southern Chile Trench (36–47°S) were investigated in an integrated approach combining description of a collection of gravity cores, measurements of physical properties, quantitative X-ray petrography and modal analysis. The sediments studied were trench hemipelagic sediments, fan deposits, and more distal hemipelagic sediments from the Nazca Plate. The trench is mostly fed by multiple point sources via submarine canyons. Sandy turbidites show a southward increase in sediment maturity. Whereas volcanic lithic fragments and plagioclase represent the dominant fraction in the north, quartz content strongly increases in the southern part of the study area, in line with source lithologies. Further north, active volcanoes in the Main Cordillera represent almost the entire provenance signal as a result of a strong contribution of highly erodible volcanic rocks. Recurrence rates of sandy and silty turbidites in the trench fan sediments indicate a link to the palaeoseismic record on land. Our study documents the potential usefulness of proximal turbidites to reconstruct palaeoseismicity, even at a scale of single segments of the plate boundary.

title: Episodic development of a convergent margin inferred from structures and processes along the southern Chile margin
abstract: Seismic reflection data acquired in the vicinity of Isla Mocha across the southern coast of Chile image structures formed along the continental margin and reveal an episodic history of accretion, nonaccretion, and possibly erosion. Structures formed at the toe of the continental slope suggest frontal accretion of ¾ to 1 ¾ km of trench fill. Seismic images also reveal that a small accretionary wedge, 20–30 km wide, abuts the truncated continental metamorphic basement that extends seaward from beneath the shelf. The small size of the accretionary wedge on three profiles examined here is not consistent with a long history of accretion with the current deformational style, as current rates of frontal accretion could have accumulated all of the existing accretionary wedge in less than 1–2 m.y. This is a small fraction of convergence history along this margin, and the current accretionary mode has not been consistently maintained in the past. The Isla Mocha region is located between the temperate climate of central Chile and the glacial climate of southern Chile, and climatic conditions in this region have likely fluctuated sufficiently to cause significant variation in trench sediment supply. Accretionary and nonaccretionary or erosional episodes are probably linked to temporal variations in trench sediment thickness, as suggested by observations along the Chile margin. Currently, thick trench sediment correlates with accretion along the southern Chile margin, and thin trench sediment correlates with nonaccretion/tectonic erosion as near the Chile Ridge and from the Juan Fernandez Ridge to northern Chile. The Isla Mocha region also lies 900 – 1000 km north of the Chile triple junction, and the Chile Ridge lies approximately 2000 km to the west and has not yet collided and affected the margin near Isla Mocha. This part of the precollision zone provides an excellent reference to examine the effects of Chile Ridge collision in the development of the Chile margin. The most apparent effect of subduction of the buoyant, young crust of the Chile Ridge is a shallow trench that is nearly devoid of sediment. Consequently, the triple junction is undergoing nonaccretion or erosion, and the accretionary complex near the triple junction remains smaller than to the north or south because the current phase of rapid accretion elsewhere in the trench has bypassed the triple junction region. The interplay of subduction zone processes, such as trench sedimentation and ridge collision, has resulted in an episodic development of the margin and produced a discontinuous record of convergence history within the accretionary wedge.

title: Seismic Images of Accretive and Erosive Subduction Zones from the Chilean Margin
abstract: Modern seismic imaging methods were used to study the subduction processes of the South American convergent margin. The data came from reflection and from wide-angle/refraction experiments acquired within the framework of the Collaborative Research Center SFB267 ‘Deformation Processes in the Andes’. Two areas of differing character and subduction type were investigated: an erosive margin to the north (19–26° S) and an accretionary margin to the south (36–40° S). Results from different seismic models yield three main transects that give an overall impression about the internal structure below the Chilean margin. At the erosive margin, we find that the upper part of the subducting oceanic lithosphere is characterized by a horst-and-graben structure that coincides with the coupling zone between the plates. Strong coupling between oceanic crust and fore-arc in the case of a horst-continent collision is also indicated by plate-parallel faults beneath the lower continental slope, which we interpret as the upper parts of the subduction channel. In this context, the subduction channel represents the downgoing Nazca Plate as well as those portions of the continental crust which moved landward. Low seismic velocities below the coastline also represent parts of the subduction channel and of the hydrofractured base of the upper crust near the plate interface. Between 45 and 60 km depth, a double reflection zone marks the upper and lower boundary of the subducted oceanic crust. Off southern Chile, the ocean bottom is characterized by relatively smooth morphology. In contrast, in the south, the trench is filled with sediments and contains an axial channel (Figs. 7.16 to 7.18) extending in N-S direction along the trench axis within the investigation area. The periodicity of the reflected seismic signal within these sediments correlates with the main glacial cycle during the Quaternary. The recent accretionary wedge is built up from strongly heterogeneous unconsolidated sediments. Frontal accretion takes place within the southern working area except for the region around the Arauco Peninsula, which shows uplift due to basal accretion and antiformal stacking. Below the Coastal Cordillera, the heterogeneity of the modern accretionary wedge and the antiformal stack structure of the Permo-Triassic accretionary wedge complicate imaging at depths greater than about 30 km. Thus, we obtain an image of the top of the subduction channel as a thin reflector segment only to about 25 km depth.

title: Geophysical Signatures and Active Tectonics at the South-Central Chilean Margin
abstract: The ISSA 2000 (Integrated Seismological experiment in the Southern Andes) and SPOC 2001 (Subduction Processes Off Chile) onshore and offshore projects surveyed the Chilean margin between 36 and 40° S. This area includes the location of the 1960 earthquake (M w = 9.5) that ruptured the margin from ∼38° S southwards for ~1000 km. Together with gravity and magnetotelluric components, the active-passive seismic experiments between 36 and 40° S provide the first, complete, high-resolution coverage of the entire seismogenic plate interface.

title: Current plate motions
abstract: We determine best-fitting Euler vectors, closure-fitting Euler vectors, and a new global model (NUVEL-1) describing the geologically current motion between 12 assumed-rigid plates by inverting plate motion data we have compiled, critically analysed, and tested for self-consistency. We treat Arabia, India and Australia, and North America and South America as distinct plates, but combine Nubia and Somalia into a single African plate because motion between them could not be reliably resolved. The 1122 data from 22 plate boundaries inverted to obtain NUVEL-1 consist of 277 spreading rates, 121 transform fault azimuths, and 724 earthquake slip vectors. We determined all rates over a uniform time interval of 3.0m.y., corresponding to the centre of the anomaly 2A sequence, by comparing synthetic magnetic anomalies with observed profiles. The model fits the data well. Unlike prior global plate motion models, which systematically misfit some spreading rates in the Indian Ocean by 8–12mm yr−1, the systematic misfits by NUVEL-1 nowhere exceed ∼3 mm yr−1. The model differs significantly from prior global plate motion models. For the 30 pairs of plates sharing a common boundary, 29 of 30 P071, and 25 of 30 RM2 Euler vectors lie outside the 99 per cent confidence limits of NUVEL-1. Differences are large in the Indian Ocean where NUVEL-1 plate motion data and plate geometry differ from those used in prior studies and in the Pacific Ocean where NUVEL-1 rates are systematically 5–20 mm yr−1 slower than those of prior models. The strikes of transform faults mapped with GLORIA and Seabeam along the Mid-Atlantic Ridge greatly improve the accuracy of estimates of the direction of plate motion. These data give Euler vectors differing significantly from those of prior studies, show that motion about the Azores triple junction is consistent with plate circuit closure, and better resolve motion between North America and South America. Motion of the Caribbean plate relative to North or South America is about 7 mm yr−1 slower than in prior global models. Trench slip vectors tend to be systematically misfit wherever convergence is oblique, and best-fitting poles determined only from trench slip vectors differ significantly from their corresponding closure-fitting Euler vectors. The direction of slip in trench earthquakes tends to be between the direction of plate motion and the normal to the trench strike. Part of this bias may be due to the neglect of lateral heterogeneities of seismic velocities caused by cold subducting slabs, but the larger part is likely caused by independent motion of fore-arc crust and lithosphere relative to the overriding plate.

title: Impact of megathrust geometry on inversion of coseismic slip from geodetic data: Application to the 1960 Chile earthquake
abstract: We analyze the role of megathrust geometry on slip estimation using the 1960 Chile earthquake (MW = 9.5) as an example. A variable slip distribution for this earthquake has been derived by Barrientos and Ward (1990) applying an elastic dislocation model with a planar fault geometry. Their model shows slip patches at 80–110 km depth, isolated from the seismogenic zone, interpreted as aseismic slip. We invert the same geodetic data set using a finite element model (FEM) with precise geometry derived from geophysical data. Isoparametric FEM is implemented to constrain the slip distribution of curve-shaped elements. Slip resolved by our precise geometry model is limited to the shallow region of the plate interface suggesting that the deep patches of moment were most likely an artifact of the planar geometry. Our study emphasizes the importance of fault geometry on slip estimation of large earthquakes.

title: The 1960 Chile earthquake: inversion for slip distribution from surface deformation
abstract: A total of 166 observations of sea-level change, 130 measurements of elevation difference, and 16 determinations of horizontal strain provide an excellent view of the (quasi-)static source process of the great 1960 Chilean earthquake. These surface deformation data were employed in classical uniform slip fault models as well as more recently developed models that allow spatial variability of slip. The best uniform slip planar (USP) model is 850km long, 130km wide, and dips 20°. Seventeen metres of fault displacement contributed to a USP moment of 9.4 times 1022 N m. The variable slip planar (VSP) model concentrates slip on a 900 km long, 150 km wide band parallel to the coast. Several peaks of slip with dimensions of 50–100 km appear in this band and are thought to represent major subduction zone asperities. Important fractures of the oceanic lithosphere bound the 1960 rupture and are offered as a potential source of fault segmentation within the Chilean subduction zone. The VSP moment for 1960 earthquake totals 9.5 times 1022 N m, about one fifth of the value estimated for the foreshock-mainshock sequence from seismic methods. Except for areas out to sea, geodetic resolution on the fault is fairly uniform. Thus, it is unlikely that slip missed by the network could increase the VSP moment much beyond 1.8 times 1023 N m. Several patches of moment, isolated from the main body at 80–110 km depth, are found down dip in the VSP model and are presumably indicative of aseismic slip. One patch at the northern end of the rupture is probably associated with the initiation phase of the mainshock, although the time sequence of the relationship is unknown. Tide gauge records suggest that another patch between 40° and 43° S, responsible for the observed strain and uplifts inland at those latitudes, is not of coseismic origin, but derives from in-place, post-seismic creep over several years. Apparently, great 1960-type events are not typical members of the ∼ 128 yr earthquake cycle in south-central Chile. The Nazca-South America boundary here is characterized by a variable rupture mode in which major asperities are completely broken by great earthquakes only once in four or five earthquake cycles. The more frequent large earthquakes, that geographically overlap the great events, fill in between the locked zones.

title: Heat flow and temperature gradients in Chile
abstract: SummaryConventional heat-flow measurements in Chile carried out by other workers are summarized. Between latitudes 26 – 29° S heat flow is consistently low (<42 mWm−2) excepting a site in the Andes slope (75.3 mWm−2). In Central Chile (33 °S) near Santiago, a value in the Andes (60.7 mWm−2) is lower than the value in the Santiago basin (78.7 mWm−2). Heat flow through the sea bottom around the Chile Ridge (about 44 – 48° S; 75 – 80° W) ranges between 25 and 414 mWm−2; heat-flow estimates based upon the location in depth of the phase of gas hydrates have also been carried out in this area. In Tierra del Fuego the only heat-flow value is 96.3 mWm−2. The present heat-flow studies in Chile do not allow any conclusions to be drawn on the general heat-flow distribution and its description within the frame of new global tectonics. Only some preliminary model results comparing heat-flow measurements in the area of the Chile Ridge to thermal effects produced by a ridge-trench collision may presently be partially adopted. A general discussion regarding the results from global seismic tomography, maximum depth of seismic coupling and thermal processes in Chile is also presented.The silica geotemperature in the Santiago basin resulting from 257 groundwater analyses is 77.4±10.4 °C; the equivalent heat flow is 92.5±16.6 mWm−2 which is in agreement with the conventional heat-flow value in this area. Geochemical thermometry indicates fluid temperature at depth higher than 200 °C in some of the 33 hot-spring areas evaluated using SiO2, Na-K-Ca and Na-Li geothermometers. The evalutation of fluid rock equilibrium and CO2 - fugacities by means of relative Na, K, Mg and Ca contents of thermal waters indicates that only in El Tatio and Puchuldiza in Northern Chile have fluids attained partial equilibrium with both K-Na and K-Mg mineral systems. Other geothermal areas in the north, and many hot springs in Central Chile, correspond to immature waters which are generally unsuitable for the evaluation of K/Na and K/Mg equilibrium temperatures. In Central Chile the evaluation of some hot-spring waters in partial equilibrium condition indicate deep temperatures between 80 °C and 245 °C.In the area of El Tatio the combined heat flow (conductive and convective) yields a value of 1465 mWm−2 with fluid circulating within 1 km of an underlying magmatic intrusion at 5 – 7 km depth. The water catchment area may be situated 20 km to the east of the geothermal area, with the underground fluid moving at a rate of about 1.3 kmy−1
Temperature logs in wells for oil prospection show that temperatures are affected by drilling disturbances. Some preliminary BHT estimates of gradients yield between 26.3°C km−1 and 72.4 °C km−1. Thermal conductivity and diffusivity data from these wells are also shown.

title: Orientation analysis of localized shear deformation in quartz fibres at the brittle–ductile transition
abstract: The crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 270° to 370°C has been investigated. To explore the active mechanism during deformation of the quartz fibres, the complete crystallographic orientation of individual recrystallized grains and subgrains was determined by using a combination of CIP (computer-integrated polarization microscopy), EBSD (electron back-scatter diffraction), and additional specially developed image analysis techniques. It was found that the quartz fibres deformed along crystallographically controlled shear bands. From inverse pole figures of normals to the shear band boundaries it is evident that these bands occur along the positive and negative rhomb planes of quartz. It is suggested that they are initiated as fractures along these planes. Inside the shear bands, small grains broken off from the host grain show passive rotation of the lattice, consistent with the shear sense of the shear bands. Continued deformation and increasing temperature lead to different recrystallization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships.

title: The thermal effects of steady-state slab-driven mantle flow above a subducting plate: the Cascadia subduction zone and backarc
abstract: Atsubductionzones,geophysicalandgeochemicalobservationsindicatethatthearcandbackarcregionsarehot,inspiteofthe cooling effects of a subducting plate. At the well-studied Cascadia subduction zone, high mantle temperatures persist for over 500 km into the backarc, with little lateral variation. These high temperatures are even more surprising due to the juxtaposition of the hot Cascadia backarc against the thick, cold North America craton lithosphere. Given that local heat sources appear to be negligible, mantle flow is required to transport heat into the wedge and backarc. We have examined the thermal effects of mantle flow induced by traction along the top of the subducting plate. Through systematic tests of the backarc model boundary, we have shown that the model thermal structure of the wedge is primarily determined by the assumed temperatures along this boundary. To get high temperatures in the wedge, it is necessary for flow to mine heat from depth, either by using a temperature-dependent rheology, or by introducing a deep cold boundary through a thick adjacent lithosphere, consistent with the presence of a craton. Regardless of the thermal conditions along the backarc boundary, flow within an isoviscous wedge is too slow to transport a significant amount of heat into the wedge corner. With a more realistic stress- and temperature-dependent wedge rheology, flow is focused into the wedge corner, resulting in rapid flow upward toward the corner and enhanced temperatures below the arc, compatible with temperatures required for arc magma generation. However, this strong flow focusing produces a nearly stagnant region further landward in the shallow backarc mantle, where model temperatures and heat flow are much lower than observed. Observations of high backarc temperatures, particularly in areas that have not undergone recent extension, provide an important constraint on wedge dynamics. None of the models of simple traction-driven flow were able to simultaneously produce high temperaturesbeneaththevolcanicarcandthroughoutthebackarc.

title: Weakening of the subduction interface and its effects on surface heat flow, slab dehydration, and mantle wedge serpentinization
abstract: The shallow part of the interface between the subducting slab and the overriding mantle wedge is evidently weakened by the presence of hydrous minerals and high fluid pressure. We use a two-dimensional finite element model, with a thin layer of uniform viscosity along the slab surface to represent the strength of the interface and a dislocation-creep rheology for the mantle wedge, to investigate the effect of this interface “decoupling.” Decoupling occurs when the temperature-dependent viscous strength of the mantle wedge is greater than that of the interface layer. We find that the maximum depth of decoupling is the key to most primary thermal and petrological processes in subduction zone forearcs. The forearc mantle wedge above a weakened subduction interface always becomes stagnant (<0.2% slab velocity), providing a stable thermal environment for the formation of serpentinite. The degree of mantle wedge serpentinization depends on the availability of aqueous fluids from slab dehydration. A very young and warm slab releases most of its bound H2O in the forearc, leading to a high degree of mantle wedge serpentinization. A very old and cold slab retains most of its H2O until farther landward, leading to a lower degree of serpentinization. Our preferred model for northern Cascadia has a maximum decoupling depth of about 70–80 km, which provides a good fit to surface heat flow data, predicts conditions for a high degree of serpentinization of the forearc mantle wedge, and is consistent with the observed shallow intraslab seismicity and low volume of arc volcanism.

title: Common depth of slab-mantle decoupling: Reconciling diversity and uniformity of subduction zones
abstract: Processes in subduction zones such as slab and mantle-wedge metamorphism, intraslab earthquakes, and arc volcanism vary systematically with the age-dependent thermal state of the subducting slab. In contrast, the configuration of subduction zones is rather uniform in that the arc is typically situated where the slab is ∼100 km deep. Toward reconciling the diversity and uniformity, we developed numerical thermal models with a nonlinear mantle rheology for seventeen subduction zones, spanning a large range of slab age, descent rate, and geometry. Where there are adequate observations, such as in Cascadia, northeast Japan, and Kamchatka, we find that surface heat flows can be explained if the interface between the slab and the mantle wedge is decoupled to a depth of 70–80 km. Models with this common decoupling depth predict that the region of high mantle temperatures and optimal fluid supply from the dehydrating slab, both required for melt generation for arc volcanism, occurs where the slab is ∼100 km deep. These models also reproduce the variations of the metamorphic, seismic, and volcanic processes with the thermal state of the slab. The shallow decoupling results in a stagnant fore arc whose thermal regime is controlled mainly by the subducting slab. The deeper coupling leads to a sudden onset of mantle wedge flow that brings heat from greater depths and the back arc, and its thermal effect overshadows that of the slab in the arc region. Our results serve to recast the research of subduction zone geodynamics into a quest for understanding what controls the common depth of decoupling.

title: Temperatures, heat flux, and frictional stress near major thrust faults
abstract: We examine the two-dimensional advective and conductive transport of heat in a region of thrust faulting. Both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor, S=1+b(zƒVsin δ)/κ below what they would be with the same heat sources but in the absence of advection. In this expression zƒ is the depth to the fault, V is the slip rate, δ is the component of the dip of the fault in the direction of underthrusting, κ is thermal diffusivity, and b is a dimensionless factor that is essentially equal to one for most forms of heating. Initial changes in temperatures near the fault are given by T(zƒ,t)=T(zƒ,0)-12Vsinδ∂T(z,0)/∂z. These simple formulae are successful because of the neglible influence of lateral conduction of heat, at least for slip rates of a few mm/yr or more. Two time constants govern the transition from the initial change in temperature to steady state: t1 = uƒ/V, where uƒ is the distance along the fault in the direction of underthrusting from the surface to the depth in question, and t2=zƒ2/κπ2, where zƒ is the depth to the fault. When elapsed times exceed the sum of these two time constants, temperatures differ from their steady state values by only about 10 percent. The numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted. Putain. An application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 MPa and are greater than 30 MPa. Calculations of temperatures appropriate for the Himalaya suggest that shear stresses of 100 MPa on the Main Central Thrust probably are required to account for the Tertiary granites of the region, if melting took place after slip began on the thrust. Similarly, the cut-off in seismicity at a depth of about 15 km in the Himalaya, if due to temperatures exceeding 350° to 450°C, implies a deviatoric stress close to 100 MPa.

title: Inferences on sediment strength and fault friction from structures at the Aleutian Trench
abstract: The structural relations of conjugate thrust faults and the master decollement imaged in a research-level, processed, multichannel seismic reflection record across the eastern Aleutian Trench allow quantitative estimates of the sediment strength and friction across thrust boundaries of the subduction zone. These estimates are derived from geometric relations between faults and from the overall geometry of accreted prisms. They suggest the presence of high pore-fluid pressures that result in low levels of friction along a master decollement and on the main bounding thrust faults in the accreted sediment. Therefore, coupling in the immediate trench area apparently does not contribute significantly to the generation of horizontal compression upslope.

title: Accretionary prisms in subduction earthquake cycles: The theory of dynamic Coulomb wedge
abstract: We expand the theory of critically tapered Coulomb wedge for accretionary prisms by considering stress changes in subduction earthquake cycles. Building on the Coulomb plasticity of the classical theory, we assume an elastic–perfectly Coulomb plastic rheology and derive exact stress solutions for stable and critical wedges. The new theory postulates that the actively deforming, most seaward part of an accretionary prism (the outer wedge) overlies the updip velocity-strengthening part of the subduction fault, and the less deformed inner wedge overlies the velocity-weakening part (the seismogenic zone). During great earthquakes, the outer wedge is pushed into a compressively critical state, with an increase in basal and internal stresses and pore fluid pressure. After the earthquake, the outer wedge returns to a stable state. The outer wedge geometry is controlled by the peak stress of the updip velocity-strengthening part of the subduction fault achieved in largest earthquakes. The inner wedge generally stays in the stable regime throughout earthquake cycles, acting as an apparent backstop and providing a stable environment for the formation of forearc basins. The new theory has important implications for the studies of the updip limit of the seismogenic zone, the evolution of accretionary prisms and forearc basins, activation of splay faults and tsunami generation, evolution of the fluid regime, and mechanics of frontal prisms at margins dominated by tectonic erosion.

title: Heat flow over the descending Nazca plate in central Chile, 32°S to 41°S: observations from ODP Leg 202 and the occurrence of natural gas hydrates
abstract: Bottom simulating reflectors (BSRs) were detected in multichannel seismic reflection data acquired in the vicinity of Isla Mocha across the southern Chile margin and near 33°S. Geothermal gradients were determined from the depth of the BSR that is interpreted to mark the thermally controlled base of a gas hydrate layer. Ground truth for the assessment and additional thermal constraints were provided by downhole measurements obtained during Ocean Drilling Program (ODP) Leg 202 in Site 1233 at 41°S and Sites 1234 and 1235 near 36°S. Both BSR-derived data and downhole temperatures were used to calculate heat flow anomalies and provide new constraints on the thermal regime of the continental slope and downgoing slab in Chile between 32°S and 41°S. Downhole chemical logs of Th, U, and K from Site 859 of ODP Leg 141 have been used to assess the radiogenic heat production in the margin wedge. Heat production is low (∼0.8 μW/m3). However, knowledge of this reduces the errors of estimating the contribution from frictional heating along the subduction thrust fault. With respect to the Eocene age of the incoming oceanic lithosphere, heat flow appears to decrease landward of the deformation front as expected due to the advective transport of heat into the subduction zone by the downgoing slab. Calculations of conductive fore-arc heat flow show that the modelled seafloor heat flow agrees with the measured heat flow only if there is negligible frictional heating. At 33°S, temperatures in the fault zone reach 100°C approximately 60 km landward of the deformation front and are coincident with the onset of earthquake activity and hence mark the up-dip limit of the seismogenic zone. The up-dip limit shifts seaward going to the south, reflecting the progressive southward decrease of lithospheric age of the subducting plate.

title: Amphibious seismic survey images plate interface at 1960 Chile Earthquake
abstract: The southern central Chilean margin at the site of the largest historically recorded earthquake in the Valdivia region, in 1960 (Mw = 9.5), is part of the 5000-km-long active subduction system whose geodynamic evolution is controversially debated and poorly understood. Covering the area between 36° and 40°S, the oceanic crust is segmented by prominent fracture zones. The offshore forearc and its onshore continuation show a complex image with segments of varying geophysical character, and several fault systems active during the past 24 m.y. 
 
In autumn 2001, the project SPOC was organized to study the Subduction Processes Off Chile, with a focus on the seismogenic coupling zone and the forearc. The acquired seismic data crossing the Chilean subduction system were gathered in a combined offshore-onshore survey and provide new insights into the lithospheric structure and evolution of active margins with insignificant frontal accretion.

title: Upper lithospheric structure of the subduction zone offshore of southern Arauco peninsula, Chile, at ∼38°S
abstract: velocities >8.4 km s � 1 . In the trench–outer rise area, the top of incoming oceanic plate is pervasively fractured and likely hydrated as shown by extensional faults, horst-and-graben structures, and a reduction of both crustal and mantle velocities. These slow velocities are interpreted in terms of extensional bending-related faulting leading to fracturing and hydration in the upper part of the oceanic lithosphere. The incoming Mocha FZ coincides with an area of even slower velocities and thinning of the oceanic crust (10–15% thinning), suggesting that the incoming fracture zone may enhance the flux of chemically bound water into the subduction zone. Slow mantle velocities occur down to a maximum depth of 6–8 km into the upper mantle, where mantle temperatures are estimated to be 400–430C. In the overriding plate, the tomographic model reveals two prominent velocity transition zones characterized by steep lateral velocity gradients, resulting in a seismic segmentation of the marine fore arc. The margin is composed of three main domains: (1) a � 20 km wide frontal prism below the continental slope with Vp � 3.5 km s � 1 ,( 2) a� 50 km area with Vp = 4.5–5.5 km s � 1 , interpreted as a paleoaccretionary complex, and (3) the seaward edge of the Paleozoic continental framework with Vp � 6.0 km s � 1 . Frontal prism velocities are noticeably lower than those found in the northern erosional Chile margin, confirming recent accretionary processes in south central Chile.

title: Towards understanding the lithospheric structure of the southern Chilean subduction zone (36°S–42°S) and its role in the gravity field
abstract: The Southern Andes differ significantly from the Central Andes with respect to topography and crustal structures and are, from a geophysical point of view, less well known. In order to provide insight into the along-strike segmentation of the Andean mountain belt, an integrated 3-D density model was developed for the area between latitudes 36 ◦ S and 42 ◦ S. The model is based on geophysical and geological data acquired in the region over the past years and was constructed using forward density modelling. In general, the gravity field of the South American margin is characterized by a relatively continuous positive anomaly along the coastline and the forearc region, and by negative anomalies along the trench and the volcanic arc. However, in the forearc region of the central part of the study area, located just to the south of the epicentre of the largest ever recorded earthquake (Valdivia, 1960), the trench-parallel positive anomaly is disrupted. The forearc gravity anomaly differences thus allow the study area to be ‐‐

title: The Nicoya Convergent Margin—A region of exceptionally low heat flow
abstract: Over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the Nicoya Peninsula is characterized by remarkably low heat flow; values over the accretionary prism average 28 mW/m², and values in the trench and the ocean crust seaward of the trench average 14 mW/m². We attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation. Thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow. Disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits.

title: Heat flow map of South America
abstract: The results of geothermal investigations carried out in South America have been compiled with the purpose of preparing regional maps of terrestrial heat flow. The compilation revealed that 655 heat flow values had been determined, giving an overall data density of 37106 km2 and a representative mean heat flow of 63+-36 mW/m2. The quality of the data set is variable, depending on the nature of the primary geothermal data, and the geographic distribution of the data set is also non-uniform. In spite of such difficulties a careful analysis of the data set, following suitable priority schemes, has allowed not only the determination of reliable mean heat flow values for a large number of major geological structures in South America, but also the preparation of mosaics of regional heat flow variations. Heat flow is extremely variable in the Cordilleran regions, with the eastern and southern parts having relatively high values compared to the western and northern parts. The general trend of increasing heat flow from the western coastal regions towards inland areas is interrupted by a N-S trending low heat flow belt in the Pre-Cordilleran basins. In the eastern part of the continent heat flow is low to normal (<75 mW/m2) but there are indications that in the Patagonian Platform it is higher than in the Brazilian Platform. There are, however, several isolated localities of high heat flow in the northeastern and south-central parts of Brazil. The Mesozoic rift basins (Potiguar, Reconcavo and Taubate) are also characterized by relatively high values. 
 
In order to examine the tectonic significance of variations in the regional geothermal regime, heat flow maps have been prepared using manual and automatic contouring methods. The comparative study of automatic contour maps generated by means of a variety of data interpolation and gridding schemes has led to the identification of some geothermal features that are believed to be related to tectonic processes affecting the South American continent. Prominent among these are E-W trending belts of low heat flow in northern Peru and in central Chile (extending into the Sierras Pampeans in Argentina), as well as high heat flow belts in northern Chile (extending into the Altiplano in Bolivia) and southern Chile (extending into western Argentina). The low heat flow belts coincide approximately with zones of sub-horizontal subduction, while the high heat flow belts are situated in regions of high-angle subduction. Some of these features correlate well with the results of studies on anelastic attenuation, electrical resistivity distribution and some patterns of global seismic tomography. On the other hand, many of these features are not evident in the recent spherical harmonic analysis of global heat flow, which suggests that the use of empirical predictors based on a heat flow-age relation in devising global heat flow maps should be restricted to tectonically stable areas.

title: Thermal structure, thickness and composition of continental lithosphere
abstract: Global compilations of surface heat flow data from stable, Precambrian terrains show a statistically significant secular change from 41±11 mW/m 2 in Archean to 55±17 mW/m 2 in Proterozoic regions far removed from Archean cratons. Using the tectonothermal age of the continents coupled with average heat flow for different age provinces yields a mean continental surface heat flow between 47 and 49 mW/m 2 (depending on the average, non-orogenic heat flow assumed for Phanerozoic regions). Compositional models for bulk continental crust that produce this much or more heat flow (i.e., K 2 O>2.3–2.4 wt%) are not consistent with these observations. More rigorous constraints on crust composition cannot be had from heat flow data until the relative contributions to surface heat flow from crust and mantle are better determined and the non-orogenic component of heat flow in the areally extensive Phanerozoic regions (35% of the continents) is determined. We calculate conductive geotherms for 41 mW/m 2 surface heat flow to place limits on the heat production of Archean mantle roots and to evaluate the significance of the pressure–temperature ( P – T ) array for cratonic mantle xenoliths. Widely variable geotherms exist for this surface heat flow, depending on the values of crustal and lithospheric mantle heat production that are adopted. Using the average K content of cratonic peridotite xenoliths (0.15 wt% K 2 O, assuming Th/U=3.9 and K/U=10,000 to give a heat production of 0.093 μW/m 3 ) and a range of reasonable crustal heat production values (i.e., ≥0.5 μW/m 3 ), we calculate geotherms that are so strongly curved they never intersect the mantle adiabat. Thus the average cratonic peridotite is not representative of the heat production of Archean mantle roots. Using our preferred estimate of heat production in the cratonic mantle (0.03 wt% K 2 O, or 0.019 μW/m 3 ) we find that the only geotherms that pass through the xenolith P–T data array are those corresponding to crust having very low heat production ( 2 O). If the lithospheric mantle heat production is higher than our preferred values, the continental crust must have correspondingly lower heat production (i.e., bulk crustal K, Th and U contents lower than that of average Archean granulite facies terrains), which we consider unlikely. If the xenolith P–T data reflect equilibration to a conductive geotherm, then Archean lithosphere is relatively thin (150–200 km, based on intersection of the P–T array with the mantle adiabat) and the primary reason for the lower surface heat flow in Archean regions is decreased crustal heat production, rather than the insulating effects of thick lithospheric roots. On the other hand, if the xenolith P–T points result from frozen-in mineral equilibria or reflect perturbed geotherms associated with magmatism, then the Archean crust can have higher heat producing element concentrations, lithospheric thickness can range to greater depths and the low surface heat flow in Archean cratons may be due to the insulating effects of thick lithospheric roots. An uppermost limit for Archean crustal heat production of 0.77 μW/m 3 is determined from the heat flow systematics.

title: Heat flow in a back-arc environment: Intermontane and Omineca Crystalline belts, southern Canadian Cordillera
abstract: A suite of 20 heat flow measurements has been completed across the Intermontane and Omineca Crystalline belts in south-central British Columbia at about 50°N. Values along the 200 km line are high ...

title: The thermal structure and thickness of continental roots
abstract: We compare heat flow data from the Precambrian shields in North America and in South Africa. We also review data available in other less well-sampled Shield regions. Variations in crustal heat production account for most of the variability of the heat flow. Because of this variability, it is difficult to define a single average crustal model representative of a whole tectonic province. The average heat flow values of different Archean provinces in Canada, South Africa, Australia and India differ by significant amounts. This is also true for Proterozoic provinces. For example, the heat flow is significantly higher in the Proterozoic Namaqua-Natal Belt of South Africa than in the Grenville Province of the Canadian Shield (61 vs. 41 mW m−2 on average). These observations indicate that it is not possible to define single value of the average heat flow for all provinces of the same crustal age. Large amplitude short wavelength variations of the heat flow suggest that most of the difference between Proterozoic and Archean heat flow is of crustal origin. In eastern Canada, there is no good correlation between the local values of heat flow and heat production. In the Archean, Proterozoic and Paleozoic provinces of eastern Canada, heat flow values through rocks with the same heat production are not significantly different. There is therefore no evidence for variations of the mantle heat flow beneath these different provinces. After removing the local crustal heat production from the surface heat flow, the mantle (Moho) heat flow was estimated to be between 10–15 mW m−2 in the Archean, Proterozoic and Paleozoic provinces of eastern Canada. Estimates of the mantle heat flow in the Kaapvaal craton of South Africa may be slightly higher (≈17 mW m−2). Large-scale variations of bulk crustal heat production are well-documented in Canada and imply significant differences of deep lithospheric thermal structure. In thick lithosphere, surficial heart flow measurements record a time average of heat production in the lithospheric mantle and are not in equilibrium with the instantaneous heat production. The low mantle heat flow and current estimates of heat production in the lithospheric mantle do not support a mechanical (conductive) lithosphere thinner than 200 km and thicker than 330 km. Temperature anomalies with surrounding oceanic mantle extended to the convective boundary layer belong the conductive layer, and hence to depths greater than these estimates. Mechanical and thermal stability of the lithosphere require the mantle part of the lithosphere to be chemically buoyant and depleted in radiogenic elements. Both characteristics are achieved simultaneously by partial melting and melt extraction.

title: Gas hydrate stability and the assessment of heat flow through continental margins
abstract: A prominent feature across some continental margins is a bottom-simulating reflector (BSR). This seismic reflection generally coincides with the depth predicted for the base of the gas hydrate stability field. Because the occurrence of gas hydrates is controlled by temperature and pressure conditions, it has been suggested that BSRs mark an isotherm and they have therefore been used to estimate the heat flow through continental margins; crucial parameters are the temperature at BSR depth and at the seafloor and the thermal conductivity structure between the BSR and the seabed. However, very often the required parameters are not available and therefore they have been derived from models for gas hydrate stability and empirical relationships to obtain thermal conductivities from seismic velocities. Here, we use downhole temperature, thermal conductivity, porosity and logging data from 10 Ocean Drilling Program (ODP) sites drilled into and through the gas hydrate field to investigate the quality of estimates. Our analyses and application of constraints to the Makran margin off Pakistan indicate the following. (i) The temperature at BSR depth could be approximated by a seawater‐methane system, although capillary forces, chemical impurities or non-equilibrium conditions can lower (or increase) the temperature. If calibration by heat probe measurements is possible, errors of geothermal gradients are less than 10 per cent, otherwise uncertainties of 20 per cent (or even higher) may arise. In addition, seasonal variations of bottom water temperature have to be considered, because they may affect thermal gradients by up to y10 per cent. (ii) The impact of typical quantities of low-thermal-conductivity gas hydrate on the bulk thermal conductivity is insignificant. (iii) The thermal conductivity profile between the BSR and the seabed can generally be approximated by a mean value. Thus, (iv) seabed measurements should be used instead of empirical relationships, which may produce errors of 5‐30 per cent. Consequently, in addition to high-quality seismic data, a prerequisite should be a large data set of thermal conductivities and oceanographic data. Heat probe measurements are recommended to constrain geothermal gradients. In this case the uncertainty of heat flow is 5‐10 per cent of the estimated heat flow. If these data are not available errors/uncertainties can reach 50‐60 per cent of the calculated value.

title: Comparison of measured and BSR-derived heat flow values, Makran accretionary prism, Pakistan
abstract: During the German research cruise SO-124 on RV Sonne (fall 1997) on the Makran accretionary wedge off Pakistan, geophysical investigations were carried out to study the thermal regime at a gas hydrate bearing sediment in a tectonically deformed accretionary wedge. On a transect perpendicular to the strike of the deformation front 42 heat flow measurements were carried out, accompanied by seismic reflection experiments. The investigations start in the south in the abyssal plain and cover the continental slope up to 2300 m water depth. The aim of this study is to compare the BSR derived heat flow (denoted as estimated heat flow) with the values from measurements at the seafloor. This requires the calculation of sediment physical properties at depth using empirical relationships between velocity and porosity. The value measured and corrected for sedimentation of 47 mW/m 2 south of the deformation front is slightly higher than values reported by Hutchison et al. ( (1981) 252–262). In all basins the estimated heatflow is significantly higher than the measured values. As a result, temperatures at the BSR extrapolated from seafloor measurements are 5–6 K lower than those taken from Gas hydrate stability considerations. As an overall trend the estimated as well as the measured heat flow show a small decrease from the deformation front to the northward thickening prism. A similar observation was made at other accretionary wedges and described by Wang et al. ( (B3) (1993) 4121–4142) and Ferguson et al. ( (B6) (1993) 9975–9984). Within the slope basins heat flow values show little variation, indicating predominantly conductive heat transport. Fluid flow might occur at the bounding faults where we have little control. The effect of rapid sedimentation on the dynamic behavior of the BSR might also have a significant influence on the estimated heat flow values. Our data set shows clearly that detailed seismic surveys and good control of the subsurface velocity are absolute necessities for the comparison of measured and BSR-derived heat flow values. However, the uncertainty of the velocity–porosity relationship together with a high and only approximately established sedimentation rate represent crucial but missing constraints which can be gained only by drilling.

title: Methane hydrate stability in seawater
abstract: Experimental data are presented for methane hydrate stability conditions in seawater (S ≈ 33.5‰). For the pressure range of 2.75–10.0 MPa, at any given pressure, the dissociation temperature of mcthane hydrate is depressed by approximately −1.1°C relative to the pure methane-pure water system. These experimental results are consistent with previously reported thermodynamic predictions and experimental results obtained with artificial seawater. Collectively these results provide a minimum constraint concerning depth ranges over which methane hydrate is stable in the oceanic environment.

title: Mineralogy, porosity and fluid control on thermal conductivity of sedimentary rocks
abstract: Estimates of thermal conductivity of the main non-clay and clay sedimentary minerals are presented, especially for the case of argillaceous ones. These estimates are averaged estimates based on the interpretation of laboratory conductivity, porosity and mineralogy measurements performed on small volumes (of the order of 100cm3) of water-saturated, moist and air-saturated samples. The sampling is representative of the main sedimentary rocks (sandstones, carbonates, evaporites and shales), and it is composed of samples considered as structurally isotropic material. First, we experimentally verify the first-order control of mineralogy, porosity and fluid content on bulk conductivity, and we demonstrate that such influences may be predicted accurately using a geometric mean model, as long as isotropic samples are used. Then we interpret the data using an inverse method, in order to estimate the average mineral conductivities of the main non-clay and clay minerals which give the best fit to the individual laboratory measurements through the geometric mean model. The analysis is based on measurements on 82 non-clay samples and 29 clay samples taken on oil-well cores or cuttings, on outcrops and on artificially recompacted samples. Estimates of average mineral conductivities returned by the inversion process, for the main non-clay minerals, are similar to generally admitted values: 7.7 W m-' K-' for quartz, 3.3 W m-I K-' for calcite, 5.3 W m-l K-' for dolomite and 6.3 W m-' K-' for anhydrite. Values obtained for the clay minerals are systematically lower than those obtained for non-clay ones, they are of the order of 1.9 W m-' K-' for illite and smectite, 2.6 W m-' K-' for kaolinite and 3.3 W m-l K-' for chlorite. These estimates, combined with porosity and mineralogy data, are used with the geometric mean model in order to verify its first-order validity for predicting thermal conductivity of small volumes of porous isotropic sedimentary rocks. The bulk conductivity is predicted with an accuracy of the order of f10 per cent for moist or water-saturated samples, and of the order of f20 per cent for air-saturated samples.

title: Strength of the lithosphere: Constraints imposed by laboratory experiments
abstract: The concept of strength envelopes, developed in the 1970s, allowed quantitative predictions of the strength of the lithosphere based on experimentally determined constitutive equations. Initial strength envelopes used an empirical relation for frictional sliding to describe deformation along brittle faults in the upper portion of the lithosphere and power law creep equations to estimate the plastic flow strength of rocks in the deeper part of the lithosphere. In the intervening decades, substantial progress has been made both in understanding the physical mechanisms involved in lithospheric deformation and in refining constitutive equations that describe these processes. The importance of a regime of semibrittle behavior is now recognized. Based on data from rocks without added pore fluids, the transition from brittle deformation to semibrittle flow can be estimated as the point at which the brittle fracture strength equals the peak stress to cause sliding. The transition from semibrittle deformation to plastic flow can be approximated as the stress at which the pressure exceeds the plastic flow strength. Current estimates of these stresses are on the order of a few hundred megapascals for relatively dry rocks. Knowledge of the stability of sliding along faults and of the onset of localization during brittle fracture has improved considerably. If the depth to the bottom of the seismogenic zone is determined by the transition to the stable frictional sliding regime, then that depth will be considerably more shallow than the depth of the transition to the plastic flow regime. Major questions concerning the strength of rocks remain. In particular, the effect of water on strength is critical to accurate predictions. Constitutive equations which include the effects of water fugacity and pore fluid pressure as well as temperature and strain rate are needed for both the brittle sliding and semibrittle flow regimes. Although the constitutive equations for dislocation creep and diffusional creep in single-phase aggregates are more robust, few data exist for plastic deformation in two-phase aggregates. Despite the fact that localization is ubiquitous in rocks deforming both in brittle and plastic regimes, only a limited amount of accurate experimental data are available to constrain predictions of this behavior. Accordingly, flow strengths now predicted from laboratory data probably overestimate the actual rock strength, perhaps by a significant amount. Still, the predictions are robust enough that uncertainties in geometry, mineralogy, loading conditions and thermodynamic state are probably the limiting factors in our understanding. Thus, experimentally determined rheologies can be applied to understand a broad range of topical problems in regional and global tectonics both on the Earth and on other planetary bodies.

title: The role of water in the deformation of dunite
abstract: Specimens of two natural dunites (Anita Bay, of 100 μm grain size, and Aheirn, of 900 μm grain size) have been deformed in experiments at temperatures between 1200° and 1400°C and constant strain rates between 10−4 and 10−6 s−1 in a gas-medium deformation apparatus at 300 MPa confining pressure. Prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200°C for >60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water. The predried specimens are much stronger than specimens tested under wet conditions in an earlier study. The predried specimens also do not show the apparent grain size dependence seen under wet conditions. Infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock, flow stress can be correlated with the intensity of a residual “water related” broad absorption. The addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component. The weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water-related species in the grain boundaries, but intragranular weakening may also be contributing. It is suggested that the concentration of the water-related species in the grain boundaries is governed by its concentration in the small melt fraction present. The amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0.01 wt %, amounts much less than most estimates of the water content of the earth's undepleted upper mantle. Such water may have an important role in determining upper mantle flow behavior. 
 
Stress-strain curves for all experiments are available with entire manuscript on microfiche.

title: Rheology of synthetic olivine aggregates: Influence of grain size and water
abstract: The influence of grain size and water content on the high-temperature plasticity of olivine aggregates was studied, using a gas-medium high-pressure deformation apparatus. The specimens used were hot-pressed, dense olivine aggregates with controlled grain size ranging from a few to 70 μm, with or without added water. Mechanical tests were made at 1573 K and 300 MPa confining pressure and at strain rates of 10−3 to 10−6 s−1. The results reveal two distinct mechanisms of deformation, depending on stress level and grain size. At relatively high stress and large grain size, the strain rate is proportional to about the cube power of the stress and is nearly independent of grain size. In this regime, microstructural observations gave evidence of intragranular deformation involving dislocation motion. At low stress and small grain size, the strain rate depends almost linearly on stress and decreases markedly with increase in grain size. In the latter regime, little evidence was found for intragranular deformation. These observations suggest that the deformation mechanism in the grain size insensitive regime is dislocation creep, while that in the grain size sensitive regime is diffusion creep. In both regimes, water was found to enhance the creep rate. The absence of grain size sensitivity in the dislocation creep regime and comparison with single-crystal data indicate that the water weakening effect is mainly an intragranular process. However, the existence also of a water weakening effect in the diffusion creep regime indicates that water also enhances diffusion. The extrapolation of the present results to coarser grain sizes indicates that the transition from dislocation to diffusion creep occurs at 0.1–1 MPa for 10-mm grain size. Therefore it is suggested that this transition may occur in the upper mantle and that, in both regimes, the presence of trace amounts of water will result in significantly lower creep strength than under strictly “dry” conditions.

title: Influence of water on plastic deformation of olivine aggregates 2. Dislocation creep regime
abstract: Triaxial compressive creep experiments have been conducted over a range of hydrous conditions to investigate the effect of water fugacity on the creep behavior of olivine aggregates in the dislocation creep regime. Samples synthesized from powders of San Carlos olivine were deformed at confining pressures of 100 to 450 MPa and temperatures between 1473 and 1573 K. Water was supplied by the dehydration of talc. Water fugacities of ∼80 to ∼520 MPa were obtained by varying the confining pressure under water-saturated conditions with the oxygen fugacity buffered at Ni/NiO. Sancles were deformed at differential stresses of ∼20 to 230 MPa. The transition from diffusion creep to dislocation creep occurs near 100 MPa for both the hydrous case and the anhydrous case. Under hydrous conditions creep experiments yield a stress exponent of n ≈ 3 and an activation energy of Q ≈ 470 kJ/mol. The creep rate of olivine is enhanced significantly with the presence of water. At a water fugacity of ∼300 MPa, samples crept ∼5–6 times faster than those deformed under anhydrous conditions at similar differential stresses and temperatures. Within the range of water fugacity investigated, the strain rate is proportional to water fugacity to the 0.69 to 1.25 power, assuming values for the activation volume of 0 to 38×10−6 m3/mol, respectively. We argue that water influences creep rate primarily through its effect on the concentrations of intrinsic point defects and hence on ionic diffusion and dislocation climb. With increasing water fugacity the charge neutrality condition changes from [FeMe•] = 2[VMe″] to [FeMe•] = [HMe′]. For the latter charge neutrality condition the concentration of silicon interstitials is proportional to fH2O1, suggesting that under hydrous conditions dislocation climb is rate limited by diffusion of Si occurring by an interstitial mechanism. Our experimentally determined constitutive equation permits extrapolation from laboratory to mantle conditions in order to assess the rheological behavior of regions of the upper mantle with different water contents, such as beneath a mid-ocean ridge and in the mantle wedge above a subducting slab.

title: Seismic consequences of warm versus cool subduction metamorphism: examples from southwest and northeast japan
abstract: Warm and cool subduction zones exhibit differences in seismicity, seismic structure, and arc magmatism, which reflect differences in metamorphic reactions occurring in subducting oceanic crust. In southwest Japan, arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth; in northeast Japan, arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth. Thermal-petrologic models predict that oceanic crust subducting beneath southwest Japan is 300 degrees to 500 degrees C warmer than beneath northeast Japan, resulting in shallower eclogite transformation and slab dehydration reactions, and possible slab melting.

title: New, improved version of generic mapping tools released
abstract: Version 3.1 of the Generic Mapping Tools (GMT) has been released. More than 6000 scientists worldwide are currently using this free, public domain collection of UNIX tools that contains programs serving a variety of research functions. GMT allows users to manipulate (x,y) and (x,y,z) data, and generate PostScript illustrations, including simple x-y diagrams, contour maps, color images, and artificially illuminated, perspective, and/or shaded-relief plots using a variety of map projections (see Wessel and Smith [1991] and Wessel and Smith [1995], for details.). GMT has been installed under UNIX on most types of workstations and both IBM-compatible and Macintosh personal computers.

title: Fault stability inferred from granite sliding experiments at hydrothermal conditions
abstract: Seismicity on crustal faults is concentrated in the depth interval 1–3 to 12–15km. Tse and Rice (1986) suggested that the lower bound on seismicity is due to a switch with increasing temperature from velocity weakening (destabilizing) to velocity strengthening (stabilizing) friction. They inferred this transition from friction data for dry granite; however, pore fluids exist at elevated temperatures throughout the crust, and may strongly influence strength and sliding behavior. We present new data from sliding experiments on granite at elevated T (23° to 600°) plus elevated PH2O (100 MPa), 
 
Our results show velocity strengthening at room temperature, but velocity weakening from 100° to 350°C (except at 250°). From 350° to 600° there are systematic trends from velocity weakening to strong velocity strengthening, and from high to low friction; neither trend was seen in tests on dry granite. The velocity dependence data imply the potential for unstable slip in the interval 100° to 350°. Using a geotherm to map temperature to depth, this interval closely matches the observed earthquake distribution.

title: Frictional slip of granite at hydrothermal conditions
abstract: Sliding on faults in much of the continental crust likely occurs at hydrothermal conditions, i.e., at elevated temperature and elevated pressure of aqueous pore fluids, yet there have been few relevant laboratory studies. To measure the strength, sliding behavior, and friction constitutive properties of faults at hydrothermal conditions, we slid laboratory granite faults containing a layer of granite powder (simulated gouge). Velocity stepping experiments were performed at temperatures of 23° to 600°C, pore fluid pressures PH2O of 0 (“dry”) and 100 MPa (“wet”), effective normal stress of 400 MPa, and sliding velocities V of 0.01 to 1 μm/s (0.32 to 32 m/yr). Conditions were similar to those in earlier tests on dry granite to 845°C by Lockner et al. (1986). The mechanical results define two regimes. The first regime includes dry granite up to at least 845° and wet granite below 250°C. In this regime the coefficient of friction is high (μ = 0.7 to 0.8) and depends only modestly on temperature, slip rate, and PH2O. The second regime includes wet granite above ∼350°C. In this regime friction decreases considerably with increasing temperature (temperature weakening) and with decreasing slip rate (velocity strengthening). These regimes correspond well to those identified in sliding tests on ultrafine quartz. We infer that one or more fluid-assisted deformation mechanisms are activated in the second, hydrothermal, regime and operate concurrently with cataclastic flow. Slip in the first (cool and/or dry) regime is characterized by pervasive shearing and particle size reduction. Slip in the second (hot and wet) regime is localized primarily onto narrow shear bands adjacent to the gouge-rock interfaces. Weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide, or from a dependence of strength on gouge particle size. Major features of the granite data set can be fit reasonably well by a rate- and temperature-dependent, three-regime friction constitutive model (Chester, this issue). We extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural, slipping faults for sliding rates as low as 31 mm/yr. We do this for two end-member cases. In the first case, pore pressure is assumed hydrostatic at all depths. Shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data, while at depths below about 9–13 km (depending on slip rate), strength becomes less sensitive to depth but sensitive to slip rate. In the second case, pore pressure is assumed to be near-lithostatic at depths below ∼5 km. Strength is low at all depths in this case (<20 MPa, in agreement with observations of “weak” faults such as the San Andreas). The predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm/yr, in rough agreement with the seismic-aseismic transition depth observed on mature continental faults. These results highlight the importance of fluid-assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry, large displacements, higher concentrations of phyllosilicates, and time-dependent fault healing.

title: A narrowly spaced double‐seismic zone in the subducting Nazca plate
abstract: High-precision relocations of intermediate-depth earthquakes (80–130 km) below the Central Andes reveal a fine-scale double-layered Wadati-Benioff zone (WBZ). Upper and lower band of seismicity are separated by about 9 km and occur at the top of the oceanic crust and in the uppermost oceanic mantle, respectively. Analysis of focal mechanisms and waveform similarities indicate that fluid processes are causing the events. Earthquakes in the oceanic crust occur on pre-existing normal faults due to hydraulic embrittlement from metamorphic dehydration, and on subvertical faults that connect the two layers in a narrow depth range. Extensional faulting predominates in both layers, indicating that slab pull forces are the dominant stress source superseding possible unbending forces in this segment of the Nazca plate.

title: Crustal Earthquake Instability in Relation to the Depth Variation of Frictional Slip Properties
abstract: Recent stability studies using constitutive relations of the type found by Dieterich, Ruina, and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes. The class of friction laws discussed has the property that the sliding stress depends on normal stress, temperature, slip rate, and slip history. For sliding at a fixed slip rate V and fixed environment (e.g., normal stress, temperature, etc.) the shear strength τ evolves toward a steady state value τss(V). Stability analysis show that for dτss(V)/dV 0 (velocity strengthening) implies stable steady state sliding, at least for a broad class of constitutive laws more fully described in the paper. Experiments by Dieterich and by Tullis and Weeks on Westerly granite with mature sliding surfaces indicate that dτss(V)/dV is negative at room temperature, whereas higher-temperature experiments by Stesky show that dτss(V)/dV becomes positive above approximately 300°C. Therefore, in the case of the earth, where temperature increases with depth, the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth, from a regime with dτss(V)/dV 0. This is not inconsistent with, but rather refines, the suggestion by Sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow. Further, our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth. The depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone. Following Mavko, a two-dimensional quasi-static strike-slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the Lachenbruch-Sass depth variation of temperature for the San Andreas fault. The resulting predictions of such features as the confinement of crustal earthquakes to shallow depths, the development of locked patches, the recurrence time for the seismic cycle, the seismic stress drop and displacement, etc., are generally in agreement with the observed characteristics of large-scale strike-slip earthquakes along the San Andreas fault. Calculations based on the laboratory data predict, for example, that stick-slip nucleates around 5–7 km, that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13–15 km depths, that rapid postseismic creep occurs over another 3–4 km depth, and that at greater depths, steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above. The most uncertain parameter is the slip weakening distance L for evolution of fault surface state; we illustrate how features of the predicted earthquake cycles vary with L over the range for which calculations are feasible.

title: The solid‐state flow of polymineralic rocks
abstract: Polymineralic rocks display three end-member types of mechanical and microstructural behavior: (1) strong minerals form a load-bearing framework that contains spaces filled with weaker minerals; (2) two or more minerals with low relative strengths control bulk rheology and form elongate boudins; (3) one very weak mineral governs bulk rheology, while the stronger minerals form clasts. A concept of strain energy partitioning in polyphase aggregates allows the expression of the bulk strength of a rock in terms of its strain rate, temperature, and the volume proportions and rheological properties of its constituent minerals. Bimineralic strength versus composition relations predicted with this model are highly nonlinear and mimic the rheological behavior of experimentally deformed bimineralic aggregates. Quantitative analysis of microstructures in quartzo-feldspathic tectonites suggests that foliation development leads to a significant reduction in rock strength. This is due to decreased stress concentration within the deforming aggregate as matrix and clast grain size is reduced and the average spacing between strong minerals increases.

title: Flow laws for rocks containing two non-linear viscous phases: A phenomenological approach
abstract: Mylonitic rocks and rock-analogue materials reveal two basic types of structure: (1) a load-bearing framework (LBF) of strong phase contains isolated pockets of weak phase; (2) interconnected layers of weak phase (IWL) separate boudins and clasts of strong phase. Aggregates with the LBF microstructure are characterized by nearly uniform strain rate. Stress is concentrated in the load-bearing framework. In aggregates with an IWL microstructure, strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase. The degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases. Based on these observations, the rheology of two-phase rock is modelled with separate functions for LBF and IWL microstructures. A new flow law is derived for rock with IWL structure in which two phases undergo dislocation creep. The flow law expresses composite creep strength in terms of temperature, bulk strain rate and the volume proportions and creep parameters of the minerals in the rock. Strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate. Composite strengths predicted with the IWL flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates. A hypothesis of viscous strain energy minimization is used to determine the relative stability of the LBF and IWL microstructures. During steady-state creep, the IWL microstructure is predicted to be stable over a broad range of two-phase compositions and mineral strength contrasts, whereas the LBF microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts. The IWL flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition, especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts.

title: On the nomenclature of mode of failure transitions in rocks
abstract: Attention is drawn to the increasing identification in the geological literature of the concept of ductility with a particular mechanism of rock deformation, namely intracrystalline plasticity. It is argued here that “ductility” only describes the capacity of a material to deform to a substantial strain without the tendency to localize the flow into bands (faults), and should not have a mechanistic connotation. A simple diagrammatic visualization of the concepts of mode-of-failure transitions is proposed. By force of repetition, the depth in the Earth to the seismic—aseismic transition is being regarded as a brittle—ductile transition in the sense of a brittle—plastic transition. This tendency is criticized and it is suggested that the nomenclature of mode of failure of rocks be rationalized, in the face of growing prospects of imprecision and misunderstandings of the communication of data and ideas.

title: Thermochronometry and microstructures of quartz-a comparison with experimental flow laws and predictions on the temperature of the brittle-plastic transition
abstract: A gradient in quartz microfabrics across a major strike-slip shear zone (with a minor vertical component), active during the Oligocene in the Eastern Alps (Alto Adige, Italy), is correlated with new zircon fission track thermochronometric data and available Rb–Sr biotite ages to constrain the depth/temperature range of the recorded rheologic regimes. Distributed deformation in the semibrittle regime (i.e. beneath the brittle–ductile transition) is effective near the closure temperature for fission tracks in zircon (which we estimate as 280±30°C), with high-stress dislocation creep of quartz, microcracking, and pressure solution being active simultaneously. Steady state dislocation creep of quartz at moderate stress in the fully plastic regime is effective at temperatures above the closure temperature for the Rb–Sr and K–Ar systems of biotite (ca. 310±30°C) and below that for the K–Ar system of white mica (ca. 350±50°C). For the inferred temperatures and correlated flow stresses derived by paleopiezometry, the majority of available experimental flow laws for wet quartzite predict strain rates on the order of 10 −13 –10 −14  s −1 , consistent with the geological constraints. This finding supports the validity of the extrapolation of experimental flow laws to natural strain rates.

title: The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700 °C
abstract: Near the eastern end of the Tonale fault zone, a segment of the Periadriatic fault system in the Italian Alps, the Adamello intrusion produced a syn-kinematic contact aureole. A temperature gradient from ∼250 to ∼700 °C was determined across the Tonale fault zone using critical syn-kinematic mineral assemblages from the metasedimentary host rocks surrounding deformed quartz veins. Deformed quartz veins sampled along this temperature gradient display a transition from cataclasites to mylonites (frictional–viscous transition) at 280±30 °C. Within the mylonites, zones characterized by different dynamic recrystallization mechanisms were defined: Bulging recrystallization (BLG) was dominant between ∼280 and ∼400 °C, subgrain rotation recrystallization (SGR) in the ∼400–500 °C interval, and the transition to dominant grain boundary migration recrystallization (GBM) occurred at ∼500 °C. The microstructures associated with the three recrystallization mechanisms and the transitions between them can be correlated with experimentally derived dislocation creep regimes. Bulk texture X-ray goniometry and computer-automated analysis of preferred [c]-axis orientations of porphyroclasts and recrystallized grains are used to quantify textural differences that correspond to the observed microstructural changes. Within the BLG- and SGR zones, porphyroclasts show predominantly single [c]-axis maxima. At the transition from the SGR- to the GBM zone, the texture of recrystallized grains indicates a change from [c]-axis girdles, diagnostic of multiple slip systems, to a single maximum in Y. Within the GBM zone, above 630±30 °C, the textures also include submaxima, which are indicative of combined basal 〈a〉- and prism [c] slip.

title: Reaction and Geometrical Softening in Granitoid Mylonites
abstract: The microstructures of granitoid mylonites from Locquirec, N. Brittany; Ben Arnaboll, NW Scotland; and Cap de Creus, NE Spain show a similarity of development as a response to both geometric and reaction softening. A combination of these two softening processes leads to a concentration of deformation in the respective shear zones. Geometric softening leads to a rotation of quartz lattices for easy slip and is accompanied by dynamic recrystallization. Metamorphic reactions involving the breakdown of orthoclase and plagioclase feldspar to fine aggregates of muscovite plus quartz provide a secondary softening process which further concentrates deformation. These reactions indicate fluid mobility in the shear zones during deformation processes.

title: Deformation mechanisms in granitic rocks at shallow crustal levels
abstract: Microstructural observations of naturally faulted granitic gneisses show that feldspar grains are weaker than quartz grains at temperatures below 325γC and at depths of less than 8–10 km. Feldspar grains sustained most of the deformation by grain-scale faulting, slip along cleavage-controlled fractures and cataclasis. Fracture of feldspar grains within fault zones promoted their alteration to kaolinite. Quartz grains also deformed by fracturing, and often healed to form quartz porphyroclasts and mosaics in a comminuted matrix of feldspar and kaolinite. Syntectonic alteration of the feldspar grains may have weakened the fault zones over time and resulted in foliated textures within the fault zones. This study of naturally deformed rocks confirms published experimental results on the behavior of granitic rocks at low temperatures and pressures and, taken together, these data show that the theology of the upper 10 km of the crust is greatly influenced by cataclastic processes in feldspar.

title: Fluid‐rock reaction weakening of fault zones
abstract: The presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well-developed fabrics. The growth of phyllosilicates is favored in meteoric water-dominated granitic fault systems, where mineral-aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions. In deeper, more alkaline, rock-dominated regimes, the reactions reverse, and feldspars tend to replace phyllosilicates. In Mg-rich mafic rocks, however, phyllosilicates (chlorite, biotite) can replace stronger framework and chain silicates in both shallower (<{approximately}10 km) meteoric H{sub 2}O-dominated and in deeper, alkaline, rock-dominated regimes. Where these phyllosilicates precipitate in active fault zones, they contribute directly to reaction softening. Because low-temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure-independent dislocation glide, the strength of phyllosilicate-rich fault rocks can be low at all depths. Low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites. Where preferred orientations are strong and contiguity of phyllosilicates is large, strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals. Fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well-defined folia can exhibit aseismic slip, especially where mafic Mg-rich rocksmore » occur along the fault (like parts of the San Andreas Fault). 104 refs., 6 figs., 1 tab.« less

title: Reaction weakening and emplacement of crystalline thrusts: Diffusion control on reaction rate and strain rate
abstract: In the southern Appalachians, the Blue Ridge–Piedmont crystalline thrust sheet was emplaced onto low-grade Late Precambrian and Paleozoic sedimentary rocks in the footwall along a basal detachment consisting of phyllosilicate-rich mylonites (phyllonites). The phyllonites developed first by mechanical breakdown of feldspar followed by chemical breakdown to white mica in the presence of a pore fluid. Diffusion of solute in the pore fluid is the rate limiting step in controlling reaction rate and also the strain rate. Assuming solute diffusion follows the Stokes–Einstein equation, the shear strain rate is given by ⅆ γ / ⅆ t = 2 n ω k T / 5 η r x 2 for shear stress ≥20 MPa, where n is a constant, ω is a geometric factor, k is Boltzmann's constant, T is absolute temperature, η is water viscosity, r is the atomic radius of the diffusing species, and x is the diffusion distance. A bulk diffusion coefficient in the range of ∼10 −10 to 10 −12  m 2 /s over distances of 10–100 m results in strain rates of 10 −14 to 10 −13  s −1 in the temperature range 200–400 °C. It is concluded that greenschist grade crystalline thrust sheets develop on pre-existing basement faults that become weak during reaction softening and localize into high strain phyllonite zones in which pore fluid diffusion controls reaction rate and strain rate.

title: Micromechanics of the brittle to plastic transition in Carrara marble
abstract: Samples of Carrara marble were deformed at room temperature to varying strains at confining pressures spanning the range in mechanical behavior from brittle to plastic. Volumetric strain was measured during the experiments, and the stress-induced microstructure was characterized quantitatively using optical and transmission electron microscopy. The range of confining pressure over which transitional (or semibrittle) deformation occurs is 30–300 MPa. The macroscopic initial yield stress is constant for confining pressures greater than 85 MPa, whereas the differential stress at the onset of dilatancy increases with pressure up to 300 MPa. The dilatancy coefficient decreases rapidly with increasing pressure up to 100 MPa, and then asymptotically approaches zero for pressures up to 300 MPa. The work hardening coefficient increases with pressure up to 450 MPa; the pressure sensitivity is greatest for pressures up to 100 MPa. Active deformation mechanisms include microcracking, twinning, and dislocation glide. Transmission electron microscopy observations indicate that dislocation glide occurs, at least on a local scale, in samples deformed in the semibrittle field at pressures as low as 50 MPa and applied differential stress well below the critical resolved shear stress for glide on the easiest slip system. Cracks and voids frequently nucleate at sites of stress concentration at twin boundaries, at twin terminations, and at the intersection of twin lamallae. Geometries suggestive of crack tip shielding by dislocations are also observed. Stereological measurements indicate that at constant strain in the semibrittle field, the stress-induced crack density and anisotropy decrease with increasing pressure. Crack density and anisotropy in samples deformed to strains of 3–5% in the semibrittle field at pressures up to 120 MPa are comparable to those in the prefailure brittle sample, although an analysis of the energetics of deformation suggests that the ratio of brittle energy dissipation to total energy dissipation is at least 60% lower. We also detect a qualitative difference in the characteristic length of the cracks in the brittle and semibrittle fields. The mean dislocation density at constant differential stress increases significantly for samples deformed at pressures of 230 MPa and greater. Our results suggest that although semibrittle flow occurs over a wide range of pressure, the most marked changes in strain partitioning, and hence the style of deformation occur over a small range in pressure.

title: The brittle‐plastic transition in experimentally deformed quartz aggregates
abstract: Deformation experiments have been conducted to provide constraints on the processes responsible for the brittle-plastic transition in quartz aggregates. A correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle-plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and/or pressure. First there is a transition from cataclastic faulting to semibrittle faulting; microstructural observations indicate that this transition occurs due to the activation of dislocations. In addition, faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide. Second, there is a transition from semibrittle faulting to semibrittle flow; this transition corresponds to a change from localized to distributed deformation. Microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime. We conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries. Third, there is a transition from semibrittle flow to dislocation creep. Microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature. In addition, microstructural observations indicate that a transition from dominantly mode I (axial) to mode II (shear) microcracking occurs with an increase in confining pressure from 0.4 to 0.8 GPa, regardless of temperature. The differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions. However, a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the Earth. The results of this study provide further evidence to indicate that the brittle-plastic transition in the continental crust occurs over a relatively wide range of conditions.

title: Textural and microstructural evidence for semi-brittle flow in natural fault rocks with varied mica contents
abstract: Fault rocks from the Siberia Fault Zone (SFZ) in southern New Zealand are derived from schists with varied mica contents. Regional evidence indicates that the rocks are exhumed from depths of 8–10 km and temperatures of 200–250 °C. Foliated cataclasites in a zone 5–40 m thick are accompanied by interlaced pseudotachylyte, and are cut through by a late-stage master fault and zones of random fabric cataclasite. Textures and microstructures in the foliated cataclasites reflect contemporaneous or cyclic operation of cataclastic, crystal–plastic and solution transfer deformation mechanisms, partitioned differently between different phases. The deformation regime is interpreted as a form of semi-brittle flow, facilitated by crystal–plastic deformation of phyllosilicate phases in a relatively weak interconnected matrix. Quartz and feldspar are deformed mainly by cataclasis. The presence of pseudotachylyte indicates the fault was seismically active, and non-localised semi-brittle flow was episodically punctuated by high strain-rate earthquake events. Late-stage formation of a discrete master fault probably reflects a change from semi-brittle flow to brittle faulting. The presently exposed level of the fault is thought to represent a section of the mid-crustal brittle–ductile transition in the seismogenic zone. Thus, this study provides a tangible natural example of theoretically and experimentally predicted fault rocks.

title: Crustal motion in the zone of the 1960 Chile earthquake: Detangling earthquake‐cycle deformation and forearc‐sliver translation
abstract: Temporary deformation in great earthquake cycles and permanent shear deformation associated with oblique plate convergence both provide critical clues for understanding geodynamics and earthquake hazard at subduction zones. In the region affected by the Mw 9.5 great Chile earthquake of 1960, we have obtained GPS observations that provide information on both types of deformation. Our velocity solutions for the first time span the entire latitudinal range of the 1960 earthquake. The new observations revealed a pattern of opposing (roughly arc-normal) motion of coastal and inland sites, consistent with what was reported earlier for the northern part of this region. This finding supports the model of prolonged postseismic deformation as a result of viscoelastic stress relaxation in the mantle. The new observations also provide the first geodetic evidence for the dextral motion of an intravolcanic arc fault system and the consequent northward translation of a forearc sliver. The sliver motion can be modeled using a rate of 6.5 mm/a, accommodating about 75% of the margin-parallel component of Nazca–South America relative plate motion, with the rate diminishing to the north. Furthermore, the new GPS observations show a southward decrease in margin-normal velocities of the coastal area. We prefer explaining the southward decrease in terms of changes in the width or frictional properties of the megathrust seismogenic zone. Because of the much younger age of the subducting plate and warmer thermal regime in the south, the currently locked portion of the plate interface may be narrower. Using a three-dimensional viscoelastic finite element model of postseismic and interseismic deformation following the 1960 earthquake, we demonstrate that this explanation, although not unique, is consistent with the GPS observations to the first order.