segmentation of the nazca and south american plates along the ecuador subduction zone from wide angle seismic profiles we describe the deep structure of the south colombian northern ecuador convergent margin using travel time inversion of wide angle seismic data recently collected offshore the margin appears segmented into three contrasting zones in the north zone affected by four great subduction earthquakes during the 20th century normal oceanic crust subducts beneath the oceanic cretaceous substratum of the margin underlined by seismic velocities as high as 6 0 6 5 km s in the central zone the subducting oceanic crust is over thickened beneath the carnegie ridge a steeper slope and a well developed high velocity cretaceous oceanic basement characterizes the margin wedge this area coincides with a gap in significant subduction earthquake activity in the south zone the subducting oceanic crust is normal the fore arc is characterized by large sedimentary basins suggesting significant subsidence velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing even if the distance between the three profiles exceeds 150 km the structural segmentation obtained along the ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation

stratigraphic responses to a major tectonic event in a foreland basin the ecuadorian oriente basin from eocene to oligocene times the eocene to oligocene sediments of the ecuadorian oriente basin record two kinds of second order stratigraphic response to the tectonic evolution lower eocene shows evidences of local scale syntectonic deposits this tectonic activity can be related to right lateral convergent movements inverting pre cretaceous extensional structures upper eocene and oligocene sediments are integrated as the expression of an isostatic rebound characterizing a basin scale syntectonic deposition this response is evidenced by a reciprocal architecture of the depositional sequences identified in the sedimentary formations these data have allowed us to propose a new geodynamic model for the paleogene evolution of the oriente basin

the great tumaco colombia earthquake of 12 december 1979 southwestern colombia and northern ecuador were shaken by a shal low focus earthquake on 12 december 1979 the magnitude 8 shock located near tumaco colombia was the largest in northwestern south america since 1942 and had been forecast to fill a seismic gap thrust faulting occurred on a 280 by 130 kilometer rectangular patch of a subduction zone that dips east beneath the pacific coast of colombia a 200 kilometer stretch of the coast tectonically subsided as much as 1 6 meters uplift occurred offshore on the continental slope a tsunami swept inland immediately after the earthquake ground shaking intensity vi to ix caused many buildings to collapse and generated liquefaction in sand fills and in holocene beach lagoonal and fluvial deposits

mass transport deposits in the northern ecuador subduction trench result of frontal erosion over multiple seismic cycles article i nfo investigations of mass transport deposits mtds and turbidite deposition in the confined north ecuador subduction trench provide access to paleoseismic information and insights into long term mechanisms for frontal tectonic erosion at a convergent margin the studied trench has been the site of four great subduction earthquakes 7 7 mw 8 8 during the 20th century the trench is isolated from major continental sediment input so that investigated mtds and turbidites are considered of local origin swath bathymetry seismic reflection and chirp data together with sedimentary cores and 14 c dating revealed that seven mtds were emplaced in distinct trench sub basins since 23 kyr and 27 turbidites deposited in the southernmost trench sub basin since 4 9 kyr our analysis shows that six mtds were derived from the margin while a single one stemmed from the outer trench wall temporal correlations between mtds emplaced within trench sub basins separated by a structural saddle indicate that the seven mtds were emplaced during five main events three were triggered locally and tentatively dated 5 8 1 6 kyr and recent whereas four were emplaced in distinct trench sub basins as a result of two regional events at 22 6 and 15 4 kyr none of the mtds occurred during the fast stage of the last sea level rise 13 to 8 kyr however dissociation of gas hydrates during the last 8 kyr stage of slow sea level rise might have contributed to trigger the three youngest mtds the large 1 5 13 5 kyr return time of the mtds contrasts with that of 189 yr of the turbidites the later is consistent with the 73 yr return time of two local mw 8 2 earthquakes implying that turbidites might have been triggered by large earthquakes the very large mtds return time is attributed to long term deformation processes and mechanical weakening of the margin outer wedge in response to repeated variations in basal friction pore pressure and margin extensional contraction strain over multiple earthquake cycles this process contributes to short term frontal erosion the rate of which is estimated to be 8 6 10 3 km 3 kyr km since at least 15 4 kyr

common pb systematics of precambrian granitic rocks of the nubian shield egypt and tectonic implications we report the pb isotopic compositions for alkali feldspars separated from 34 granitic rocks from the late proterozoic assemblages of the eastern desert and the sinai peninsula egypt in the nubian segment of the arabian nubian shield the eastern desert shows a range of initial pb isotopic compositions 206 pb 204 pb 17 375 19 176 207 pb 204 pb 15 462 15 629 208 pb 204 pb 37 023 38 349 that extend from model mantle toward upper crustal values initial pb isotopic compositions do not define geographic provinces except at aswan and surrounding areas where the most radiogenic values were obtained aswan 207 pb 204 pb 15 611 gebel el hudi 207 pb 204 pb 15 629 and wadi mariya 207 pb 204 pb 15 615 samples from these areas are separated from the less radiogenic samples to the east by trails of serpentinites that we interpret from landsat thematic mapper images as the extension of the allaqi heiani ophiolitic belt our data support previous interpretations of the belt as a suture location and suggest that its western margin might coincide with the boundary between the nubian shield and the old african continent we interpret the pb isotopic compositions from the eastern desert and the previously identified group i and ii pb stacey and others 1980 from the arabian nubian shield to indicate various degrees of mixing between mantle derived juvenile oceanic component s and pre pan african crustal material on 207 pb 204 pb 208 pb 204 pb plots the feldspar and galena data display variations characteristic of modern island arc magmas consistent with models that relate shield assembly to accretion of island arcs the crustal pb isotopic signature of sample from aswan and surrounding areas is best explained by interaction of arc magmas with an overriding crustal plate whereas the less radiogenic nature of the eastern desert pb9s is probably related to subduction of sediments derived from adjacent continent or deposition of these sediments in near arc basins accounting for recycled pre pan african crustal material in the shield results in normal growth rates 0 44 km 3 yr using reasonable assumptions for initial pb isotopic compositions and concentrations

the brittle plastic transition and the depth of seismic faulting a simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow the two are separated by an abrupt brittle plastic transition which is assumed to be indicated by the lower limit of seismicity experimental studies however as well as the deformation structures of mylonites indicate that a broad transitional field of semi brittle behavior lies between these extremes this is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws for quartzofeldspathic rocks the semi brittle field lies between t1 the onset of quartz plasticity at about 300 c and t2 feldspar plasticity at about 450 c a model is presented in which the transition t1 does not correspond to a transition to bulk flow but to a change from unstable velocity weakening friction to stable velocity strengthening friction t1 thus marks the depth limit of earthquake nucleation but large earthquakes can propagate to a greater depth t3 t3 t2 which corresponds to the lower limit of dynamic frictional behavior in the semi brittle field and approximately to the peak in strength the zone between t1 and t3 is one of alternating behavior with flow occurring m the interseismic period and with co seismic dynamic slip occurring during large earthquakes this zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting the transition t1 is also marked by a change in the generation mechanism of fault rocks from abrasive wear above which produces cataclastites to adhesive wear below which is proposed as an important generation mechanism of mylonites in the upper part of the semi brittle field

new insights on the interseismic active deformation along the north ecuadorian south colombian nesc margin 1 the north ecuadorian south colombian subduction zone was the site of the 1906 mw 8 8 megathrust earthquake this main shock was followed by three large events in 1942 1958 and 1979 whose rupture zones were located within the 500 km long 1906 rupture area acombined onshore andoffshore temporary seismic network covering from thetrench to the andes was deployed during 3 months in the area of large earthquakes in order to obtain a detailed knowledge of the seismic background activity resulting earthquakes location and mechanisms bring new insights on interseismic active deformation distribution in the three main tectonic units of the margin namely the interplate seismogenic zone the fore arc region which is part of the north andean block and the downgoing oceanic nazca plate the interplate seismic activity presents along strike variations suggesting that the seismicity and the associated stress buildup along the plate interface depend on the time elapsed since the last large earthquakes according to our results the updip and downdip limits of the seismogenic zone appear to be located at 12 and 30 km depth respectively shallow to intermediate depth seismicity indicates a slab dip angle of 25 north of the carnegie ridge the wadati benioff plane is defined beneath the fore arc down to 100 km depth facing the ridge the wadati benioff plane extends beneath the andes down to 140kmdepth thisobservationconflictswiththehypothesisofthepresenceofaflatslabat a depth of 100 km facing the ridge in the overlying fore arc region the crustal seismicity occurs down to 40 km depth and is mainly concentrated in a roughly nw se 100 km wide stripe stretching from the coast at about 1 n to the andes the location of this active deformation stripe coincides with observed tectonic segmentation of the coastal domain as evidenced by the presence of an uplifting segment to the south and a subsiding segment to the north of the stripe it also corresponds to a 30 change in the trend of the andes suggesting that the curvature of the volcanic arc might play an important role in the deformation of the fore arc region

andean flat slab subduction through time the analysis of magmatic distribution basin formation tectonic evolution and structural styles of different segments of the andes shows that most of the andes have experienced a stage of flat subduction evidence is presented here for a wide range of regions throughout the andes including the three present flat slab segments pampean peruvian bucaramanga three incipient flat slab segments carnegie guanacos tehuantepec three older and no longer active cenozoic flat slab segments altiplano puna payenia and an inferred palaeozoic flatslab segment early permian san rafael based on the present characteristics of the pampean flat slab combined with the peruvian and bucaramanga segments a pattern of geological processes can be attributed to slab shallowing and steepening this pattern permits recognition of other older cenozoic subhorizontal subduction zones throughout the andes based on crustal thickness two different settings of slab steepening are proposed slab steepening under thick crust leads to delamination basaltic underplating lower crustal melting extension and widespread rhyolitic volcanism as seen in the caldera formation and huge ignimbritic fields of the altiplano and puna segments on the other hand when steepening affects thin crust extension and extensive within plate basaltic flows reach the surface forming large volcanic provinces such as payenia in the southern andes this last case has very limited crustal melt along the axial part of the andean roots which shows incipient delamination based on these cases a palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism the geological evolution of the andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system

seismicity and geometry of a 110 km long blind thrust fault 2 synthesis of the 1982 1985 california earthquake sequence during 1982 1985 three 5 4 mw 6 5 earthquakes migrated 65 km along the north half of a string of quaternary folds at the east front of the california coast ranges there is no surface fault associated with these earthquakes but the fold chain oriented parallel to the san andreas fault 30 km to the west takes up contraction normal to the san andreas fault identification of active blind thrust faults is hindered by the absence of a fault trace but may be revealed by the presence of growing folds at the surface and by earthquakes at depth because of excellent seismic geodetic and geologic data this earthquake sequence illuminates the geometry of the blind faults and enables us to probe their mechanics we relocate seismicity and compute focal mechanisms in a three dimensional velocity space and model the vertical deformation associated with the 1983 m 6 5 coalinga earthquake the aftershock zones abut at echelon offsets in the fold axes and the mainshocks display reverse slip perpendicular to the axes suggesting that the folds conceal a contiguous segmented thrust fault background seismicity concentrates at bends and breaks in the fold chain sites that may correspond to tears and ramps in the thrust fault at depth seismic reflection profiles reveal thrust and reverse faults dipping toward the san andreas fault at depths of 5 10 km with several kilometers of cumulative slip and high angle reverse faults in the anticlines with several hundred meters cumulative slip coseismic fold uplift accompanied the coalinga and kettleman hills north dome earthquakes suggesting that fold growth is episodic and coupled closely to repeated earthquakes on the underlying thrusts the north half of the fold chain has been the site of several 6 m 6 5 earthquakes since 1885 thus kettleman hills middle dome the next fold segment to the south has an elevated seismic potential the aftershock zones of the three main shocks are diffuse and occupy a region much larger than the site of seismic slip aftershocks occur 5 7 km from the fault regions where on the basis of a boundary element model the shear strain increase caused by the fault slip exceeds 20 ppm equivalent to about 0 7 mpa we argue that the broad aftershock zone is a product of high sustained off fault stress caused by repeated displacement on faults that do not cut the earth s surface the fault tip stresses lead to the formation of secondary faults which can become sites of aftershocks and postseismic creep

fault plane orientations of intermediate depth earthquakes in the middle america trench 1 intermediate depth earthquakes are often attributed to dehydration embrittlement reactivating preexisting weak zones the orientation of presubduction faults is particularly well known offshore of middle america where seismic reflection profiles show outer rise faults dipping toward the trench and extending 20 km into the lithosphere if water is transported along these faults and incorporated into hydrous minerals the faults may be reactivated later when the minerals dehydrate in this case the fault plane orientations should be the same in the outer rise and at depth after accounting for the angle of subduction to test this hypothesis we analyze the directivity of 54 large mw 5 7 earthquakes between 35 and 220 km depth in the middle america trench for 12 of these earthquakes the directivity vector allows us to identify the fault plane of the focal mechanism between 35 and 85 km depth we observe both subhorizontal and subvertical fault planes the subvertical fault planes are consistent with the reactivation of outer rise faults whereas the subhorizontal fault planes suggest the formation of new faults deeper than 85 km we only observe subhorizontal faults indicating that the outer rise faults are no longer being reactivated the similarity with previous results from the colder tonga kermadec subduction zone suggests that the mechanism generating these earthquakes and controlling fault plane orientations depends on pressure rather than temperature or other tectonic parameters and that the observed rupture characteristics constitute a basic feature of intermediate depth seismicity exclusively subhorizontal faults may result from isobaric rupture propagation or the hindrance of seismic slip on preexisting weak subvertical planes

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

historical 1942 ecuador and 1942 peru subduction earthquakes and earthquake cycles along colombia ecuador and peru subduction segments two large shallow earthquakes occurred in 1942 along the south american subduction zone inclose proximity to subducting oceanic ridges the 14 may event occurred near the subducting carnegie ridge off the coast of ecuador and the 24 august event occurred off the coast of southwestern peru near the southern flank of the subducting nazca ridge source parameters for these for these two historic events have been determined using long periodp waveforms p wave first motions intensities and local tsunami data we have analyzed thep waves for these two earthquakes to constrain the focal mechanism depth source complexity and seismic moment modeling of thep waveform for both events yields a range of acceptable focal mechanisms and depths all of which are consistent with underthrusting of the nazca plate beneath the south american plate the source time function for the 1942 ecuador event has one simple pulse of moment release with a duration of 22 suconds suggesting that most of the moment release occurred near the epicenter the seismic moment determined from thep waves is 6 8 1020n m corresponding ot a moment magnitude of 7 8 7 9 the reported location of the maximum intensities ix for this event is south of the main shock epicenter the relocated aftershcks are in an area that is approximately 200 km by 90 km elongated parallel to the trench with the majority of aftershocks north of the epicenter in contrast the 1942 peru event has a much longer duration and higher degree of complexity than the ecuador earthquake suggesting a heterogeneous rupture seismic moment is released in three distinct pulses over approximately 74 seconds the largest moment release occurs 32 seconds after rupture initiation the seismic moment as determined from thep waves for the 1942 peru event is 10 25 1020n m corresponding to a moment magnitude of 7 9 8 2 aftershock locations reported by the iss occur over a broad area surrounding the main shock the reported locations of the maximum intensities ix are concentrated south of the epicenter suggesting that at least part of the rupture was to the south we have also examined great historic earthquakes along the colombia ecuador and peru segments of the south american subduction zone we find that the size and rupture length of the underthrusting earthquakes vary between successive earthquake cycles this suggests that the segmentation of the plate boundary as defined by earthquakes this century is not constant

origin of a crustal splay fault and its relation to the seismogenic zone and underplating at the erosional north ecuador south colombia oceanic margin 1 splay faults within accretionary complexes are commonly associated with the updip limit of the seismogenic zone prestack depth migration of a multichannel seismic line across the north ecuador south colombia oceanic margin images a crustal splay fault that correlates with the seaward limit of the rupture zone of the 1958 mw 7 7 tsunamogenic subduction earthquake the splay fault separates 5 6 6 km s velocity inner wedge basement rocks which belong to the accreted gorgona oceanic terrane from 4 to 5 km s velocity outer wedge rocks the outer wedge is dominated by basal tectonic erosion despite a 3 km thick trench fill subduction of 2 km high seamount prevented tectonic accretion and promotes basal tectonic erosion the low velocity and poorly reflective subduction channel that underlies the outer wedge is associated with the aseismic decollement thrust subduction channel fluids are expected to migrate upward along splay faults and alter outer wedge rocks conversely duplexes are interpreted to form from and above subducting sediment at 14 to 15 km depths between the overlapping seismogenic part of the splay fault and the underlying aseismic decollement coeval basal erosion of the outer wedge and underplating beneath the apex of inner wedge control the margin mass budget which comes out negative intraoceanic basement fossil listric normal faults and a rift zone inverted in a flower structure reflect the evolution of the gorgona terrane from cretaceous extension to likely eocene oblique compression the splay faults could have resulted from tectonic inversion of listric normal faults thus showing how inherited structures may promote fluid flow across margin basement and control seismogenesis

deep structures of the ecuador convergent margin and the carnegie ridge possible consequence on great earthquakes recurrence interval 1 the deep structure of the ecuador subduction zone and adjacent carnegie ridge cr was investigated using on shore off shore wide angle seimics a crustal model obtained by 2 d inversion of traveltimes reveals the overthickened 14 km oceanic crust of the cr that underthrusts the high velocity 6 km s basement of the upper plate margin wedge interpreted as part of the accreted oceanic terranes described on shore the plate interface dips 4 to 10 east from the trench to a depth of 15 km shadow zones observed on the margin obs records are interpreted as a low velocity zone consisting of a thin layer of underthrust sediments and the 3 km thick cr layer 2 whose average seismic velocity of 5 1 km s is slower than that of the margin wedge increased interplate coupling related to the subduction of the thick buoyant cr may account for an apparent local increased recurrence interval between great interplate earthquakes

joint inversion of multichannel seismic reflection and wide angle seismic data improved imaging and refined velocity model of the crustal structure of the north ecuador south colombia convergent margin 1 improving seismic imaging of the crust is essential for understanding the structural factors controlling subduction zones processes we developed a processing work flow based on the combined analysis of multichannel seismic reflection mcs and wide angle wa reflection refraction data to derive both shallow and deep velocities suitable for prestack depth migration and to construct a blocky velocity model integrating all identifiable seismic phases contained in mcs and wa data we apply this strategy to the study of the north ecuador sw colombia subduction margin to improve the imaging and geostructural interpretation of a splay fault and surrounding outer and inner margin wedges results show improvements over tomographic inversion of wa data only such as 1 sediment velocity variation across the trench and margin slope that correlates with lateral lithologic changes tectonic compaction and effect of mass wasting processes 2 a two layer velocity structure of the inner wedge basement that is consistent with the crust of an oceanic plateau 3 a complex velocity structure of the outer wedge basement that consists of a deep high velocity 5 0 5 5 km s 1 core and a low velocity zone 3 8 5 0 km s 1 associated with the major splay fault 4 a 1 3 km thick low velocity 3 5 4 0 km s 1 subduction channel that extends beneath the margin outer wedge both the splay fault and subduction channel are expected to direct fluid flows and 5 downdip velocity increase 5 6 km s 1 in the subducting oceanic crust associated with a low 7 8 km s 1 upper mantle velocity possibly reflecting changes in rock nature or properties

morphological and microtectonic analysis of quaternary deformation from puna and santa clara islands gulf of guayaquil ecuador south america abstract this paper presents the neotectonic study of santa clara and puna islands sited in the gulf of guayaquil eastern part both islands are located on the south western segment of the fault zone bounding to the east the north andean block fault motion and morphostructural analysis were carried out from pleistocene age terrain a two step deformation characterises the south puna tectonics the first step involves the zambapala cordillera uplift that post dates pleistocene sediments and pre dates a marine terrace correlated with the m i s 11 or 13 440 550 ka the second step is the formation of a pull apart that shows evidence of 2 9 km dextral offset since the m i s 11 or 13 giving an offset mean rate of 5 3 to 6 6 mm yr this rate is higher than the one calculated on the pallatanga fault northeast of the study area in the western andean cordillera suggesting that deformation is split in different fault segments from the gulf of guayaquil to the continent the zambapala cordillera uplift and transpression deformation requires a compressive event that may have been induced by the subduction process during the early pleistocene

seismological evidence on the geometry of the orogenic system in central northern ecuador south america analysis of the spatial distribution of seismicity beneath central ecuador from a temporary network gives new insights on two main points of the ecuadorian tectonics major structures in the ecuadorian andes are east dipping late jurassic to paleogene sutures reactivated by present day compression the oceanic plate is plunging continuously down to a depth of about 200 km with a dip of 25 35 we also show that the coastal plain acts as a buttress transmitting stresses to the andes beneath which deformation is concentrated

depth of seismic coupling along subduction zones 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 wm 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

thermoluminescence dating of the malan loess at zhaitang china although the widespread loess deposits of north china have been well described there has been limited information on the absolute age of the various layers loess a wind carried deposit exposed to sunlight and subsequently buried should be amenable to dating by thermoluminescence through use of the fine grain technique ages from 18 to 84 ka have been obtained for a sequence in the malan loess exposed in the zhaitang section near beijing the dates are consistent with the evidence from geology and magnetostratigraphy and provide a chronology for the zhaitang section

are rupture zone limits of great subduction earthquakes controlled by upper plate structures evidence from multichannel seismic reflection data acquired across the northern ecuador southwest colombia margin 1 subduction of the nazca plate beneath the ecuador colombia margin has produced four megathrust earthquakes during the last century the 500 km long rupture zone of the 1906 mw 8 8 event was partially reactivated by three thrust events in 1942 mw 7 8 1958 mw 7 7 and 1979 mw 8 2 whose rupture zones abut one another multichannel seismic reflection and bathymetric data acquired during the sisteur cruise show evidence that the margin wedge is segmented by transverse crustal faults that potentially correlate with the limits of the earthquake coseismic slip zones the paleogene neogene jama quininde and esmeraldas crustal faults define a 200 km long margin crustal block that coincides with the 1942 earthquake rupture zone subduction of the buoyant carnegie ridge is inferred to partially lock the plate interface along central ecuador however coseismic slip during the 1942 and 1906 earthquakes may have terminated against the subducted northern flank of the ridge we report on a newly identified manglares crustal fault that cuts transversally through the margin wedge and correlates with the limit between the 1958 and 1979 rupture zones during the earthquake cycle the fault is associated with high stress concentration on the plate interface an outer basement high which bounds the margin seaward of the 1958 rupture zone may act as a deformable buttress to seaward propagation of coseismic slip along a megathrust splay fault coseismic uplift of the basement high is interpreted as the cause for the 1958 tsunami we propose a model of weak transverse faults which reduce coupling between adjacent margin segments together with a splay fault and an asperity along the plate interface as controlling the seismogenic rupture of the 1958 earthquake

nonlinear variations of the physical properties along the southern ecuador subduction channel results from depth migrated seismic data abstract we use two high quality pre stack depth migrated multichannel seismic profiles acquired to quantify physical properties variations of underthrust sediments along the first 30 km of subduction off the erosional southern ecuadorian margin seismic data show three zones along the subduction channel referred to as zones i ii and iii characterized by distinct velocity and velocity derived physical properties which are in agreement with values estimated from experimental results of deformation in granular media these three zones result from transformational changes of underthrust sediments governed by fundamentally different physical processes that control their mechanical behavior at increasing confining pressures based on our observations and its comparison with experimental results we argue that the transformations undergone by underthrust sediments as they dip into the subduction zone are the following within zone i progressively increasing velocity and decreasing velocity derived porosity indicates continuous sediment compaction which must be accompanied by effective fluid drainage along the decollement and or across the accretionary wedge the underthrust material is here unconsolidated from a mechanical point of view laboratory experiments indicate that the dominant processes at this range of pressures are grain rolling particle rotation and frictional slip at grain contacts within zone ii velocity and porosity remains constant for 16 km sis 72 and 12 km sis 18 this suggests undrained conditions resulting in growing fluid overpressure at the subduction channel grain deformation is similar to zone i within zone iii velocity increases and porosity falls rapidly indicating sediment compaction and subsequent release of over pressured fluids where grain deformation is likely to be elastic this might be the dominant process until the grains attain their crushing strength resulting in granular cataclasis and eventually in the collapse of the system we suggest that over pressured fluid release may induce hydrofracturation and it is likely to increase inter plate coupling down from zone iii

geomorphic evidence of active deformation and uplift in a modern continental wedge top foredeep transition example of the eastern ecuadorian andes abstract the eastern ecuadorian andes appear as a fold and thrust belt adjacent to a continental foredeep represented by one of the world s largest tropical alluvial megafans the pastaza megafan debouching into the amazonian lowland the apex of the pliocene pleistocene megafan situated in the present day wedge top subandean zone has been cut by an erosion surface the western part of which has been uplifted of 500 m along the frontal thrust forming a poorly dissected plateau the mera plateau this erosion surface erased most of the previous fluvial landscape but preserved a large thrust related anticlinal hinge deforming less erodible underlying strata the mirador fold and smaller sized anticlines this surface has been then incised by two antecedent major rivers the pastaza and the napo and few tributaries the plateau edge is marked by a series of large scale gently sloping landslides clustered along a 70 km long concave eastward line associated with the frontal thrust fault the newly formed immature rivers issued from the landslides or sourced within east dipping remnants of the erosion surface downstream of the landslide line constitute the greatest part of the streams feeding the ecuadorian amazonian basin at 70 to 100 km from the landslide line the drainage abruptly changes from highly immature to mature with a well defined hinge line representing the outer limit of landslide and tectonic control the diversions of the pastaza river indicate ongoing fold growth since at least the late pleistocene in the eastern cordillera and the early holocene in the mera plateau the preserved terraces of the pastaza valley are all degradational and are ascribed to periods of tectonic seismic activity alternating with periods of tectonic quiescence or decreased seismic activity rather than to climatic events 14c dating of the plateau erosion surface and of the upper pastaza terraces indicates that the minimum average incision rate since 18 000 years bp varies locally in the upper pastaza valley from 0 5 to 0 67 cm year 1 increasing from 18 000 years bp to now a comparison of these incision rates with fold and thrust fault uplift rates indicates that incision in the upper pastaza valley was a result of rapid uplift up to 1 cm year 1 along the mirador fold and thrust which caused a restoration of the local equilibrium profile of the upper reach combined with smaller local fault uplift along the westernmost thrust faults the uplift of the whole mera plateau with respect to the upper amazonian basin gives a minimum average uplift rate of 2 8 cm year 1 since 18 000 years bp the overall uplift of the mera plateau and the eastern cordillera is likely to have been caused by a regional scale low angle thrust ramp emerging as the frontal thrust fault

the nature and provenance of accreted oceanic terranes in western ecuador geochemical and tectonic constraints western ecuador consists of a complex tectonic melange of oceanic terranes accreted to the continental margin from late cretaceous to eocene time new geochemical data from these accreted terranes arising from a 5 year british geological survey mapping programme indicate that they comprise rocks from a variety of oceanic tectonic settings from thickened and relatively unsubductable oceanic plateau basalts through island arc tholeiites with occasional more calc alkaline lavas to back arc basin basalt sequences this study has enabled us to construct a new geodynamic model for the cretaceous tertiary evolution of the northern andes and has placed important new constraints on the extent of oceanic plateau sequences in colombia and around the caribbean the age and nature of sediments combined with evidence for the age of peak metamorphism suggests that a prolonged 15 20 ma accretionary event occurred in late cretaceous time and involved the collision of an oceanic plateau represented by the pallatanga unit with the continental margin this accreted unit can be correlated with similar oceanic plateau sequences from the western cordillera of colombia and those within and around the caribbean region the naranjal and macuchi island arcs and the associated la portada back arc basin developed along the accreted margin from late campanian to eocene time and these arcs accreted to the continental margin along with oceanic plateau material represented by the pinon unit and pedernales esmeraldas sequences during eocene time the development of island arcs which separate the two accretionary events implies that the most westerly coastal oceanic plateau sequences both in ecuador pinon and pedernales esmeraldas and in colombia gorgona and serrania de baudo cannot belong to the caribbean colombian oceanic plateau ccop it therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the northern andes it is possible that the ccop formed over the galapagos hotspot as previously proposed but the more westerly coastal plateau was derived from a more southerly hotspot source region such as sala y gomez in the se pacific

development of the gulf of guayaquil ecuador during the quaternary as an effect of the north andean block tectonic escape 1 interpretation of industrial multichannel seismic profiles and well data are used to identify the main tectonic features of the gulf of guayaquil gg area these include two e w trending major detachments the posorja and the jambeli detachment systems which represent half grabens with oppositely dipping detachments to the south and to the north respectively the ne sw trending puna santa clara fault system developed as a transfer fault system between the posorja and the jambeli detachments the esperanza and the jambeli basins exhibit 3 4 km of sediment that accumulated during the past 1 6 1 8 myr the puna santa clara fault system bounds the esperanza and the jambeli basins evidencing that the evolution of these basins is tightly controlled by the two detachments at depth to the west the n s trending domito fault system bounding the posorja detachment system and the esperanza basin to the west acted as a transfer zone between the shelf area and the continental slope the pliocene series show no significant variations in thickness throughout the gulf of guayaquil area suggesting that no important tectonic deformation occurred from 5 2 to 1 8 1 6 ma the major period of tectonic deformation in the gulf of guayaquil area occurred during the pleistocene times three main tectonic steps are identified from early pleistocene to 180 ka major subsidence occurred along the esperanza and jambeli basins from 180 to 140 ka most of the gulf of guayaquil area was above sea level during the isotope substage 6 low stand from 140 ka to present tectonic activity is restricted along the normal faults bounding the esperanza basin the tenguel fault and the puna santa clara and domito fault systems a n s trending tensional stress regime characterizes the pleistocene times throughout the northward drifting of the north andean block is proposed to control the tectonic evolution and associated subsidence of the gulf of guayaquil area it is also accepted that the collision of the carnegie ridge with the trench axis has to play a major role in controlling the north andean block northward drift because the carnegie ridge subduction is possibly an ongoing process which began prior to the pliocene we postulate the along strike morphology of the ridge at the origin of interplate coupling variations the subduction of an along strike positive relief of the ridge is proposed at the origin of the major tectonic reorganization of the gg area occurring at 1 8 1 6 ma

circum pacific seismic potential 1989 1999 the seismic potential for 96 segments of simple plate boundaries around the circum pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5 10 and 20 years i e 1989 1994 1989 1999 and 1989 2009 this study represents the first probabilistic summary of seismic potential on this scale and involves the comparison of plate boundary segments that exhibit varying recurrence times magnitudes and tectonic regimes presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments as well as accounting for individual variations in recurrence time along a specific fault segment and uncertainties in the determination of the average recurrence time

shallow seismogenic zone detected from an offshore onshore temporary seismic network in the esmeraldas area northern ecuador for a given site many factors control the seismic risk earthquake magnitude hypocentral distance rupture mechanism site effects and site vulnerability are among the most important this article deals with one of these factors the depth of the seismogenic zone in the northern ecuadorian subduction system beneath a highly vulnerable site the city of esmeraldas and its industrial complex the ecuadorian oil refinery and shipping terminal to address this problem we analyzed data from a three weeks passive seismological experiment conducted in the spring of 1998 using 13 ocean bottom seismometers and 10 portable land stations a preliminary interpretation of wide angle data obtained in the fall of 2000 in the manta area 100 km south of the study area unambiguously indicates the presence of a velocity inversion in the ecuadorian margin velocity structure this velocity inversion is characterized by a shadow zone of 1 s on the record sections and is interpreted as the result of a velocity contrast between the upper plate structure and the sedimentary and basaltic layer ii of the subducted oceanic nazca plate one dimensional velocity models are deduced from these wide angle data and are used for earthquake location in the esmeraldas area this highly improved the hypocentral parameter determinations the updip limit of the seismogenic zone is found at a depth of 12 km 35 km eastward of the trench and the depth of the seismogenic zone below the esmeraldas city is found at 20 km this shallow depth of the seismogenic zone dramatically increases the seismic hazard of the area

recent crustal deformation and the earthquake cycle along the ecuador colombia subduction zone recent results from global positioning system gps measurements show deformation along the coast of ecuador and colombia that can be linked to the rupture zone of the earthquake in 1979 a 3d elastic boundary element model is used to simulate crustal deformation observed by gps campaigns in 1991 1994 1996 and 1998 deformation in ecuador can be explained best by 50 apparent locking on the subduction interface although there have not been any historic large earthquakes mw 7 south of the 1906 earthquake rupture zone 50 apparent elastic locking is necessary to model the deformation observed there in colombia only 30 apparent elastic locking is occurring along the subduction interface in the 1979 earthquake rupture zone mw 8 2 and no elastic locking is necessary to explain the crustal deformation observed at two gps sites north of there there is no evidence from seismicity or plate geometry that plate coupling on the subduction zone is reduced in colombia however simple viscoelastic models suggest that the apparent reduction in elastic locking can be explained entirely by the response of a viscous upper mantle to the 1979 earthquake these results suggest that elastic strain accumulation is occurring evenly throughout the study area but postseismic relaxation masks the true total strain rate

the rupture process of the great 1979 colombia earthquake evidence for the asperity model we have determined the asperity distribution in the segment of the ecuador colombia subduction zone that was ruptured by the 1979 mw 8 2 and 1958 mw 7 7 earthquakes for the 1979 earthquake we are able to document directivity in the p wave source time functions and consequently have spatially located the moment release by combining results from both p wave and surface wave directivity we have defined three regions 1 from 0 to 56 km northeast of the epicenter with low moment release and a small seismic displacement 2 the asperity from 56 to 116 km northeast of the epicenter with most of the moment release and a displacement of 6 m and 3 a small displacement region from 116 to 180 240 km northeast of the epicenter thus the 1979 rupture zone is composed of an asperity 60 km in length bounded by two weak small displacement regions the displacement at the asperity is equal to the cumulative tectonic displacement between 1979 and 1906 the year of the previous great earthquake in this segment the 1958 earthquake is characterized by the rupture of a small asperity length scale approximately 25 km in the epicentral region the 1958 event continued to rupture past this small epicentral asperity and stopped near the epicenter of the following 1979 event this detailed study of the spatial variation in moment release offers strong support for the asperity model of large earthquake occurrence

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

dubious case for slab melting in the northern volcanic zone of the andes it was first suggested by r w kay that adakites may represent melts of subducted slab since then the term adakite has become synonymous with slab melts based on their unusual geochemical signature this contribution using the northern volcanic zone of the andes as an example aims 1 to expose the weakness in simply associating a geochemical signature with a genetic mechanism and 2 to underline the importance of using several integrated lines of evidence in assessing the viability of slab melting we conclude that although slab melting probably occurs in some arcs regional geochemical trends in the northern volcanic zone and their relationship to the subduction zone architecture are not indicative of slab melting and can be accounted for by normal arc magmatic processes acting on wedge derived basaltic magmas

transition from calc alkalic to adakitic magmatism at cayambe volcano ecuador insights into slab melts and mantle wedge interactions in volcanic arcs two main types of magmatism are recognized 1 widespread calc alkalic magmatism generated by hydrous partial melting of a metasomatized mantle wedge and 2 less common adakitic volcanism produced by subducted slab melting the cayambe volcanic complex in ecuador shows a progressive temporal change from an older calc alkalic volcano viejo cayambe to a younger adakitic edifice nevado cayambe this evolution may be related to the unusual geodynamic setting of the ecuadorian andes controlled mainly by the subduction of the nazca plate including the carnegie ridge thickened oceanic crust the viejo cayambe magmas appear to be generated from a mantle wedge source slightly metasomatized by slab derived melts conversely the nevado cayambe magmas imply either stronger and more advanced interactions between slab melts and mantle peridotite or that adakitic magma could reach the surface due to higher degrees of slab melting fractional crystallization assimilation and mixing processes also contributed to lava diversity we propose that the magmatic evolution of the cayambe volcanic complex is due to increasing efficiency of interaction between slab melts and the mantle wedge because of higher degrees of slab melting in response to the subduction of the carnegie ridge

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

influence of the subduction of the carnegie volcanic ridge on ecuadorian geology reality and fiction the proposed ages for the collision of the carnegie ridge with the south america trench offshore ecuador range from 1 to 15 ma in this time frame many geological features of ecuador are commonly linked to the subduction of the carnegie ridge 1 after the ridge collided with the trench at ca 15 ma the subsequent interplate coupling produced high exhumation rates of volcanic materials at ca 9 ma 2 the oblique convergence of the carnegie ridge would have caused the northward drift of the north andean block and the opening of the gulf of guayaquil 3 during the late miocene the subduction of the carnegie ridge would have triggered a regional tectonic inversion along the forearc 4 along the collision front of the ridge with the trench subduction related erosion is occurring and the ecuadorian continental margin is being uplifted in the present day 5 the chemistry of the active volcanic arc is explained as resulting from the arrival of the carnegie ridge into the trench for instance the adakitic signal which appears at 1 5 ma is thought to be ridge induced 6 the buoyancy of the subducted carnegie ridge would explain the flatness of the slab beneath ecuador in this paper we review the geological evolution of the northern andes in order to establish which of these geological events may be related to the subduction of the carnegie ridge this review suggests that there is no clear deformation linked with the subduction of the carnegie ridge or with its landward prolongation postulated at depth