bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleoseismology in coastal Chile
Authors: Vita‐Finzi Claudio
Published: February 2004

The fossil shorelines of the Chilean coast reflect interseismic as well as coseismic movements. The earthquake record is further obscured by the variable pattern of coastal deformation that accompanies large earthquakes, such as the longitudinal warping associated with the 1960 Valdivia (Mw= 9.5) earthquake. The number of events recorded is a minimum, as relaxation can lead to the reoccupation of shorelines, and paleoearthquake magnitude cannot be determined with confidence, as the amount of uplift or subsidence depends on location within the deformed zone and on the timing of the measurements during the deformation cycle. However, provided radiometric dating makes it possible to discriminate between successive earthquakes, variations in the amount of uplift can be used for modeling the deformation numerically to the ultimate benefit of seismic hazard assessment.
Canonical model of volcano acoustics
Authors: Buckingham Michael J. , Garcés Milton A.
Published: February 2004

A full wave‐theoretical model is developed for the acoustic field generated by an explosive point source embedded in a magma column that is open to the atmosphere. The Green's functions for the field in the magma and the atmosphere are derived on the basis of several simplifying assumptions concerning the geometry of the conduit, the boundary conditions, and the geoacoustic properties of the magma. A kinematic model for the acoustic signature of the explosive source is proposed, which, when combined with the Green's functions, provides full analytical expressions for the acoustic field in the magma and in the atmosphere as (complex) functions of frequency. By Fourier inversion the airborne pressure spectrum is transformed into a pressure time series. The predicted sound pulse in the atmosphere and its energy spectrum are highly dispersive, showing complicated structure that arises from the coherent addition of many normal modes of oscillation (i.e. depth and radial resonances) in the magma column. At high frequencies, for which the aperture of the vent is many wavelengths across, each mode is launched into the atmosphere as a parallel‐sided beam of sound with a characteristic angle of elevation, which, through Snell's law, is determined by the speed of sound in the magma relative to that in air. At somewhat lower frequencies, the modal beams of sound undergo angular spreading due to diffraction at the edge of the vent. In the lowest‐frequency regime, where the wavelength is comparable with the aperture, the airborne field shows little angular structure. A comparison between airborne acoustic data that we recorded in July 1994 at the western vent of Stromboli Volcano and the predictions of the theory, using parameters that are characteristic of Stromboli, show compelling agreement. The theoretical and observed power spectra both display the following features: (1) a concentration of energy below 20 Hz, associated with the first four longitudinal resonances; (2) radial resonances between 35 and 65 Hz; and (3) a broad minimum around 30 Hz, arising because the source lies near nulls in longitudinal modes that would otherwise be excited. The conclusion is that the airborne sound signature from an explosive volcanic event may be inverted to provide estimates of the depth and radius of the magma conduit, the depth, spectral shape and peak shock‐wave pressure of the source, and the viscosity of the magma.
Three‐dimensional numerical experiments of strain partitioning at oblique plate boundaries: Implications for contrasting tectonic styles in the southern Coast Ranges, California, and central South Island, New Zealand
Authors: Braun Jean , Beaumont Christopher
Published: February 2004

A three‐dimensional numerical model is used to investigate the nature and distribution of crustal‐scale deformation and strain partitioning across obliquely convergent, transpressive, plate boundaries. A uniform elastic‐plastic crustal layer, obeying Murrell's extension to three dimensions of the Griffith failure criterion, was deformed under basal kinematic boundary conditions that correspond to the oblique convergence between two mantle lithospheres, one of which detaches and subducts beneath the other. The results show deformation concentrated in relatively narrow shear zones. The character (planar versus curved) and dip of these shear zones (or crustal‐scale faults) depend on the relative amount of convergence versus transcurrent motion imposed across the plate boundary. In models dominated by transcurrent boundary conditions, two separate sets of coexisting structures develop: (1) a pair of planar, relatively shallow‐dipping thrust zones arranged in a “V” structure that accommodate convergent motion; and (2) a set of curved, near‐vertical strike‐slip zones arranged in a “flower” structure that accommodates the transcurrent motion. In models dominated by convergence, both contractional and transcurrent deformations are accommodated by oblique slip across a single pair of structures that have the same geometry as the thrust shear zones. There is a gradational behavior between these end‐member partitioned and nonpartitioned modes for intermediate boundary conditions. In all cases the dip of the shear zones predicted by the numerical model is in good agreement with the dip of ideal faults predicted by Mohr's incipient faulting hypothesis. The two basic modes are interpreted to underlie the contrasting tectonic styles of the southern Coast Ranges, California, and central South Island, New Zealand. Factors other than obliquity of convergence which play a role in strain partitioning are discussed.
Late Quaternary stream incision and uplift in the forearc of the Cascadia subduction zone, western Oregon
Authors: Personius Stephen F.
Published: February 2004

Documentation of a latest Pleistocene/earliest Holocene episode of strath formation and fluvial aggradation in the Oregon Coast Range provides a datum from which long‐term bedrock stream incision rates are determined. Variations in long‐term incision rates probably reflect cumulative differential uplift in the forearc of the Cascadia subduction zone, although factors such as bedrock and climatic controls and isostatic adjustments to erosion obscure the precise relationship between surface uplift and stream incision. Patterns of differential incision are most striking near the latitude of Newport, where a steep gradient divides a region of higher rates (∼0.6–0.9 mm/yr) in the northern Coast Range from a region of lower rates (∼0.1–0.3 mm/yr) in the central Coast Range. The steep incision gradient is nearly coincident with abrupt changes in marine terrace (∼80–125 kyr) uplift rates, the locations of Quaternary faults, and the southern flank of a saddle of low historic (∼40–70 years) uplift. The exact causes of these variable patterns of incision/uplift are unknown. Analogies with uplift patterns in other subduction zones and comparisons with other neotectonic data in the region indicate that patterns of differential incision probably are caused by variations in permanent strain accumulation along the Cascadia subduction zone. Such variations may be related to differences in seismic moment release during individual earthquakes, to changes in plate geometry or rates of wedge accretion, to segmentation of earthquake ruptures, and/or to deformation on active structures in the North American plate and accretionary wedge.
Forty million years of magmatic evolution in the Mariana arc: The tephra glass record
Authors: Lee Jongman , Stern Robert J. , Bloomer Sherman H.
Published: February 2004

Tephra glasses retrieved from 10 Deep Sea Drilling Project (DSDP) cores around the Mariana arc system make up a remarkable record of explosive volcanism in the Marianas over the past 40 m.y. Major element compositions for approximately 1800 tephra glasses, from basalt through rhyolite, are reported and used to examine the nature and history of this activity. Three maxima of volcanic explosivity, presumably related to especially vigorous volcanism in the Mariana arc, are identified, with the biggest maximum at around 18–11 Ma and two other maxima at 35–24 and 6–0 Ma; the two younger maxima are contemporaneous with peaks in explosive volcanism observed for other western Pacific arcs. Explosive arc volcanism has been predominantly tholeiitic since shortly after arc inception; no boninitic glasses were found. The tephra glasses belong to the low‐ to medium‐K suite, except during an enigmatic phase of medium‐ to high‐K explosive volcanism during the late Miocene (11–7 Ma). Even though Mariana tephra glasses are largely similar in composition to Mariana arc lavas, the tephra show a much larger compositional range than that found for subaerial Mariana arc lavas, which totally lack dacitic to rhyolitic volcanic products. The tephra glasses define a bimodal population in terms of silica content, with a pronounced minimum, or “Daly gap,” around 65–66% SiO2. Mariana tephra glasses are fractionated, with the least evolved glass being Fe‐rich and having a magnesium number (Mg #) (100Mg/Mg+Fe2+) of 55. Basaltic tephra glasses contain ≤16% Al2O3 (average 14.3% Al2O2) and are not high‐alumina basalts; this contrasts with the observation that modern Mariana arc lavas contain 15–21% (average 17.4%) Al2O3. The high‐alumina basalt lavas of the Mariana arc probably reflect plagioclase accumulation and not liquid compositions. All tephra glasses plot near low‐pressure cotectics and reaction curves on the subprojection olivine‐clinopyroxene‐quartz; mafic and felsic samples define distinct trends. The mafic trend reflects fractional crystallization of mantle‐derived basaltic magma, whereas the felsic trend may be due either to anatexis of Mariana arc crust or to fractionation of mafic melts. The tephra glass data reinforce the model that the magmatic evolution of the Mariana arc has been dominated by low‐pressure fractionation, perhaps accompanied by anatexis. Episodic changes in melting regime to generate Miocene potassic tephra may be related to changing mantle sources and processes related to episodes of back arc basin spreading. These episodic changes are superimposed on a long‐term increase in potassium that reflects progressive metasomatism of the mantle source. Long‐term increases in K2O contents for Mariana arc magmas inferred from the tephra glass record are 0.004 wt % m.y.−1 (mafic), 0.011 wt % m.y.−1 (intermediate), and 0.023 wt % m.y.−1 (felsic).
Very broadband seismic analysis of the 1992 Flores, Indonesia, earthquake (Mw = 7.9)
Authors: Beckers Jos , Lay Thorne
Published: February 2004

The December 12, 1992, Flores Island earthquake (Mw = 7.9), which caused a destructive tsunami, is located in the back arc of the transition zone between the Sunda and Banda arcs, a region in active collision with the Australian continental margin. We study the earthquake using four seismological techniques spanning a period range of 1–1000 s and infer that the Flores event may be characterized as a shallow back arc thrust with fault plane strike 70°±30°, dip 28°±10° and rake 80° to 140° (linear trade‐off with strike), a rupture consisting of three discrete pulses with a total duration of 70 s (centroid time 26.3 s) and an estimated moment release between 7.5×1020 and 8.0×1020 N m. From the slip vector we infer that compressional forces play an important role in the deformation mechanism of the back arc north of Flores Island. Body waves constrain the hypocentral depth to be 16 km with rupture mainly propagating updip to shallow depth and bilaterally along strike with most subevents located toward the northeast. Evidence for rupture directivity from surface wave deconvolutions is inconclusive, but the overall source function is consistent with body wave results. Our body wave finite fault model is in rough agreement with two models proposed to explain tsunami run‐up observations. Normal mode modeling of W phase observations at periods longer than 70 s shows that slow slip is negligible. From this and the slip distribution determined from the body wave analysis we infer that the seismic source is an unlikely candidate to explain anomalously high tsunami run‐ups measured on northeastern Flores Island.
Correlation and emplacement of a large, zoned, discontinuously exposed ash flow sheet: The 40Ar/39Ar chronology, paleomagnetism, and petrology of the Pahranagat Formation, Nevada
Authors: Best Myron G. , Christiansen Eric H. , Deino Alan L. , Grommé C. Sherman , Tingey David G.
Published: February 2004

Many single‐crystal 40Ar/39Ar ages and thermoremanent magnetization directions have resolved the problematic stratigraphie correlation of the laterally and vertically zoned rhyolite ash flow sheet of the Pahranagat Formation in the southern Great Basin. This outflow sheet was previously designated by four different stratigraphie names in different locations over its highly discontinuous exposure area of 33,000 km2. We show that it is a single cooling unit emplaced at 22.639+0.009 Ma around its source, the Kawich caldera. The volume of the outflow sheet was about 1600 km3 after compensation for 50% post volcanic east‐west extension. A comparable volume of tuff likely accumulated inside the Kawich caldera. Modal and chemical compositions of bulk tuff and cognate pumice fragments, together with compositions of phenocrysts, show the preemption magma body was zoned from high‐silica rhyolite (two feldspars, quartz, biotite, and titanomagnetite) to underlying, silica‐poor, more mafic rhyolite and trachydacite (plagioclase, minor biotite, titanomagnetite, amphibole, and clinopyroxene). Initial evacuation of the uppermost evolved zone produced proximal outflow hundreds of meters thick of relatively densely welded, pumice‐poor, high‐silica rhyolite tuff. As eruption progressed, tens of meters of more mafic ejecta were deposited in distal areas and locally near the caldera and consist of less welded, pumice‐rich ash flow tuff derived by physical mixing of pyroclasts from all zones of the magma chamber. This mixing during eruption invalidates direct comparison of the composition of tuff and a particular part of the magma chamber. The Pahranagat ash flow sheet provides a rigorous test case for application of high‐precision correlation tools because of the zonal emplacement of ejecta from the compositionally stratified magma chamber together with the subsequent tectonic dismemberment and erosion of the sheet that created widely scattered exposures.
Hydrothermal activity on a 105‐year scale at a slow‐spreading ridge, TAG hydrothermal field, Mid‐Atlantic Ridge 26°N
Authors: Lalou Claude , Reyss Jean‐Louis , Brichet Evelyne , Rona Peter A. , Thompson Geoffrey
Published: February 2004

New radiometric dates are presented for the hydrothermal deposits in the Alvin and Mir relict hydrothermal zones located on the lower east wall of the rift valley within the TAG hydrothermal field and placed in context of the evolution of the TAG field as a whole over the past 105 years. The field includes an active high‐temperature sulfide mound situated near the base of the east wall and a low‐temperature zone at middepth on the east wall all within a 5 by 5 kilometer area. The oldest hydrothermal deposits sampled in the Mir relict zone, the low‐temperature zone, and an inactive sulfide mound in the Alvin relict zone are low‐temperature manganese crusts dated at 140,000, 125,000 and 74,000 years, respectively. High‐temperature activity began about 100,000 years ago in the Mir zone. A 50,000 year‐high‐temperature event took place at the active sulfide mound, in the Mir zone, and at the inactive sulfide mound in the Alvin zone. This event is followed by sporadic activity at the Mir zone and the active sulfide mound, until 10,000 and 5000 years ago, respectively. A reactivation of the system affected the Mir zone from about 2000 to 600 years ago. The presently active sulfide mound was reactivated only 60 years ago, after a quiescence of about 4000 years. The radiochronologic record reveals that the hydrothermal activity has been episodic with a major high‐temperature event throughout the TAG field at 50,000 years and other more localized events.
Large‐scale geomorphology: Classical concepts reconciled and integrated with contemporary ideas via a surface processes model
Authors: Kooi Henk , Beaumont Christopher
Published: February 2004

Linear systems analysis is used to investigate the response of a surface processes model (SPM) to tectonic forcing. The SPM calculates subcontinental scale denudational landscape evolution on geological timescales (1 to hundreds of million years) as the result of simultaneous hillslope transport, modeled by diffusion, and fluvial transport, modeled by advection and reaction. The tectonically forced SPM accommodates the large‐scale behavior envisaged in classical and contemporary conceptual geomorphic models and provides a framework for their integration and unification. The following three model scales are considered: micro‐, meso‐, and macroscale. The concepts of dynamic equilibrium and grade are quantified at the microscale for segments of uniform gradient subject to tectonic uplift. At the larger meso‐ and macroscales (which represent individual interfluves and landscapes including a number of drainage basins, respectively) the system response to tectonic forcing is linear for uplift geometries that are symmetric with respect to baselevel and which impose a fully integrated drainage to baselevel. For these linear models the response time and the transfer function as a function of scale characterize the model behavior. Numerical experiments show that the styles of landscape evolution depend critically on the timescales of the tectonic processes in relation to the response time of the landscape. When tectonic timescales are much longer than the landscape response time, the resulting dynamic equilibrium landscapes correspond to those envisaged by Hack (1960). When tectonic timescales are of the same order as the landscape response time and when tectonic variations take the form of pulses (much shorter than the response time), evolving landscapes conform to the Penck type (1972) and to the Davis (1889, 1899) and King (1953, 1962) type frameworks, respectively. The behavior of the SPM highlights the importance of phase shifts or delays of the landform response and sediment yield in relation to the tectonic forcing. Finally, nonlinear behavior resulting from more general uplift geometries is discussed. A number of model experiments illustrate the importance of “fundamental form,” which is an expression of the conformity of antecedent topography with the current tectonic regime. Lack of conformity leads to models that exhibit internal thresholds and a complex response.
Observations of regional phase propagation across the Tibetan Plateau
Authors: McNamara Daniel E. , Owens Thomas J. , Walter William R.
Published: February 2004

We present observations of regional phase velocity and propagation characteristics using data recorded during a 1‐year deployment of broadband digital seismic stations across the central Tibetan Plateau along the Qinghai‐Tibet highway from Golmud to Lhasa. Previous seismological studies within this region have had to rely on earthquakes recorded almost exclusively at stations outside of the plateau. We have the opportunity to study numerous source‐receiver paths confined entirely within the Tibetan Plateau. Our analysis concentrates on travel time, amplitude, and frequency content measurements of the Pg, Pn and Sn phases. Pn can be clearly picked for all observed paths and propagates at an average velocity of 8.16±0.07 km/s within the Tibetan Plateau. Sn, however, shows dramatic variations in propagation efficiency across the Tibetan Plateau that is strongly dependent on frequency. We observe that Sn rapidly decreases in frequency and amplitude as it passes through the northern portion of the plateau. We show that in general, Sn propagation efficiency decreases with increasing frequency content. We use 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high‐frequency Sn propagation. Specifically, we show that a larger portion of the northern Tibetan Plateau attenuates Sn energy than was previously suggested. In the southern plateau, where high‐frequency Sn is observed, we computed an average velocity of 4.59±0.18 km/s. We also observed that the Pn velocity within this region of inefficient high‐frequency Sn propagation is nearly 4% slower than the Pn velocity computed for paths restricted to the southern plateau and that the crust is about 10 km thinner than in the south. The coincident locations of inefficient Sn propagation and slow Pn velocity is commonly observed in regions of active tectonics. Our results add constraints to the velocity structure of the lithosphere beneath the Tibetan Plateau and require first‐order lateral variations in the uppermost mantle structure, despite the relatively uniform topography of the plateau.
The 1993 Killari earthquake in central India: A new fault in Mesozoic basalt flows?
Authors: Seeber Leonardo , Ekström Goran , Jain Sudhir K. , Murty C. V. R. , Chandak Navin , Armbruster John G.
Published: February 2004

The Mw = 6.1 1993 Killari earthquake in central India was one of the deadliest earthquakes to occur in a stable continental area. It had a centroid 2.6 km deep and ruptured to the surface. A good fit of teleseismic waveforms is obtained with a simple 4.2‐s‐long pulse that releases a moment Mo = 1.8 × 1018 nm from a reverse fault rupture with strike, dip, and rake of 126°, 46°, and 100°, respectively. A ≈10‐km‐wide meizoseismal area is tightly defined by relative casualty count; in the center of this area, aftershock hypocenters (Baumbach et al., 1994) outline a southwest dipping rupture that extends 5.5–8 km along strike and from the surface 8–10 km along dip. These dimensions correspond with an average displacement of 1.5–0.8 m and a stress drop of 10–4.5 MPa, respectively. We mapped a 1‐km‐long zone of compressional scarps which are spatially correlated with the rupture outlined by hypocenter. In the central portion of this zone, the scarps are multiple and face in opposite directions. Deformation features indicate north–northeast directed shortening of at least 0.5 m. A leveling profile of an irrigation canal about 3 km northwest of the scarps displays a one‐wavelength warp, about 1 km long and 1.3 m in peak‐to‐peak amplitude, that may reflect deformation associated with the rupture. Two trenches across the scarps exposed faults that offset the soil‐rock interface as much as 50 cm. We found no convincing evidence suggesting that these faults existed prior to the earthquake. In the basalt, these faults reactivated exfoliation fractures in shear but were not associated with a zone of preexisting breccia and mineralization. The hypothesis that the 1993 rupture is a new fault in the Deccan Traps is consistent with lack of geomorphic evidence of prior faulting and with the lack of accumulated deformation or tilting in basalt layers exposed as close as few tens of meters from the scarp. No convincing clues about a seismogenic fault have yet been detected from either historic seismicity or geology in the 1993 source area, but a burst of seismicity 1 year before the mainshock and 2.5 years after the first impounding of a nearby reservoir was interpreted by some as a possible precursor to a larger earthquake before the 1993 mainshock. Precursory seismicity and artificial effects that cause significant mechanical changes in the crust may provide an indication of future potentially damaging earthquakes in stable continental areas.
On the mechanism of seismic decoupling and back arc spreading at subduction zones
Authors: Scholz C. H. , Campos J.
Published: February 2004

To address the problem of the great variability of the mechanical state of subduction zones, we investigate the mechanics of back arc spreading and seismic decoupling. Back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench‐normal tension reaches a critical value. Seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition. Two forces are important in this problem. The first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench. The second is a sea anchor force exerted on the slab that resists its lateral motion, assumed to occur at the upper plate velocity. Both forces contribute to the coupling problem: only the sea anchor force is responsible for back arc spreading. The unbalanced slab pull force is determined from a force balance for subduction, the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid. The model predicts three regimes: seismically coupled compressional arcs with advancing upper plates; seismically decoupled extensional arcs with retreating upper plates, and strongly extensional arcs which also have back arc spreading. This model is applied in detail to the Izu‐Bonin‐Marianas system. It shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1×1013 N m−1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed. It shows why the plate interface near Guam is seismically coupled, while the plate boundary everywhere farther north is decoupled. When applied globally, it successfully predicts the state of seismic coupling and back arc spreading in more than 80% of the world's subduction zones. Of the remaining, half can be seen to contain additional complications not included in the model. About 10% of cases remain unexplained, but some of these may have incorrectly determined seismic coupling coefficients.
Crustal thickness variations across the Colorado Rocky Mountains from teleseismic receiver functions
Authors: Sheehan Anne F. , Abers Geoffrey A. , Jones Craig H. , Lerner‐Lam Arthur L.
Published: February 2004

Variations in crustal thickness from the Great Plains of Kansas, across the Colorado Rocky Mountains, and into the eastern Colorado Plateau are determined by receiver function analysis of broadband teleseismic P waveforms recorded during the 1992 Rocky Mountain Front Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiment. The receiver functions are calculated using a time domain deconvolution approach and are interpreted in terms of a single crustal layer, with thickness determined by a grid‐search comparison of observed receiver functions with synthetics. The average crustal thicknesses determined by these methods are Kansas Great Plains, 43.8±0.4 km; Colorado Great Plains, 49.9±1.2 km; Colorado Rocky Mountains, 50.1±1.3 km; and northeast Colorado Plateau, 43.1±0.9 at latitudes of 38°–40°N. The main variations in crustal thickness that we observe are between the Kansas Great Plains and the Colorado Great Plains and between the Rocky Mountains and the Colorado Plateau. There is not a significant crustal thickness difference between the Colorado Great Plains and the Colorado Rocky Mountains. Together with gravity data and mass balance calculations, these results are incompatible with the hypothesis that the compensation of the Rocky Mountains relative to the Great Plains is accommodated purely by an Airy‐type crustal root or any other mechanism that restricts compensation solely to the crust and requires significant support for the excess topography of the Rocky Mountains to come from the mantle. Models with a rigid elastic plate may match receiver function estimates of crustal thickness but underpredict the amplitude of the gravity low over the Rockies. Our favored model includes lateral variations in crustal velocities obtained from refraction studies and crustal thickness variations constrained by the receiver functions. These models indicate that there is a profound transition in mantle density structure near the eastern range front.
Uplift and denudation of the central Alaska Range: A case study in the use of apatite fission track thermochronology to determine absolute uplift parameters
Authors: Fitzgerald Paul G. , Sorkhabi Rasoul B. , Redfield Thomas F. , Stump Edmund
Published: February 2004

Apatite fission track thermochronology (AFTT) on granitic samples collected in the central Alaska Range in conjunction with geologic constraints from basins to the north (Nenana Basin) and south (Cook Inlet) of the range is used to constrain the timing, amount, rate, and pattern of surface uplift, rock uplift, and denudation since the late Miocene. The conversion from a thermal frame of reference (apatite fission track data) to an absolute frame of reference (with respect to mean sea level), which requires constraining the paleoland surface elevation, the paleomean annual temperature, and the paleogeothermal gradient, is evaluated and shown to be viable in the context of an exhumed apatite partial annealing zone (PAZ). Apatite ages at Denali (Mount McKinley) range from 16 Ma near the summit (∼6 km elevation) to 4 Ma at ∼2 km elevation. A distinctive break in slope in the apatite age profile at an elevation of 4.5 km, also marked by a change in confined track length distributions, marks the base of an exhumed apatite PAZ. Rock uplift and denudation are greatest at Denali, decreasing southward away from the McKinley strand of the Denali fault system as shown by progressively older apatite ages (7–35 Ma) from a suite of samples along the Kahiltna Glacier. A correlative decrease in topography occurs southward from the fault. The central Alaska Range lies within an arc defined by the Denali fault, with the highest peaks (including Denali) concentrated at the arc apex. Patterns of rock uplift and denudation within the central Alaska Range mimic topography. Between early and late Miocene, and possibly earlier, the central Alaska Range was most likely an area of relative tectonic and thermal stability. Rock uplift, denudation, and mean surface uplift of the Denali region began by the Late Miocene (∼5–6 Ma), being ∼8.5 km, ∼5.7 km, and ∼2.8 km, respectively, at average rates of ∼1.5 km/m.y., ∼1 km/m.y., and ∼0.5 km/m.y. The amount of rock uplift, denudation, and surface uplift decreases to ∼3 km, ∼2 km, and ∼1 km at Little Switzerland, some 45 km south of the Denali fault. We conclude that the topographic and rock uplift patterns of the central Alaska Range, the shape and proximity of the McKinley strand of the Denali Fault to these patterns, the timing of the onset of rock uplift and denudation at ∼5–6 Ma, and a significant change in relative plate motion between North America and the Pacific plates circa 5.6 Ma are all inherently related.
Distribution of volcanoes in active margins
Authors: de Bremond d'Ars Jean , Jaupart Claude , Sparks R. Stephen J.
Published: February 2004

We report a comprehensive study of the distribution of volcanoes in 16 active plate margins, corresponding to a total of 479 volcanic systems. The active volcanic arcs are found to have a ribbon geometry with an average length / width ratio of around 10. The shape of the volcanic arc is compared to the shape of the associated trench by projecting one onto the other. The projection direction agrees with the direction of plate convergence, which shows that plate motion is the main factor constraining magma generation on the lithospheric scale. There is no characteristic spacing between volcanoes. In each arc, the distribution of volcano spacing is best represented by a Gamma distribution which corresponds to randomly generated points in the same geometrical conditions. In order to explain how such distributions may be generated, we have investigated in the laboratory the gravitational instability of a layer of buoyant liquid which is fed at a constant rate at the bottom of denser fluid. Different dynamic regimes are reached for different values of the viscosity ratio between the two fluids. For low viscosity contrasts, there is only one initial instability event, with buoyant plumes organized in a periodic pattern. These plumes are fed continuously by the source, even at large times. For high viscosity contrasts, which is relevant to the geological problem, plumes are produced intermittently by individual instability events. They take the form of “cavity” plumes fed by narrow tails. Once the plumes have reached the surface, they continue to be fed from below by material rising through their shrinking tail. In successive instability events, plume generation is not repeated at the same locations which leads to a complex plan‐form. With time, and hence with increasing number of instability events, the cumulative distribution of plume spacing changes from periodic to random. Material from a single plume is able to sustain volcanic eruptions for a period longer than the time between two instability events, and hence the distribution of active volcanic centers reflects the cumulative distribution of several instability events. This is consistent with the observed distributions.
Toward a better understanding of the Lowrie‐Fuller test
Authors: Xu Song , Dunlop David J.
Published: February 2004

We develop a theory of acquisition and alternating field (AF) demagnetization of anhysteretic remanence (ARM) and saturation isothermal remanence (SIRM) in multidomain (MD) grains in order to better understand the Lowrie‐Fuller test. Our theory shows that the relative stabilities of low‐field ARM and high‐field SIRM against AF demagnetization are determined by the distribution ƒ(hc) of microcoercivity hc in a sample, as found earlier by Bailey and Dunlop. When f(hc) is nearly constant, weak‐field ARM is more resistant to AF demagnetization than SIRM. In contrast, when ƒ(hc) varies exponentially or is a Gaussian distribution, SIRM is more AF resistant than ARM. These contrasting stability trends are conventionally called single‐domain (SD)‐type and MD‐type Lowrie‐Fuller results, respectively, but in reality, both types occur in the MD size range. We propose instead the descriptive terms L‐type result (low‐field remanence, i.e., ARM, more stable) and H‐type result (high‐field remanence, i.e., SIRM, more stable). The Lowrie‐Fuller test does not distinguish one type of domain structure from another, but it does depend indirectly on grain size. We show that the distribution ƒ(hc) in a given sample is determined primarily by the grain size d and the dislocation density ρ. A nearly constant ƒ(hc) occurs in grains with small d and/or ρ, but a Gaussian f(hc) is approached with increasing d and/or ρ. The transition from L‐type to H‐type behavior in the Lowrie‐Fuller test occurs at a critical grain size dt ≈ 2/(ρw), where w is the domain‐wall width. The lower the dislocation density, the larger the transition size in the Lowrie‐Fuller test. This simple relationship explains the increase in the transition size from about 5–10 μm observed for crushed magnetite grains to ≈ 100 μm for hydrothermally grown magnetites, which have lower dislocation densities than crushed grains.
Geochemical controls on the kinetics of quartz fracture at subcritical tensile stresses
Authors: Dove Patricia M.
Published: February 2004

A new kinetic model links physical and chemical controls on the subcritical fracture kinetics of quartz from the assumption that molecular level reactions governing fracture and dissolution proceed by similar pathways. The model formulation combines fracture theory with a mechanistically based description of chemical, thermal, and tensile stress effects on reactivity in aqueous environments. Water, as a vapor or liquid, promotes rupture of Si‐O bonds by end‐member processes: (1) reaction of a protonated surface with molecular water and (2) reaction of hydroxyl ions at an ionized surface. In humid environments, reaction frequency is determined by water accessibility to the crack tip. In wetted environments, the relative contributions of these mechanisms are determined by bulk solution composition which affects surface ionization and sol vent‐surf ace interactions. The macroscopic fracture rate law is given in meters per second by the fractional sum of these end‐member reaction mechanisms per a first‐order equation. Agreement of this empirical rate expression with reported measurements of quartz fracture rates suggests the model is robust. It gives a good fit to fracture rates over 6 orders of magnitude and explains increasing rates with increasing solution pH, the dependence of rate upon crystallographic direction, and thermal dependence of rate over 20° to 80°C. Findings in this study suggest that (1) fracture models based upon changes in surface free energy with solution composition are macroscopic descriptions of solvent‐surface interactions and parallel the mechanistic model presented here; (2) faster fracture rates observed in basic solutions are not facilitated by decreases in the activation barrier but are due to a transition in the solvent‐surface reaction to give a higher reaction frequency and (3) power law expressions applied to fracture rate versus stress intensity measurements may not have direct mechanistic significance since log‐linear relations describe rate data equally well. Results of the model indicate that common chemical constituents in groundwater could affect mineral reactivity in near‐surface and deeper earth environments. This quartz fracture rate model may lead to a better understanding of the time‐dependent integrity of rock and glassy materials in contact with the fluids of natural systems.
Statistical trends in the intraplate stress field
Authors: Coblentz David D. , Richardson Randall M.
Published: February 2004

The World Stress Map (WSM) database contains thousands of intraplate stress indicators, with the potential to provide important constraint for arguments about the relationship between tectonic stresses and both the kinematics and dynamics of plate motion. Previous studies, which relied almost exclusively on visual inspection of the data, established the existence of broad regions of uniform maximum horizontal compressive stress orientation (SHmax) and stress regimes on a regional scale. In the present study, we present a statistical analysis of the WSM stress indicators with the aim of quantifying trends in both the SHmax orientations and stress regimes. The analysis was carried out within 5° × 5° bins which provide a resolution of several hundred kilometers. Only the 4537 high‐quality WSM indicators with rating of A to C were used in the analysis. We present results for two types of analysis on the information contained within the bins. First, we evaluate the spatial distribution of the average stress regime (normal, strike‐slip, or thrust). Second, we apply the Rayleigh test, a standard statistical method in the analysis of directional data, to the distribution of SHmax orientations to test the null hypothesis that the orientations are random. An important aspect of our study is the quantification of the conclusions drawn from visual inspection of the World Stress Map. Our results indicate that broad regions of uniform SHmax orientations exist in most continental regions at high confidence levels (90% and 95%) and are less robust in the slowest moving continental plates. We also quantify the predominance of strike‐slip and compressional stress regimes in continental regions. Importantly, our analysis provides information about trends in the SHmax orientations in regions where large amounts of scatter in the directional data prevented conclusions being drawn from visual inspection of the data, for example, in western North America and continental Australia. Furthermore, we find a strong correlation between average SHmax orientations and both the ridge push torque and the absolute plate velocity azimuths. Our observation that a greater number of SHmax orientations correlate with the ridge push torque directions is further evidence that the intraplate stress field is strongly influenced by the ridge push force.
Localized boundary layer below the mid‐Pacific velocity anomaly identified from a PcP precursor
Authors: Mori Jim , Helmberger Donald V.
Published: February 2004

Dense record sections from deep earthquakes in Fiji and Argentina recorded on hundreds of short‐period stations in California at distances of 81° to 85° are used to investigate the detailed P wave velocity structure above the core‐mantle boundary (CMB). In the Fiji data a secondary phase arriving 2 to 4 s after the direct P is identified as a precursor to PcP. This phase provides good evidence for a reflection off the top of a thin low‐velocity layer above the CMB. Comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5%–10% compared to the overlying mantle. A record section from an Argentina event does not show the PcP precursor, indicating that the low‐velocity layer is not a global feature. This thin low‐velocity layer is in the same place as a much larger S wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the CMB under the mid‐Pacific.
Holocene slip rate and earthquake recurrence of the northern Calaveras Fault at Leyden Creek, northern California
Authors: Kelson Keith I. , Simpson Gary D. , Lettis William R. , Haraden Colleen C.
Published: February 2004

The northern Calaveras fault traverses a heavily populated area in the eastern San Francisco Bay region and has not had a large earthquake in more than 130 years. To obtain data on the number, timing, and recurrence of large paleoearthquakes, we conducted paleoseismologic investigations at Leyden Creek, which crosses the fault in the rugged southern East Bay Hills. The site is characterized by a prominent west facing scarp and five fluvial terraces on the western (upstream) side of the fault. On the eastern (downstream) side of the fault, the creek flows through a narrow bedrock canyon that constricts the modern valley and has constrained the location of a late Pleistocene paleovalley. The margin of a buried bedrock valley west of the fault trends nearly perpendicular to the fault and is offset 54 (+18, −14) m in a right‐lateral sense from the narrow bedrock canyon. Based on radiocarbon ages for alluvial sediments predating and postdating this paleovalley margin, we estimate an age of 11.5 (+3, −1) ka for the valley margin and a Holocene slip rate of 5 ± 2 mm/yr for the fault at Leyden Creek. Slickensides exposed in multiple trenches across the fault show that the most recent movement was predominantly lateral with a minor component of down‐to‐the‐west slip. Multiple displaced scarp‐derived colluvial deposits are interpreted as results of five or six surface ruptures within the past 2500 years. Twenty‐one radiocarbon samples from scarp‐derived colluvium and interfingered alluvial deposits suggest an average interval between surface rupture earthquakes of 250 to 850 years.
Methane‐rich fluids in the oceanic crust
Authors: Kelley Deborah S.
Published: February 2004

Analyses of fluid inclusions in plutonic rocks recovered from the slow‐spreading Southwest Indian Ridge (SWIR) record CH4 concentrations of 15–40 times those of hydrothermal vent fluids and of basalt‐hosted volcanic gases and provide the first direct sampling of CO2‐CH4‐H2O‐H2‐C‐bearing fluids in the oceanic crust. Compositional, thermal, and spatial analyses of these fluids are used to model the evolution of volatiles during the crystallization and cooling of mid‐ocean ridge magma chambers and to assess the potential importance of carbon‐bearing fluids in geochemical processes in the lower crustal component of hydrothermal systems. Results from these analyses show that the earliest fluids to be exsolved from the melts are dominated by CO2‐rich vapors, which with progressive fractionation evolved to more H2O‐rich compositions. These later fluids were most likely exsolved under immiscible conditions and involved the development of CO2‐H2O‐rich vapors and CO2‐H2O‐NaCl brines that were trapped during mineral growth, as well as during later high‐temperature fracturing events. CO2 + CH4 ± H2O‐rich fluids in olivine and plagioclase minerals that contain up to 30–50 mol % CO2 and 33 mol % CH4 may reflect respeciation of magmatic CO2 during cooling and attendant graphite precipitation. Phase equilibria suggest that these fluids reequilibrated at ∼500°–600°C and at fO2's ∼ −3 log units below, to close to QFM conditions. Alternatively, the inclusions may record respeciation of magmatic fluids attendant with the inward diffusion of H2 into the inclusions and reduction of entrapped CO2 during degassing and cooling of the gabbros throughout the subsolidus regime. Subsequent seawater reaction, at minimum temperatures of ∼400°C, with mafic‐rich layers within the gabbroic rocks or with ultramafic material that underlies the plutonic sequence resulted in the formation of CH4‐H2O fluids that contain up to 40 mol % CH4, molecular H2, and graphite(?) daughter minerals. These data provide strong evidence that the CH4‐H2O‐rich fluids produced during serpentinization reactions were trapped under equilibrium conditions in the presence of graphite at very near to QFM conditions. The ubiquitous occurrence of CH4‐rich fluids in oceanic crustal layer 3 rocks from the SWIR indicates that these fluids may be a significant and previously unrecognized source for these volatile species in some hydrothermal systems venting on the seafloor and that the deep‐seated fluids most likely play an important role in the transfer of carbon from the lithosphere to the hydrosphere.
Strain distribution across the Australian‐Pacific plate boundary in the central South Island, New Zealand, from 1992 GPS and earlier terrestrial observations
Authors: Pearson C. F. , Beavan J. , Darby D. J. , Blick G. H. , Walcott R. I.
Published: February 2004

As part of a geodetic experiment aimed at understanding the deformation associated with the Australian‐Pacific plate boundary in the South Island of New Zealand, in 1992 we reoccupied using Global Positioning System (GPS) techniques a first‐order triangulation and trilateration network established in 1978 between Christchurch on the east coast and Hokitika on the west. The network crosses the South Island a few tens of kilometers southwest of the region where the plate boundary changes from a single, throughgoing oblique slip fault, the Alpine fault, to a series of subparallel strike‐slip faults, the Marlborough faults. The GPS data have been analyzed as daily network solutions, with about 12 stations in each solution. RMS repeatabilities for stations with multiple occupations are 4 mm, 8 mm, and 15 mm in the north, east, and vertical components, respectively. The observed strain across the network is consistent with the entire NUVEL‐IA Pacific‐Australia plate velocity being accommodated on land across this part of the South Island. Shear strain rates derived from the GPS and terrestrial data show that the highest strain rate (> 0.4 μrad/yr) occurs in the region of the Southern Alps and Alpine fault. This rate is about two thirds of the rate predicted from the NUVEL‐IA plate velocity model assuming that all the plate boundary deformation occurs across this region, implying that about two thirds of the plate motion is accommodated in the vicinity of the Alpine fault. Significant shear strain rates greater than 0.2 μrad/yr are also observed farther east, particularly in the region of the Porters Pass‐Amberley fault zone, demonstrating that this zone accommodates a significant part of the plate boundary deformation. Using dislocation modeling, we show that the variation in fault‐parallel shear strain rates across the Alpine fault is reasonably consistent with a nonoptimized dislocation model involving a 50° dipping fault that is presently locked to a depth of ∼12 km. One explanation for this observation is that the upper ∼12 km of the crust in this area is storing most of the right‐lateral shear component of the relative plate motion as elastic energy that will be released in a future major earthquake. The variation in the other component of shear strain, which may be interpreted as fault‐normal relative contraction, is not explained by a simple dislocation model, implying that the normal component of plate motion is taken up in other ways.