摘要:

In the region of the Los Padres‐Tehachapi geodetic network, the San Andreas fault (SAF) changes its orientation by over 30° from N40°W, close to that predicted by plate motion for a transform boundary, to N73°W. The strain orientation near the SAF is consistent with right‐lateral shear along the fault, with maximum shear rate of 0.38±0.01 μrad/yr at N63°W. In contrast, away from the SAF the strain orientations on both sides of the fault are consistent with the plate motion direction, with maximum shear rate of 0.19±0.01 μrad/yr at N44°W. The strain rate does not drop off rapidly away from the fault, and thus the area is fit by either a broad shear zone below the SAF or a single fault with a relatively deep locking depth. The fit to the line length data is poor for locking depthdless than 25 km. For d of 25 km a buried slip rate of 30 ± 6 mm/yr is estimated. We also estimated buried slip for models that included the Garlock and Big Pine faults, in addition to the SAF. Slip rates on other faults are poorly constrained by the Los Padres‐Tehachapi network. The best fitting Garlock fault model had computed left‐lateral slip rate of 11±2 mm/yr below 10 km. Buried left‐lateral slip of 15±6 mm/yr on the Big Pine fault, within the Western Transverse Ranges, provides significant reduction in line length residuals; however, deformation there may be more complicated than a single vertical fault. A subhorizontal detachment on the southern side of the SAF cannot be well constrained by these data. We investigated the location of the SAF and found that a vertical fault below the surface trace fits the data much better than either a dipping fault or a fault zone located south

ISSN:0148-0227    

DOI:10.1029/JB095iB02p01139

年代:1990

数据来源: WILEY

摘要:

The theory and mechanics of a statistical smoothing algorithm for estimating stress fields based on observed data are described. Incorporated into the algorithm are tunable parameters which allow the user to adjust the smoothness and fidelity of the fitted stress field. In addition, the user has the option of using robustness weights, or to weight the predicted stress field by assigning quality ranks to each data point. Fitted stress fields can be displayed as either a gridded map (axes of maximum, or minimum, horizontal stress are shown as short bars located at each point in an evenly spaced grid), or as a stress trajectory map. Examples of predicted stress fields for southern California and western Canada, based on borehole elongation data, are displayed as stress trajectory maps. A predicted paleostress field for the Spanish Peaks intrusion complex in Colorado, based on vertical dike orientations, is displayed as a gridded map. The smoothing algorithm is not limited to the analysis of stress orientation data. Any data consisting of a set of undirected lines measured at discrete locations are appropriate for input; possible candidates include strain data, mineral or intersection lineations, and lineaments.

ISSN:0148-0227    

DOI:10.1029/JB095iB02p01155

年代:1990

数据来源: WILEY

摘要:

The Michipicoten greenstone belt (MGB), Wawa gneiss terrane (WGT), and Kapuskasing structural zone (KSZ) in Ontario, Canada, are regarded as a 25‐km‐thick partial cross section through the crust of the Superior Province. We measured compressional wave velocities (Vp) at confining pressures up to 600 MPa for representative rock samples from these terranes.Vpfor MGB is variable (6.1–7 km/s) and depends on composition and anisotropy. Compressional wave velocities for low anisotropy quartzofeldspathic gneisses and intrusive rocks from the WGT and the KSZ show little variation over a wide range of silica content but mafic gneisses and anorthositic rocks show a wide velocity range over a narrower silica range, a trend corroborated by data for other high‐grade rocks. A model of the continental crust based on the laboratory data shows three megalayers with differing seismic characteristics. The shallowest levels (MGB) are seismically heterogeneous with high average velocities (6.6 km/s at 100 MPa) because of the dominance of metabasalts. The balance of the upper crust (WGT) is seismically homogeneous as it is dominated by a variety of isotropic quartzofeldspathic rocks with average velocities of 6.35 km/s at 100 MPa and 6.6 km/s at 600 MPa. The uppermost lower crust (KSZ) is layered, seismically heterogeneous and transversely isotropic. The average velocity at 600 MPa is 6.84 km/s. The boundary between WGT and KSZ is marked by a velocity increase of between 0.2 and 0.3 km/s, an increase similar to that commonly associated with midcrustal discontinuities (i.e., Conrad) in shield areas. Here, this boundary corresponds to the amphibolite‐granulite facies transition and to a change from a tonalitic middle crust to a lithologically heterogeneous uppermost lower crust. Refraction results show an anomalous high‐velocity zone in the upper crust under the KSZ that, on the basis of laboratory velocity data, can be correlated with KSZ lithologies. These data indicate that KSZ rocks can be traced to the west to depths as great as 20 km. Levels deeper than 20 km, not exposed at the surface in the KSZ, are characterized by velocities greater than 7.0 km/s and may be dominated by mafic and anorthositic lithologies or by more garnet‐rich rocks. The velocity model of the Superior Province crust based on geological reconstruction and laboratory velocity data is in general agreement with the velocity structure to depths of about 25 km determined for the crust below the MGB from refraction experiments. The velocity structure of this model is also generally similar to the velocity structure of

ISSN:0148-0227    

DOI:10.1029/JB095iB02p01167

年代:1990

数据来源: WILEY

摘要:

Seismograms from 52 aftershocks of the 1971 San Fernando earthquake recorded at 25 stations distributed across the San Fernando Valley are examined to identify empirical Green's functions, and characterize the dependence of their waveforms on moment, focal mechanism, source and recording site spatial variations, recording site geology, and recorded frequency band. Recording distances ranged from 3.0 to 33.0 km, hypocentral separations ranged from 0.22 to 28.4 km, and recording site separations ranged from 0.185 to 24.2 km. The recording site geologies are diorite gneiss, marine and nonmarine sediments, and alluvium of varying thicknesses. Waveforms of events with moment below about 1.5×1021dyn cm are independent of the source‐time function and are termed empirical Green's functions. Waveforms recorded at a particular station from events located within 1.0 to 3.0 km of each other, depending upon site geology, with very similar focal mechanism solutions are nearly identical for frequencies up to 10 Hz. There is no correlation to waveforms between recording sites at least 1.2 km apart, and waveforms are clearly distinctive for two sites 0.185 km apart. The geologic conditions of the recording site dominate the character of empirical Green's functions. Even for source spatial separations of up to 20.0 km, the empirical Green's functions at a particular site are consistent in frequency content, amplification, and energy distribution. Therefore, it is shown that empirical Green's functions can be used to obtain site response functions. The observations of empirical Green's functions are used as a basis for developing the theory for using empirical Green's functions in deconvolution for source pulses and synthesis of seismograms of larger earthquak

ISSN:0148-0227    

DOI:10.1029/JB095iB02p01187

年代:1990

数据来源: WILEY

摘要:

The boundary element‐image method is proposed as a method for generating synthetic seismograms in a system of piecewise homogeneous layers. In contrast with many other methods (such as finite difference) in which the boundary conditions are represented simply by a change of material properties, the present approach uses the boundary conditions explicitly in order to develop a system of integral equations. As in the indirect boundary element method, the scattered seismic waves are presumed to result from the action of fictitious sources. However, as in the image method, these fictitious sources are located outside of the domain through which the scattered waves propagate. This combination of the image and boundary element methods thereby avoids introducing additional singularities (in the form of fictitious sources) into the wave equation for the scattered waves. Although this method can be formulated in a general way, specific computational advantages occur when the basis and testing functions are chosen as harmonically related complex sinusoids. Physically, this corresponds to a plane wave expansion of the scattered waves. Computationally, when considering piecewise linear interfaces, this results in a (possibly) large system of simultaneous algebraic equations having matrix elements which can be calculated analytically. This method is applied to the reflection of seismic waves from a V‐shaped reflector subjected to two different incident waves: a plane wave and a cylindrical wave created by a line source. The synthetic seismograms for both cases are presented (in the form of time sections) and then processed in an attempt to reconstruct the geometry of the reflector (in the form of a depth secti

ISSN:0148-0227    

DOI:10.1029/JB095iB02p01215

年代:1990

数据来源: WILEY

摘要:

Heat flow studies were conducted in January–February 1987, off the Atlantic Coast of Mexico on board the R/VAkademik Nikolai Strakhov. Two areas were surveyed, one transecting the Salt Dome Province and the Campeche Canyon, in the Gulf of Mexico, and the other, on the eastern flank of the Yucatan Peninsula. Conductive heat flow through the bottom sediments was determined as the product of vertical temperature gradient and in situ thermal conductivity, measured with a thermal probe using a multithermistor array and real‐time processing capabilities. Forward two‐dimensional modeling allows us to estimate heat flow variations at both sites from local disturbances and to obtain average heat flow values of 51 mW/m2for the transect within the Gulf of Mexico and 38 and 69 mW/m2for two basins within the Yucatan area. Sea bottom relief has a predominant effect over other environmental factors in the scatter of heat flow determination in the Gulf of M

ISSN:0148-0227    

DOI:10.1029/JB095iB02p01223

年代:1990

数据来源: WILEY

摘要:

The Alula‐Fartak transform fault is associated with a broad, steep‐sided valley running NNE‐SSW across the mouth of the Gulf of Aden. A discussion of the tectonics of the fracture zone is based principally on GLORIA side scan sonar and gravity data. The transform valley is 180 km long and 25 km across, has 3.5 km of relief, and is bounded by transverse ridges. The northern two‐thirds is highly asymmetrical in cross section. The eastern wall dips 38° and appears to be a single scarp exposing a section of oceanic crust 3100 m thick. The western wall is much less steep and accommodates several normal faults oriented at about 15° to its general trend. A very fine, long sonar target running along part of the valley delineates the main active strike‐slip fault. The sonar data also reveal many fine faultlike targets between the main walls, running approximately at right angles to the trend of the valley, which may be anti‐Riedel shears. A 50‐km‐long, 7.5‐km‐wide, and 1.5‐km‐high median ridge runs along the southern third of the transform valley. Gravity data show that the valley is underlain by a significant mass deficiency thought to be a large serpentinite intrusion. The median ridge is suggested to be the surface manifestation of a large serpentinite diapir. The ridges flanking the transform valley are above a level compatible with isostatic equilibrium and are interpreted as flexural features raised in response to the upward loads imposed on the lithosphere by the fracture zone valley and its underlying mass deficiency. Very shallow crust is located close to the ridge‐transform intersections. This is considered to result from static flexure induced by the negative load of the transform valley superimposed on dynamic compensation of the spreading center valley. Evidence of a c. 3.5 m.y. B.P. change in spreading direction suggests the transform has experienced a recent component of opening, accounting for sever

ISSN:0148-0227    

DOI:10.1029/JB095iB02p01239

年代:1990

数据来源: WILEY

摘要:

A mathematical model is presented for the cooling of an active flow with two separate thermal components. One component is a crust that cools by radiation and generally thickens with time. The other component is an inner core that is vertically isothermal and partially exposed at the top surface, where heat is lost by radiation. This model provides a more realistic description of active lava flows than the existing models that assume thermal homogeneity in each vertical cross section perpendicular to the direction of flow advance. Characteristic time scales for heat loss from the core are comparable to typical durations for the emplacement of single‐lobed basaltic flows. The two‐component model predicts fractional areas of exposed core between 0.001 and 0.1, consistent with several types of observations of lava flows. Predicted temperature changes for the crust and core, as a function of distance from the vent, also comparable favorably to the limited observations available for active flows. Depending on eruption duration, fraction of exposed core, flow thickness, and initial eruption temperature, the thermal interaction between the crust and the core can have a significant effect on the core temperature, the growth of the crust, and various averages of thermal losses by radiation. This analysis suggests that flow dimensions may be strongly influenced by thermal dynamics in the core if emplacement duration is long, the fraction of exposed core is maintained at a high level, or the initial eruption temperature is

ISSN:0148-0227    

DOI:10.1029/JB095iB02p01255

年代:1990

数据来源: WILEY

摘要:

All subaerial lavas at Mauna Kea Volcano, Hawaii, belong to the postshield stage of volcano construction. This stage formed as the magma supply rate from the mantle decreased. It can be divided into two substages: basaltic (∼240–70 ka) and hawaiitic (∼66–4 ka). The basaltic substage (Hamakua Volcanics) contains a diverse array of lava types including picrites, ankaramites, alkalic and tholeiitic basalt, and high Fe‐Ti basalt. In contrast, the hawaiitic substage (Laupahoehoe Volcanics) contains only evolved alkalic lavas, hawaiite, and mugearite; basalts are absent. Sr and Nd isotopic ratios for lavas from the two substages are similar, but there is a distinct compositional gap between the substages. Lavas of the hawaiitic substage can not be related to the older basalts by shallow pressure fractionation, but they may be related to these basalts by fractionation at moderate pressures of a clinopyroxene‐dominated assemblage. We conclude that the petrogenetic processes forming the postshield lavas at Mauna Kea and other Hawaiian volcanoes reflect movement of the volcano away from the hotspot. Specifically, we postulate the following sequence of events for postshield volcanism at Mauna Kea: (1) As the magma supply rate from the mantle decreased, major changes in volcanic plumbing occurred. The shallow magma chamber present during shield construction cooled and crystallized, and the fractures enabling magma ascent to the magma chamber closed. (2) Therefore subsequent basaltic magma ascending from the mantle stagnated within the lower crust, or perhaps at the crust‐mantle boundary. Eruptions of basaltic magma ceased. (3) Continued volcanism was inhibited until basaltic magma in the lower crust cooled sufficiently to create relatively low‐density, residual hawaiitic melts. Minor assimilation of MORB‐related wall rocks, reflected by a trend toward lower206Pb/204Pb in evolved postshield lavas, may have occurred at this time. A compositional gap developed because magma ascent was not possible until a low‐density hawaiitic melt could escape from a largely crystalline mush. Eruption of this melt created aphyric hawaiite and mugearite lavas which incorporated cumulate gabbro, wherlite, and dunite xeno

ISSN:0148-0227    

DOI:10.1029/JB095iB02p01271

年代:1990

数据来源: WILEY

摘要:

An analysis is presented for convective stress coupling to an elastic lithosphere as it applies to tectonic deformation on Venus. Theoretical solutions are introduced for the response of a mathematically thick elastic plate overlying a Newtonian viscous medium with an exponential depth dependence of viscosity. A Green's function solution is obtained for the viscous flow driven by a harmonic density distribution at a specified depth. It is shown that during the transient phase of the solution, the field quantities (stress, velocity, displacement) in both media depend upon the rheology of both media, whereas in steady state the solutions become uncoupled. Due to this uncoupling, stresses obtained from a purely viscous solution can be applied as boundary conditions at the base of an isolated elastic plate. I show for such a plate that (1) shear stress imparted at the base of the lower boundary can induce large in‐plane stretching mode forces and (2) these forces can be estimated directly from free‐air gravity anomalies without detailed knowledge of the density distribution in the underlying flow. Free‐air gravity anomalies for Venus imply in‐plane forces of about 2–3×1012N/m. In‐plane forces of this magnitude are capable of extensive disruption of the Venusian lithosphere. To demonstrate this, an elastic‐plastic analysis is carried out for the deformation of a model Venus lithosphere; the calculation is based on a lithospheric strength envelope and uses gravity and topography as a guide to the magnitude of stresses imparted by convective flow. The results predict that dynamic uplift of Venusian topography must be accompanied by extensive brittle failure and viscous flow in the lithosphere. Geoid anomalies and geoid‐to‐topography ratios for Venusian topographic features are large compared to values for thermal swells in Earth's ocean basins; this suggests that dynamic support of topography must be an important process on Venus. Further, a low‐viscosity zone is probably not present beneath the Venusian lithosphere. A comparative analysis suggests that tectonic deformation of the lithosphere associated with dynamically supported topography on

ISSN:0148-0227    

DOI:10.1029/JB095iB02p01301

年代:1990

数据来源: WILEY