摘要:

The method of least squares collocation has been used to investigate the regional recovery of the gravity field from satellite gravity gradiometer (SGG) data and gravity vector data derived from, e.g., satellite‐to‐satellite tracking (SST). We chose a region centered over the European Alps where the gravity anomalies showed a (large) standard deviation of 60 mGal. Mean gravity anomalies were used to generate SGG data at a satellite altitude of 200 km. As SGG data we used the second‐order derivatives in radial direction, across‐track, and the mixed radial/cross‐track derivative, all assumed to have an associated noise equal to 0.01 EU (Eötvös unit =EU, 1 EU=10−9s−2). As gravity data we used the three components of the gravity vector, assuming that the associated errors were uncorrelated noise with standard deviation equal to 1 mGal. The satellite data in the test area were used in combination to predict 0.5° mean gravity anomalies and geoid heights. Ground truth data were used to investigate the quality of the recovery. The difference between observed and computed values have a standard deviation equal to 22 mGal in the best case. The use of the gravity vector data gave no improvement when added to the SGG data. The use of topographic information with 5 arc min resolution (ETOPO5U) gave, after removal of severe errors, a 17‐mGal standard deviation of observed minus computed values. This is a much smaller improvement than expected but is due to errors in the

ISSN:0148-0227    

DOI:10.1029/95JB00748

年代:1995

数据来源: WILEY

摘要:

Temperatures were measured in six boreholes (∼380 m deep) in northcentral Oklahoma (36.36 °N, 96.70 °W) in May 1993. Temperatures in the upper 150 m of the boreholes appeared to be anomalously warm when the average thermal gradient was extrapolated from below 150 m depth. The average thermal gradient below 150 m depth was 37.5°C/km; the average thermal gradient from 20 to 150 m depth was 27.4°C/km. Several hundred laboratory measurements of thermal conductivity were made on cores collected at the study site; 65 specific heat capacity measurements were also made. In situ porosity was estimated from density logs and measurements of rock matrix density. Average thermal diffusivity of the upper 110 m of the stratigraphic section was estimated to be 20.3±2.0 m2/yr (6.43×10−7m2/s). Heat flow was estimated to be 52±6 mW/m2from 20 to 110 m depth and 69±7 mW/m2from 277 to 305 m depth. The observed energy imbalance and anomalously warm temperatures in the upper 150 m of the boreholes could not be explained solely by hypotheses related to topographic gradients, vegetation, heat refraction, groundwater flow, or land use changes. The only hypothesis which satisfactorily explained all of the observations was an apparent increase (1.25–1.50 ±0.5°C) in ground surface temperature (GST) related to a climatic warming starting in the middle to early 19th century or before (1835, +50, −150 years). When constraints from surface air temperatures (SATs) were used to interpret borehole temperatures, a better match to observations was obtained, suggesting that changes in SATs at the study site were tracked b

ISSN:0148-0227    

DOI:10.1029/95JB02625

年代:1995

数据来源: WILEY

摘要:

We compute crustal motions in Alaska by calculating the finite element solution for an elastic spherical shell problem. The method we use allows the finite element mesh to include faults and very long baseline interferometry (VLBI) baseline rates of change. Boundary conditions include Pacific‐North American (PA‐NA) plate motions. The solution is constrained by the oblique orientation of the Fairweather‐Queen Charlotte strike‐slip faults relative to the PA‐NA relative motion direction and the oblique orientation from normal convergence of the eastern Aleutian trench fault systems, as well as strike‐slip motion along the Denali and Totschunda fault systems. We explore the effects that a range of fault slip constraints and weighting of VLBI rates of change has on the solution. This allows us to test the motion on faults, such as the Denali fault, where there are conflicting reports on its present‐day slip rate. We find a pattern of displacements which produce fault motions generally consistent with geologic observations. The motion of the continuum has the general pattern of radial movement of crust to the NE away from the Fairweather‐Queen Charlotte fault systems in SE Alaska and Canada. This pattern of crustal motion is absorbed across the Mackenzie Mountains in NW Canada, with strike‐slip motion constrained along the Denali and Tintina fault systems. In south central Alaska and the Alaska forearc oblique convergence at the eastern Aleutian trench and the strike‐slip motion of the Denali fault system produce a counterclockwise pattern of motion which is partially absorbed along the Contact and related fault systems in southern Alaska and is partially extruded into the Bering Sea and into the forearc parallel the Aleutian trench from the Alaska Peninsula westward. Rates of motion and fault slip are small in western and northern Alaska, but the motions we compute are consistent with the senses of strike‐slip motion inferred geologically along the Kaltag, Kobuk Trench, and Thompson Creek faults and with the normal faulting observed in NW Alaska near Nome. The nonrigid behavior of our finite element solution produces patterns of motion that would not have been expected from rigid block models: strike‐slip faults can exist in a continuum that has motion mostly perpendicular to their strikes, and faults can exhibit along‐strike differences in

ISSN:0148-0227    

DOI:10.1029/95JB00237

年代:1995

数据来源: WILEY

摘要:

The west central African region is characterized by various geological features: Cretaceous rifts (Benue), Tertiary domal uplift (Adamawa volcanic uplift), Tertiary‐Recent volcanoes (Cameroon Volcanic Line or CVL), Tertiary sedimentary basins (Chad basins), and cratonic region (Congolese craton). In this study, we investigate the relationship between these tectonic features and the flexural rigidity of the lithosphere in Cameroon, in terms of effective elastic thickness (Te), by the use of the coherence function analysis. For that purpose, we use a new dataset of ∼32,000 gravity and topography points from Cameroon and the adjacent countries. TheTecontour map deduced from this study shows a good relationship between the tectonic provinces and the rigidity of the lithosphere, the minima (14–20 km) are beneath the active rifted and volcanic areas (Benue, CVL, and Adamawa), and the maxima (∼40 km) correspond to the Archean reworked unit in Congo. A spectral analysis of the gravity data is performed to determine the crust‐mantle boundary in these tectonic provinces. The crustal thickness (Tc) contour map shows a variation from 14 km to about 45 km, consistent with other geophysical data. The lower values (14–20 km) are obtained in central Cameroon on the Adamawa uplift, and the highest values are found in southern Cameroon (Archean reworked Congolese craton). ComparingTeandTcvalues shows that there is generally a positive correlation between the two parameters, with an exception in Chad where this correlation is rath

ISSN:0148-0227    

DOI:10.1029/95JB01149

年代:1995

数据来源: WILEY

摘要:

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

ISSN:0148-0227    

DOI:10.1029/95JB02279

年代:1995

数据来源: WILEY

摘要:

Continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities. Such a deformation pattern is displayed by the Borborema shear zone system in northeastern Brazil. It involves continental‐scale, curvilinear, E‐W trending right‐lateral transcurrent shear zones that branch off from a major NE trending, right‐lateral strike‐slip deformation zone formed at the northern termination of the São Francisco craton and that finally terminate in transpressive metasedimentary belts. We suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphère composed of a stiff domain (craton) and rheologically weaker domains (basins). The effect of a low‐viscosity domain on the deformation of a continental plate and, in particular, the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling. The low‐strength domain induces an enhanced strain localization, and the geometry of the shear zone is significantly modified. It is either split, forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low‐viscosity domain, or completely rotated toward the weak block. The perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation, which is controlled by its initial viscosity contrast relative to the surrounding lithosphere, its orientation relative to the convergence direction, and its distance from the shear zone initiated at the termination of the stiff block. The interaction between imposed boundary conditions (tectonic forces and plate geometry) and the internal structure of the plate may give rise to highly heterogeneous strain fields, as exhibited by the Borborema shear zone system, in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones. A heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity, with shear zones bordering nearly undeformed blocks, and in deformation regimes and vertical strains that would result in contrasting metamorphic and

ISSN:0148-0227    

DOI:10.1029/95JB02042

年代:1995

数据来源: WILEY

摘要:

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 1×1013N m−1and 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 coef

ISSN:0148-0227    

DOI:10.1029/95JB01869

年代:1995

数据来源: WILEY

摘要:

In order to predict metamorphic conditions near the slab/mantle‐wedge interface and possible melting relations beneath the volcanic arc in southern Alaska, we have constructed a two‐dimensional kinematic thermal model of the southern Alaska subduction zone that includes an abrupt change in slab dip at 40‐km depth, time‐dependent convergence rate, and variable shear heating. The numerical results indicate that the shallow‐dipping slab segment in the southern Alaska subduction zone increases slab‐surface temperatures up to 180 ° C at depths<40 km and up to 90 °C at greater depths. Shear heating may increase slab‐surface temperatures an additional 140 °C at depths<40 km and 70 °C at greater depths. Calculated pressure‐temperature (PT) paths indicate that oceanic crust subducting beneath the southern Alaska forearc is undergoing blueschist‐facies metamorphism. Calculated slab PT paths do not intersect the wet basaltic solidus for shear stresses ≤ 10% lithostatic pressure, suggesting that observed arc magmas are not derived from partial melting of subducting oceanic crust. The mantle wedge adjacent to the subducting slab lies at suosolidus conditions, and at depths<75 km, numerous hydrous phases are stable including serpentine, talc, amphibole, and chlorite. LowPwave velocities observed in the mantle wedge adjacent to the subducting slab probably reflect extensive hydration of mantle‐wedge peridotite caused by fluids liberated from the subducting oceanic crust and sediments. Calculated PT conditions in the core of the mantle wedge permit partial melting if H2O is present either as a free fluid ph

ISSN:0148-0227    

DOI:10.1029/95JB02506

年代:1995

数据来源: WILEY

摘要:

The thin‐plate method of modeling neotectonics uses isostasy and vertical integration of lithospheric strength to reduce three‐dimensional problems to two dimensions. We introduce new thin‐shell continuum elements and fault elements which satisfy the completeness and compatibility requirements, and permit extension of these methods to the lithospheres of spherical planets. Even a coarse grid of these elements with elastic rheology can reproduce low‐order toroidal free oscillations. In the program SHELLS, a realistic frictional/dislocation‐creep rheology is handled by iteration; this method converges to solutions which can be numerically tested to confirm that they satisfy both local equilibrium and the balance of torques on whole plates. Because this code can incorporate both natural plate shapes (with internal faults) and realistic rheologies, it yields models that are readily tested by their predictions of geodetic velocities, stresses, and fault s

ISSN:0148-0227    

DOI:10.1029/95JB02435

年代:1995

数据来源: WILEY

摘要:

An important but poorly known part of the earthquake hazard at near‐coastal cities of western North America from southern British Columbia to northern California is from great thrust earthquakes on the Cascadia subduction zone. Although there have been no such events in the historical record, there is good geological evidence that they have occurred in the past. The downdip landward limit of the seismogenic or seismic rupture zone on the subduction thrust fault has been estimated for the whole Cascadia margin from (1) the locked zone from dislocation modeling of current deformation data, and (2) the thermal regime, taking the downdip limit of seismic behavior on the fault to be controlled by temperature. The geodetic data include ten leveling lines, tide gauges at six locations along the coast, one high precision gravity line, seven horizontal strain arrays, and a continuously recording Global Positioning System (GPS) network. There is present uplift for most of the coast at a rate of a few millimeters per year, decreasing inland, and shortening across the coastal region at about 0.1 µstrain/yr (i.e., 10 mm/yr over a distance of 100 km). The present interseismic uplift is consistent with the great earthquake coseismic subsidence inferred from buried coastal salt marshes and other paleoseismicity data. The modeled width of the locked zone that is taken to be accumulating elastic strain averages 60 km fully locked, plus 60 km transition (90 km fully locked with no transition gives similar interseismic deformation). It is wider off the Olympic Peninsula of northern Washington and narrower off central Oregon to northern California. This unusually narrow downdip extent compared to many other subduction zones is a consequence of high temperatures associated with the young oceanic plate and the thick blanket of insulating sediments on the incoming crust. The variations in the modeled locked zone from geodetic data correspond well to variations along the margin of downdip temperatures on the fault as estimated from numerical thermal models, taking the maximum temperature for the fully locked seismogenic zone to be 350°C with a transition zone to 450°C. The temperatures on the subduction thrust fault and thus the downdip extent of the seismogenic zone depend on five local subduction parameters: (1) the age of the subducting plate, (2) the plate convergence rate, (3) the thickness of insulating sediments on the incoming crust, (4) the dip angle profile of the fault, and (5) the thermal properties of the overlying material. The landward limit to the seismogenic zone, extending little if at all beneath the coast, limits the ground motion from great subduction earthquakes at the larger Cascadia cities that lie 100–200 km inland. The narrow width also limits the earthquake size but events of magnitude well over 8 are still possible; the maximum depends on the along‐margin length. If the whole Cascadia margin seismogenic zone fails in a single event, empirical fault area versus magnitude relations give earthquakes as larg

ISSN:0148-0227    

DOI:10.1029/95JB01970

年代:1995

数据来源: WILEY