bluuebunny/crossref_metadata_2025 huggingface dataset:
Errors in plate rotations as described by covariance matrices and their combination in reconstructions
Authors: Jurdy Donna M. , Stefanick Michael
Published: February 2008

We use covariance matrices to describe the errors in plate rotations and to combine the contrbutions of individual plate pairs for plate reconstructions. First, for the rotation of a pair of plates the covariance matrix which describes the errors in the rotation is defined. Then we show how individual covariance matrices can be combined in a circuit of successive plate rotations to assess the errors in a plate reconstruction. Using the covariance matrix for accumulated errors from a sequence of rotations, we can determine the uncertainties in the position of selected points on one plate in the plate circuit to another that is assumed to be fixed. As an example, the position of the Pacific plate relative to North America is reconstructed at chron 6 (20 Ma). This choice is made to allow a direct comparison with published error assessments. Using the accumulated covariance matrix, we find similar uncertainties in the positions of their selected points on the Pacific plate. As a further test, this reconstruction is repeated using seafloor spreading data that directly relate Antarctica to Africa, bypassing the link of India. This eliminates the largest source of error in the plate circuit, that due to the short ridge segment between Africa and India. This revised, shortened circuit results in generally smaller errors in the positions of points on the Pacific plate. The covariance matrices of the two alternative circuits are used to construct the optimal estimate and its associated error. The optimal estimate and its error region intersect the two alternative reconstructions and their error regions.
Fluid flow in crystalline rocks: Relationships between groundwater spring alignments and other surface lineations at Altnabreac, United Kingdom
Authors: Brereton N. R. , McEwen T. J. , Lee M. K.
Published: February 2008

The Strath Halladale Granite in the region around Altnabreac, northern Scotland, United Kingdom, has been studied with a view to establishing a relationship between the regional distribution of faults and fracture zones, surface discharges of groundwater, and groundwater flow systems. A major component of the groundwater flow is through the rock fractures. Because of the extensive superficial cover the surface expression of major fractures was difficult to identify from the limited surface exposures. Geophysical surveys and aerial photography enabled the authors to define lineations which could be related to the presence of fractures. The areal distribution of groundwater spring discharges was mapped using thermal infrared line scan techniques. The distribution of these springs has been studied to assess their relationships to surface lineaments and to correlations with geophysical and fracture mapping data.
Detection of a locked zone at depth on the Parkfield, California, segment of the San Andreas Fault
Authors: Harris Ruth A. , Segall Paul
Published: February 2008

The Parkfield, California, segment of the San Andreas fault is transitional in character between the creeping segment of the fault to the northwest and the locked Carrizo Plain segment to the southeast. The rate of shallow fault slip decreases from 25–30 mm/yr northwest of the epicenter of the 1966 Parkfield earthquake to zero at the southeastern end of the 1966 rupture zone. Data from a network of trilateration lines spanning the San Andreas fault near Parkfield and extending to the Pacific coast near San Luis Obispo shed light on the rate of fault slip at depth since the 1966 earthquake. In this study, average rates of line length change and shallow fault slip were inverted to determine the slip rate at depth on the Parkfield fault segment. The fault is taken to be a vertical surface with unknown distribution of strike‐slip displacement in an elastic half‐space. A striking result of the inversions is that all solutions providing acceptable fits to the data exhibit a locked zone essentially coincident with the rupture surface of the 1966 Parkfield earthquake. The data require that the locked zone extend nearly as far north as the 1966 epicenter; however, the vertical extent of the locked zone is not well resolved. Over much of the Parkfield segment the fault is slipping faster at the earth's surface than it is at seismogenic depths. In order to fit the trilateration measurements it is necessary to include a component of contraction normal to the trend of the San Andreas. The inversion results suggest a spatially uniform normal strain of −0.06 μstrain/yr. The orientation of the contraction is compatible with geologic and seismic evidence of active folding and reverse faulting in the region. The magnitude of the contraction is consistent with convergence rates inferred from global plate motion models.
Till beneath ice stream B: 1. Properties derived from seismic travel times
Authors: Blankenship D. D. , Bentley C. R. , Rooney S. T. , Alley R. B.
Published: February 2008

Seismic experiments conducted on ice stream B, part of the marine ice sheet of West Antarctica, show a meters‐thick layer immediately beneath the 1000‐m‐thick ice. A seismic experiment consisting of wide‐angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer. Inversion of seismic travel times yields a compressional wave speed of less than 1700 m s−1 and a shear wave speed less than 160 m s−1 for the layer. These very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure. Based on wave speeds in other saturated, unconsolidated sediments, we believe that a porosity substantially greater than 0.32, probably around 0.4, and an excess of overburden pressure over pore pressure of only 50 kPa (0.5 bar) characterize the layer at this location.
Observations on the distribution and characteristics of potholes on surging glaciers
Authors: Sturm Matthew
Published: February 2008

Potholes are depressions, often water‐filled, 10–100 m across, found on the surfaces of glaciers. Most fields of potholes are found on surging glaciers, suggesting that potholes form as a result of surging. A photographic inventory of hundreds of glaciers in Alaska and Yukon Territory allowed this hypothesis to be tested. Out of 26 pothole fields observed, 19 were on known surging glaciers, two on probable surging glaciers, and the rest in areas where surging glaciers are common. The majority of pothole fields were found near the equilibrium line. Photographs show that potholes formed within several years following the 1937 surge of the Black Rapids Glacier in a zone where there had been intense crevassing. Between 1949 and 1986 the total number of potholes on the glacier decreased from approximately 700 to less than 200, while their mean size increased from 20 to 30 m. Some of these potholes have been observed to drain englacially. The movement of these potholes were measured photogrammetrically to be less than 10 m/yr, the majority of the motion resulting from ice flow. Intense crevassing resulting from surging appears to be necessary for the formation of potholes, but not sufficient, as only 21 out of 256 surging glaciers contain potholes.
Subglacial till: A physical framework for its properties and processes
Authors: Clarke Garry K. C.
Published: February 2008

The continuum theory of mixtures provides a framework around which present knowledge of subglacial till can be organized. The proposed framework is provisional in two senses: the constitutive description of till will require improvement, and not all potentially significant processes have been included. Till is treated as a three‐component mixture of water, clasts (coarse‐grained solids), and fine‐grained matrix solids. These components obey mass balance equations, and their behavior is subject to constitutive relations that describe such processes as Darcian water transport, fine‐grained sediment transport, consolidation, shear deformation, dilatancy, and comminution. Five state variables uniquely determine the compressibility, permeability, shear strength, and other physical properties. Numerical solutions of the state evolutions equations illustrate that the properties of till are highly responsive to changes in the subglacial environment.
How do glaciers surge? A review
Authors: Raymond Charles F.
Published: February 2008

The quasi‐periodic oscillations between normal and fast motion exhibited by surge‐type glaciers provide the best observational opportunity to determine limiting conditions that allow fast motion. The measurements from Variegated Glacier prove that its surge motion is caused by rapid sliding induced by high water pressure. This arises from a major restructuring of the basal hydraulic system, which impedes water discharge prior to and during surge. Although the evolving glacier geometry and stress distribution play a principal enabling role, the seasonal timing of two distinct surge pulses, each initiated in winter and terminated in summer, indicates a major influence from variable external water inputs. This influence is not considered in existing surge models and should promote caution in the use of data from temperate and subpolar surge‐type glaciers to deduce surge potential in polar ice masses. The spatial spreading of surge motion from a zone of local initiation occurs by stress redistribution, which may spread the surging zone rapidly upglacier or downglacier inside a region of active ice, and by mass redistribution with compressional thickening at the surge front, which enables down‐glacier propagation into less active ice. The data from other surge‐type glaciers, including the extensive data from Medvezhiy Glacier, are not inconsistent with the above processes but are inadequate to establish whether completely different mechanisms operate in some surges. The way by which water accumulates and produces fast sliding is not established in detail for any surge‐type glacier and may be different on different glaciers depending, for example, on the presence or absence of unconsolidated debris between the ice and rock.
Mutual interactions of redox couples via electron exchange in silicate melts: Models for geochemical melt systems
Authors: Schreiber Henry D. , Merkel Robert C. , Schreiber V. Lea , Balazs G. Bryan
Published: February 2008

The mutual interactions via electron exchange of redox couples in glass‐forming melts were investigated from both theoretical and experimental points of view. A thermodynamic approach for considering the mutual interactions leads to the conclusion that the degree of mutual interaction in the melt should be proportional in part to the difference in relative reduction potentials EM′ of the interacting redox couples. Experimental studies verify this conclusion for numerous redox couples in several composition/temperature/oxygen fugacity regimes. Geochemical systems simultaneously possess many potentially multivalent elements; the stabilized redox states in the resulting magmas can be explained in part by mutual interactions and by redox buffering through the central Fe(III)‐Fe(II) couple in the melts.
Ultrasonic studies of silicate melts
Authors: Rivers Mark L. , Carmichael Ian S. E.
Published: February 2008

The ultrasonic velocity and absorption of 32 silicate melts have been determined as a function of temperature (1175–1925 K) and frequency (3–12 MHz). The samples include the components SiO2, TiO2, Al2O3, FeO, MgO, CaO, SrO, BaO, Li2O, Na2O, K2O, Rb2O, and Cs2O, and range from simple alkali silicates to multicomponent synthetic and natural melts. The relaxed sound speed varies systematically with substitution of the alkali and alkaline earth oxides, decreasing monotonically with increased cation radius. An algorithm for the compositional dependence of the ultrasonic velocity at
, where xi,υ is the volume fraction of component i and
is independent of composition, is capable of predicting relaxed melt velocities to within 4%. The temperature dependence of the sound speeds in the nondispersive region is quite small, ≈1 part in 104 K−1. A simple linear model for the compositional dependence of the derived property (δV/δP)T has an uncertainty of 13%, approximately equal to that expected from the propagation of errors involving sound speed, density, thermal expansion, and heat capacity. The contributions of the changes in sound speed and density with temperature are nearly the same in their influence on the temperature dependence of the bulk modulus of natural melts. The relaxation properties c/c0 (ratio of the sound speed to the low‐frequency value) and αλ (absorption per wavelength) of all of the liquids studied are very similar when plotted against ωτ, the product of the angular frequency and the shear relaxation time. Both properties are consistent with τ equal to 1% of the product of the low‐frequency shear viscosity and bulk modulus. Dispersion begins when ωτ ≃ 0.1 and the limiting high‐frequency sound speed c∞ is about 2.3 times c0. The product αλ attains a maximum value of about 1.4 when ωτ ≃ 0.8. The data cannot be explained by a single relaxation time in the melts; rather, a spectrum of relaxation times is indicated.
Elastic wave velocity structure of the crust and upper mantle beneath the North China Basin
Authors: Shedlock Kaye M. , Roecker Steven W.
Published: February 2008

Travel time residuals from locally and teleseismically recorded earthquakes are used to determine the three‐dimensional elastic wave velocity structure beneath the central North China basin and surrounding mountains. We calculated P wave and S wave travel time residuals for 200 aftershocks of the 1976 Tangshan earthquake and P wave travel time residuals for 194 teleseisms recorded by the Beijing network between 1976 and 1983. We performed iterative one‐ and three‐dimensional block inversions for crust and uppermost mantle structure based solely on the aftershock travel time residuals. We also performed iterative three‐dimensional block inversions for crust and upper mantle structure based solely on the teleseismic travel time residuals and based on the combined aftershock and teleseismic travel time residuals. We determined that the average thickness of the crust beneath the North China basin and surrounding mountains is about 35 km. Beneath the onshore portion of the North China basin, however, the crust is closer to 32 or 33 km thick. The velocity structure of the upper 20 km or so of the crust is well correlated with the surface geology and parallels the short‐wavelength (a few tens of kilometers) features of the surface topography. Rocks with low seismic velocities lie in or beneath the basin; rocks with high seismic velocities lie in or beneath the mountains. The uppermost mantle, and possibly even the lower crust, appears to be a zone of transition in velocity structure between that of the upper crust and the mantle. The trends of relative highs and lows in the velocity structure of the mantle are nearly opposite that of the crust. The mantle material underlying most of the North China basin has higher than average seismic velocities, while the material beneath the western mountains has lower than average velocities. The velocity structure of the mantle parallels the long‐wavelength (tens to hundreds of kilometers) features evident in the gravity and topography of North China. The high‐velocity mantle material presently beneath the basin probably represents mantle processes not directly related to the formation of the North China basin. We infer that the transition zone between the short‐wavelength and long‐wavelength velocity features represents a zone of decoupling between the lithosphere and asthenosphere. Specifically, the lithosphere beneath the North China block is moving eastward with respect to the asthenosphere.
Tectonics and conductivity structures in the Southern Washington Cascades
Authors: Stanley William D. , Finn Carol , Plesha Joseph L.
Published: February 2008

The tectonic setting of the southern Washington Cascades has been studied with the aid of magnetotelluric (MT) and other geophysical data. The main feature of interest in the geophysical data is a broad high‐conductivity anomaly mapped with MT and geomagnetic variation (GMV) data. This anomaly is located roughly within the triangle formed by the volcanoes Mount Rainier, Mount St. Helens, and Mount Adams but exceeds beyond Mount Rainier to the northwest. We interpret the cause of the anomaly to be conductive rocks with resistivities of 1–4 ohm m and thicknesses possibly greater than 15 km. These conductive rocks are found 2–8 km beneath the overlying less conductive volcanic and sedimentary rocks at the surface. Two aeromagnetic lows follow the trend of the conductivity anomaly, and linear belts of strike‐slip seismicity are coincident with both these magnetic lows. One of the aeromagnetic lows is coincident with the western margin of the conductivity anomaly. The geophysical data appear to outline a suture zone of probable Eocene age caused by accretion of a large seamount complex (Siletzia) and that may contain large thicknesses of compressed forearc basin and accretionary prism sedimentary rocks of Cretaceous to Eocene age. Part of the shallower conductive rocks may be associated with carbonaceous continental and transitional marine sedimentary rocks of the Puget Group. The contact between the hypothesized compressed basin and the accreted terranes to the west may localize the release of shear stresses in this region of oblique subduction. Several possible explanations for the conductivity anomaly are considered in addition to the compressed basin hypothesis.
A geophysical investigation of the Northern Panamint Valley, Inyo County, California: Evidence for possible low‐angle normal faulting at shallow depth in the crust
Authors:   MIT 1985 Field Geophysics Course, Biehler Shawn
Published: February 2008

Gravity, magnetic, seismic refraction, electrical resistivity, and magnetotelluric measurements in northern Panamint Valley indicate that the alluvial and pluvial fill in the basin is thin and that basement under the valley floor lies at a shallow depth. Contours of the Bouguer gravity anomaly field do not define a low over the valley and do not mimic the mapped boundaries of alluvial fill. Electrical resistivity and seismic refraction measurements indicate that the playa in northern Panamint Valley is underlain by no more than about 200 m of sediment. This is consistent with the absence of any local lows in the Bouguer gravity anomaly field over the valley exceeding about 15 mGal in amplitude. There are no anomalies more than 50 nT in amplitude along an east‐west magnetic profile across the valley floor. In contrast, upward continuations of magnetic profiles measured across basalt outcrops present at the valley margins still contain anomalies exceeding 75 nT in amplitude at heights of 300 m. Because the inferred thickness of sediment under a width between 5 and 9 km of the valley is less than 300 m, it is insufficient to conceal the signal expected from such magnitized basalt buried under the sediment. We interpret this finding as indicating that the valley is not underlain by basalt except at its margins, which implies that the west side of the valley has moved between 6 and 10 km away from the Panamint Range, in a direction parallel to the Hunter Mountain fault (∼N55°W). A balanced cross section across the valley constrains the required low‐angle normal fault to be between 0.5 and 3 km beneath the valley floor.
Earthquake deformation cycle on the San Andreas Fault near Parkfield, California
Authors: Segall Paul , Harris Ruth
Published: February 2008

Six moderate earthquakes are thought to have ruptured the Parkfield segment of the San Andreas fault since 1857. The similar characteristics of the three seismically recorded events, together with the quasi‐regular 22‐year recurrence interval, have led to the forecast of a similar event in 1988 ± 5 years. In this study the potential for the hypothesized earthquake is assessed by estimating the time required for interseismic straining to recover the strain energy released by the most recent, June 1966, Parkfield earthquake. For a simple model of the earthquake cycle this interval is proportional to the ratio of the 1966 seismic moment to the interseismic moment deficit rate. Baseline lengths measured before and after the 1966 earthquake are inverted to estimate the magnitude and distribution of the combined coseismic and transient postseismic slip. Although the geodetic data determine only broad spatial averages of the fault slip, they do require that slip at depth exceeded the surface offsets measured in 1966. Inversions with extreme prior models are calculated to assess the range of seismic moment and moment deficit rates consistent with the available geodetic data. Inversions assuming only that the slip distributions are to some degree smooth yield strain energy recovery intervals of 14–25 years. Extreme values range from as little as 6 years to as much as 29 years. In comparison, historic recurrence intervals of Parkfield earthquakes range from 12 to 32 years. These results are consistent with the expectation of a magnitude 5½–6 earthquake near Parkfield by 1993.
Three‐dimensional P and S wave velocity structures beneath Taiwan: Deep structure beneath an arc‐continent collision
Authors: Roecker S. W. , Yeh Y. H. , Tsai Y. B.
Published: February 2008

The island of Taiwan is a byproduct of one of the few active collisions between a continent, represented by the Eurasian continental shelf, and an island arc, represented by the northern extension of the Luzon arc on the Philippine Sea plate. To understand better the evolution and current tectonics of this collision, we selected 1260 well‐recorded earthquakes from an initial set of 50,000 located by the Taiwan Telemetered Seismograph Network for the determination of one‐ and three‐dimensional P and S wave velocity structures beneath the island. The results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones. In the east, the Philippine oceanic plate is underthrusting the Eurasian plate east of a north‐south boundary that is well defined by both seismic activity and a region of high velocity. In the south, the Eurasian continental plate is underthrusting the Philippine Sea plate south of an east‐west boundary at about 23°N that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities. The dip of the subducted continent is shallow until it reaches the Luzon island arc 50 km east of the main island. Structure under the main part of the island north of 230N reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range. The dip of this low‐velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km. This seismic zone lies in a velocity “saddle” and marks an apparent offset in the central low‐velocity region at 24°N. Evidence for the subduction of the Eurasian continent beneath Taiwan therefore exists everywhere beneath the island, and the low‐velocity regions in the mantle support, but do not require, the subduction of some 6–16 km of lower continental crust to depths of at least 50 km. The distinct change in both the velocity structure and the seismic activity at 23°N and the offset of the low‐velocity regions at 24°N argue for abrupt changes in the nature of subduction across these latitudes. These changes may be caused by an interaction of the Luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface. Finally, the velocity structure of the subducting Philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest‐velocity regions of the plate, but are probably located near the upper edge of the plate. There is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented, and that subduction is presently occurring as far south as 22°N.
Morphometric variability within the axial zone of the southern Juan de Fuca Ridge: Interpretation from Sea MARC II, Sea MARC I, and deep‐sea photography
Authors: Kappel Ellen S. , Normark William R.
Published: February 2008

The morphometric characteristics of the axial regions of oceanic spreading centers are determined by (1) the type of volcanic flows, (2) the relation between primary volcanic relief (on a scale of a few meters to tens of meters) and degree of sediment cover, and (3) the extent of surficial expression and timing of tectonic disruption of the young oceanic crust. Even within a single, continuous, linear spreading‐ridge segment with relatively uniform axial valley dimensions over a distance of 50 or more kilometers, such as along the southern Juan de Fuca Ridge, the changes in morphometric characteristics along axis within the youngest crust indicate distinct variation in tectonic and volcanic activity over short distances within short time periods. An integrated analysis of Sea MARC I, Sea MARC II, and photographic data for the southernmost continuous segment of the Juan de Fuca Ridge shows that generalizations about tectonic and volcanic processes at spreading ridges must consider both the temporal scale of processes as well as the physical scales of observations if predictive models are to be successful. Comparison of the morphometric expression within the major hydrothermal vent area and the rest of the southernmost ridge segment suggests that the mapped distribution of hydrothermal vents may reflect the extent of survey effort rather than uniqueness of geologic setting.
Stable isotope studies of vent fluids and chimney minerals, southern Juan de Fuca Ridge: Sodium metasomatism and seawater sulfate reduction
Authors: Shanks Wayne C. , Seyfried William E.
Published: February 2008

Sulfur isotope values (δ34S) of H2S in vent fluids from the southern Juan de Fuca Ridge hydrothermal sites range from 4.0 to 7.4‰ and are variably 34S‐enriched with respect to coexisting inner wall chimney sulfides. Chimney sulfides range from 1.6 to 5.7‰. The chimneys consist of Fe‐sphalerite zoned to inner zinc sulfide and chalcopyrite (± isocubanite)‐pyrrhotite lining channels. Sulfide from inner walls of type A chimneys have the lightest δ34S values. Type B chimneys (porous, unzoned, low‐Fe‐sphalerite) have the isotopically heaviest chimney sulfides and occur at vent sites distal to the along‐axis shallow point of the ridge crest, hence distal to the magma chamber. These variations are largely ascribed to sulfate reduction by ferrous iron in the hydrothermal fluid in chimneys or substrate mounds, probably due to transitory entrainment of ambient sulfate‐bearing seawater. The δ18O values of end‐member hydrothermal fluids range from 0.6 to 0.8‰, significantly lower than the δ18O values at 21°N vent fluids. The δD values of the fluid samples range from −2.5 to 0.5‰. Isotopic differences from the 21°N fluids may be due to slightly higher water/rock ratios, approximately 1.0, in the southern Juan de Fuca Ridge hydrothermal system. Admixture of a small amount of residual brine from an earlier phase separation event may have contributed water with low δD values. Sulfate reduction occurs in the deep (2.3 km) hydrothermal reaction zone; a small amount of seawater sulfate passes through the zone of anhydrite precipitation during recharge of the hydrothermal system and is reduced by reaction with pyrrhotite in basalt. Sulfide from pyrrhotite is mixed with 34S‐enriched sulfate‐derived sulfide to produce pyrite having δ34S values of about 3.0‰ and H2S having values of about 2.1‰ in the ascending fluid. Requisite acidity is provided by Na‐metasomatism at about 370°C, yielding albite‐ and epidote‐rich alteration phases.
A kinematic model for the evolution of the Gorda Plate
Authors: Stoddard Paul R.
Published: February 2008

The magnetic lineation pattern produced by the Gorda Rise reflects a complex tectonic history marked by propagating ridges and nonrigid deformation of the Gorda plate. The Pacific‐side lineation pattern is matched using a kinematic model which incorporates northward and southward propagating ridge segments and ridge rotation. By superimposing flexural‐slip style deformation on the resulting Gorda‐side lineations, and allowing convergent motion at the Mendocino Fracture Zone, the Gorda magnetic pattern is also successfully predicted. A similar model, but one which does not allow convergence at the Mendocino Fracture Zone, fails to produce the observed lineation pattern. Model predictions of the amount of material that would be “obducted” at the Mendocino due to convergence between the Gorda and Pacific agree with bathymetric and density estimates of the amount of material located along the Mendocino Ridge.
Crustal deformation in Great California earthquake cycles
Authors: Li Victor C. , Rice James R.
Published: February 2008

Periodic crustal deformation associated with repeated strike‐slip earthquakes is computed for the following model; A depth L (≤ H) extending downward from the Earth's surface at a transform boundary between uniform elastic lithospheric plates of thickness H is locked between earthquakes. It slips an amount consistent with remote plate velocity Vpl after each lapse of earthquake cycle time Tcy. Lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress. The plates are coupled at their base to a Maxwellian viscoelastic asthenosphere through which steady deep‐seated mantle motions, compatible with plate velocity, are transmitted to the surface plates. The coupling is described approximately through a generalized Elsasser model. We argue that the model gives a more realistic physical description of tectonic loading, including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle, than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone. Parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 San Andreas ruptures. We fix Vpl = 35 mm/yr, Tcy = 160 years, and L = 9–11 km based on earthquake nucleation depths. The geodetic data are then found to be described reasonably, within the context of a model that is locally uniform along strike and symmetric about a single San Andreas fault strand, by lithosphere thickness H = 20–30 km and Elsasser relaxation time tr = 10–16 years. We therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust‐mantle transition zone and has an effective viscosity between about 2 × 1018 and 1019 Pa s, depending on the thickness assigned to the asthenospheric layer. Predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces, although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands.
Creation and preservation of subduction‐related inverted metamorphic gradients
Authors: Peacock Simon M.
Published: February 2008

Peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost U.S. Cordillera. Inverted metamorphic gradients are preserved in the Pelona‐Orocopia Schist (southern California), the Catalina Schist (California borderland), the Central Metamorphic Belt (Klamath province), the South Fork Mountain Schist (northern California Coast Ranges), and possibly the Shuksan Suite (northern Washington). With the exception of the South Fork Mountain Schist, peak metamorphic temperatures increase structurally upward from ∼400°C to ∼650°C over 1–2 km, indicating metamorphic gradients in excess of −100°C km−1. Estimates of metamorphic gradients in some terranes, such as the Catalina Schist, depend critically on the effects of postmetamorphic faulting. The tectonic settings and high metamorphic pressures (∼800 MPa) suggest that the inverted metamorphic gradients formed in paleosubduction zones. Four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls, whereas the Pelona‐Orocopia Schist formed beneath granitic crystalline rocks. The high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones. Two separate two‐dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved. Rapid subduction (∼10 cm yr−1) beneath young oceanic lithosphere (<10 Ma) results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of −100°C km−1in the top portion of the downgoing slab. In order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall. Accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and/or greenschist facies devolatilization reactions that weaken the descending slab. Alternatively, inverted metamorphic gradients may represent the continuous accretion of material at rates of 1–10 mm yr−1under conditions of declining temperature. Continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 Ma. Continued subduction of ∼1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient. Synsubduction uplift and/or moderately fast postsubduction uplift rates of ∼1 mm yr−1are required to preserve the inverted metamorphic gradient. The models predict that (1) the high‐grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction, and (2) inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km.
Spatial patterns of aftershocks of deep focus earthquakes
Authors: Willemann Raymond J. , Frohlich Cliff
Published: February 2008

This paper presents precise relative relocations for aftershocks of 59 intermediate and deep earthquakes, using P, pP, and PKP arrival times read by us or reported in the literature. These aftershocks included 36 “rupture subevents,” occurring within 1 min of the initial event, and 71 “true aftershocks,” occurring later and identified statistically as being related to the initial event. We compare the relative locations with the orientations of the Wadati‐Benioff zones and with the available focal mechanisms using statistical methods not previously applied to earthquake data. Surprisingly, neither true aftershocks nor rupture subevents cluster along the nodal planes of intermediate or deep earthquake focal mechanisms. Aftershocks more than 20 km from the initial events occur preferentially in the plane of the Wadati‐Benioff zone, while those lying closer have isotropically distributed directions. Rupture subevents occur after the travel time of the S wave from the initial event. Larger‐magnitude earthquakes generally possess rupture subevents lying farther from the initial event, whereas true aftershocks can occur 50 km or more from initial events having magnitudes as small as 5.3. Except following a few unusually large earthquakes with focal depths of about 100 km, we find no rupture subevents or true aftershocks more than about 80 km from the initial hypocenter. The existence of many aftershocks far from nodal planes does not favor models in which deep earthquake failure is simply slip along a planar fault. Rather, the aftershocks may occur in response to a general redistribution of stress caused either by the occurrence of the initial event or, possibly, by nearby continuing phase transitions. Alternatively, the failure zones of deep earthquakes may be surfaces which are frequently curved by 40° or more from the orientation of the initial nodal plane, perhaps due to inhomogeneous stress distributions.
Late Paleozoic magnetization of mineralized and unmineralized ordovician carbonates from east Tennessee: Evidence for a post‐ore chemical event
Authors: Bachtadse Valerian , Van der Voo Rob , Haynes Frederick M. , Kesler Stephen E.
Published: February 2008

Thermal demagnetization studies on samples from mineralized and unmineralized rocks in the Lower Ordovician Knox Group of east Tennessee reveal a mean direction of D/I = 157°/+18° (α95 = 7.7°, k = 52.9) after 60% unfolding, combining stable endpoint and remagnetization circle data. Although not significant at the 95% confidence level, the fold test is suggestive of a synfolding (Alleghenian) age of the magnetization. The magnetization cannot be postfolding, i.e., Permian or younger, because the in situ directions do not resemble those for post‐Carboniferous times. The magnetization is unlikely to be much older than the folding because the directions with respect to bedding yield a paleopole characteristic of the Early Permian, but unlike poles for pre‐Late Carboniferous times. Characteristic magnetizations from mineralized and unmineralized collapse breccias are in good agreement and fail the conglomerate test. The best estimate for the paleopole resulting from this study falls at 40°N, 126°E, within the Late Carboniferous segment of the North American Paleozoic apparent polar wander path. Thus the magnetization is interpreted as synfolding and secondary due to widespread Kiaman remagnetization. Blocking temperatures of up to 520°C and the acquisition of isothermal remanent magnetization suggest magnetite as the carrier of the magnetization. Scanning electron microscopy and energy dispersive analysis of magnetic extracts from limestone samples reveal abundant magnetite spheres. Secondary dolomites have the same magnetic characteristics as the primary limestones, and it is inferred that similar magnetite spheres are the carriers of their magnetization. Because this magnetization, and therefore its carrier, occurs in sphalerite and secondary dolomite, it must be coeval with or younger than the ore, which itself is thought to be of middle to late Paleozoic age. It is argued that the magnetization was acquired during growth of magnetite, associated with a late Paleozoic chemical event.
Paleomagnetism and age of the Archean Usushwana Complex, southern Africa
Authors: Layer Paul W. , Kröner Alfred , McWilliams Michael , Burghele Alexander
Published: February 2008

Paleomagnetic and 40Ar/39 Ar age data are reported from the Usushwana Complex, a layered gabbro and pyroxenite intrusive exposed in Swaziland and eastern South Africa (26.5°S, 31.0° E). The main gabbro body and an associated granophyre return a mean Rb‐Sr whole‐rock and Sm‐Nd mineral age of 2875±40 Ma. Alternating field and thermal analysis of specimens from 18 sites (70 samples) yields a mean direction D = 304°, I = 53°, (k = 32, α95 = 6°). This direction is significantly different from that obtained from the ∼ 2100 Ma Bushveld Complex. Rare dikes of probable Bushveld age cut the Usushwana Complex, and a low coercivity overprint seen in specimens from a few sites has a Bushveld direction and almost certainly reflects this event. Thermal overprinting is recorded by 40Ar/39 Ar hornblende and pyroxene ages between 2094±54 Ma and 2386±58 Ma that we interpret to have occurred at low temperature but over a significant period of time. Isothermal remanent magnetization acquisition experiments and laboratory unblocking temperatures suggest that the primary remanence carrier is single domain magnetite. The paleomagnetic pole position calculated from the Usushwana Complex (9° N, 347° E) is significantly different from other late Archean poles. Although the Usushwana pole is close to the pole from the Palabora Igneous Complex of South Africa (1900 Ma), there is no isotopic evidence indicating a thermal event of that age in the Usushwana Complex.
Age and spreading history of the Cayman Trough as determined from depth, heat flow, and magnetic anomalies
Authors: Rosencrantz Eric , Ross Malcolm I. , Sclater John G.
Published: February 2008

The opening of the Cayman Trough records aspects of relative motion between the North American and Caribbean plates essential to accurate reconstruction of Caribbean evolution. Assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates. Depths to basement show that the crust has subsided away from the present axis in two stages: a younger stage characterized by substantial and asymmetric (greater to the east) subsidence, preceded by an older stage of slight increases of depth with distance. Depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm/yr since 25–30 Ma and at 20–30 mm/yr prior to 30 Ma. Marine magnetic anomalies show lineations perpendicular to the trough, including a distinct central anomaly, but the pattern is neither well defined nor symmetrical about the present axis. However, partial anomaly sequences can be identified on both sides of the axis, and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps, mostly eastward. The overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm/yr since and prior to 26 Ma. Both subsidence and magnetics indicate that the trough opened by at least 45–50 Ma.
Introduction and Tribute to H. W. Menard 1920–1986
Authors: Orcutt John A.
Published: February 2008

Henry William Menard, professor of geology at the Scripps Institution of Oceanography, was one of the pioneers and leaders of the great period of exploration of the ocean floor that began after World War II. He led or participated in 25 deep‐sea expeditions, mainly in the North Pacific and South Pacific oceans. On the first of these, the Midpac Expedition in 1950, he and his colleagues at the Scripps Institution and the U.S. Navy Electronics Laboratory, Ed Hamilton and Bob Dietz, discovered the Mid‐Pacific Mountains, a 4000‐m‐high mountain range under the sea, which extends for nearly 4000 km from the Hawaiian Island chain to Wake Island. During Midpac, Russell Raitt collected 16 refraction stations showing, for the first time, the uniformity of the ocean crust and Art Maxwell and Roger Revelle discovered that oceanic heat flow was nearly the same as that measured on continents.
Eocene reorganization of the Pacific‐Farallon Spreading Center north of the Mendocino Fracture Zone
Authors: Caress David W. , Menard H. W. , Hey R. N.
Published: February 2008

During a 1984 Sea Beam and magnetics survey in the North Pacific we collected data in the area north of the Mendocino fracture zone where a change in the direction of seafloor spreading occurred between magnetic anomalies 24 and 21 and also conducted detailed surveys of the bends in the Surveyor and Mendocino fracture zones associated with this change in plate motion. We have analyzed these data combined with all available magnetics and bathymetric data from previous cruises to determine whether the ridge reorientation and reorganization occurred by way of rift propagation or rift rotation through differential asymmetric spreading. The reorganization began just after anomaly 24 to the north of the Surveyor fracture zone but did not begin between the Mendocino and Surveyor fracture zones until the time of anomaly 22, a 2.5‐m.y. delay produced by the old Mendocino transform trend constraining the Farallon plate lithosphere between the Mendocino and Surveyor transforms to move in the old spreading direction until the fracturing through of a new Mendocino transform fault in the new orientation. Most of the reorientation of spreading rift segments occurs within 2–3 m.y.; the entire reorganization requires about 7 m.y. We find direct evidence for seven propagation episodes accounting for the primary reorientation of 75% of the length of the ridge system, and nowhere is the data more consistent with rift rotation than with rift propagation as the principal means of reorientation. We conclude that the primary mechanism by which the reorganization occurred was rift propagation. Assuming that rift propagation can account for all of the ridge reorganization, we present forward models of magnetic anomaly profiles which succeed in fitting the data well. We then present a model for the tectonic evolution of the reorganization which fits the data well using 11 episodes of both northward and southward propagation.
Morphology and crustal structure of the Kane Fracture Zone Transverse Ridge
Authors: Abrams Lewis J. , Detrick Robert S. , Fox Paul J.
Published: February 2008

The Kane Fracture Zone (KFZ) transverse ridge is an anomalously shallow ridge which parallels the KFZ for over 200 km east of its intersection with the Mid‐Atlantic Ridge rift valley. Sea Beam bathymetry and gravity data have been used to determine the morphology and density structure of the ridge, and travel time data from two seismic refraction experiments have been modeled to constrain its seismic velocity structure. The transverse ridge first appears on older lithosphere opposite the eastern ridge‐transform intersection. It rises to a maximum height of about 2000 m above the bordering KFZ trough 40 km east of the intersection area. Although bathymetry data along the western limb of the KFZ are relatively sparse, a ridge of similar dimensions is not present. The KFZ transverse ridge has a decidedly flexural shape in cross section with a high, steep, south facing wall and a gently sloping northern side. The grain of the ridge‐parallel topography north of the KFZ continues undisrupted across the transverse ridge suggesting that the ridge has formed by vertical uplift of this crust. Seismic ray tracing and gravity analyses indicate that the KFZ transverse ridge is not locally compensated by an overthickened crust, but is underlain by crustal thicknesses and velocities similar to that of normal oceanic crust on either side of the KFZ. Anomalously thin crust is only present immediately beneath the KFZ trough and along the south facing wall of the transverse ridge., These constraints, and the formation of the KFZ transverse ridge on the older plate, preclude several previously proposed models for its origin including flexural bending due to differential subsidence across the fracture zone, constructional volcanism, and serpentine diapirism. Although no single mechanism provides a completely satisfactory explanation for the origin of the KFZ transverse ridge, the most likely mechanism appears to be a combination of thermal and viscodynamic forces operating near the ridge‐transform intersection.