Thomas L. Henyey

Preliminary results from a geophysical study across a modern continent-continent collisional plate boundary - the Southern Alps, New Zealand

Abstract The Southern Alps of South Island, New Zealand, is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement. A joint US-NZ geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three-dimensional model of the deformation. The measurements undertaken include active source and passive seismology, magnetotelluric and electrical studies, and petrophysics. Preliminary models for the active source seismic measurements across South Island confirm, in general terms, a thickened crust under the Southern Alps, a high-velocity lower crustal layer, and a major crustal discontinuity associated with the Alpine fault. The anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements, shear wave splitting and resistivity. The mid-crust under the Southern Alps coincides with a major electrical conductivity high, which possibly corresponds to fluid in the crust. The top lies at about 15 km, close to the base of shallow seismicity east of the Alpine fault. Offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to South Island. These data are showing complex structure, particularly off western and southeastern South Island. The complexity in structure, high-quality data and consistency in results from several techniques indicates that the South Island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular, and the continental lithosphere in general.

Significance of seismic reflections beneath a tilted exposure of deep continental crust, Tehachapi Mountains, California

The focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface, with the Tehachapi Mountains in California being the case in point. As a prime example of the lower crust, these mountains expose Cretaceous gneisses that formed 25–30 km down in the Sierra Nevada batholith and appear to be underlain by the ensimatic Rand schist. Integrated geophysical and geological studies by the CALCRUST program have produced a cross section through this post-Mid-Cretaceous structure and suggest a general model for its development. Seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the San Joaquin Basins Tejon embayment. Two south dipping reverse faults on the rim of the Tejon embayment were discovered in the reflection data and verified in the field. The faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the Rand schist. The upper and lower crusts are separated by a zone of laterally discontinuous reflectors. Reflections from the lower crust form a wedge, the base of which is a nearly flat Moho at 33 km. Regional geological relations and gravity models both suggest that the reflective zone corresponds to the Rand schist and the newly recognized faults account for its Neogene exposure. Alternatively, the reflective zone maybe part of the gneiss complex, suggesting that the schist either lies deeper or is not present under the gneisses. If the Rand schist underlies the Tehachapi Mountains and Mojave region to their south, a model for their evolution can be constructed from regional geological relations. It seems that during Late Cretaceous Laramide subduction the protolith of the schist was thrust eastward beneath the Mojave. Along this portion of the Cordilleran batholithic belt the subduction was evidently at very low angles. The bottom of the batholith was removed and replaced by a thick section of schist, fluids from which weakened the overlying batholith. This thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat-lying slabs. When emplacement of the schist ended in latest Cretaceous/earliest Paleocene, the underlying mantle rose, compensating for the extension and providing material for magmatic underplating. In the Neogene, transpression and rotation of the upper crust along the San Andreas and Garlock faults resulted in the exposure of the schist.

Tectonic Elements of the Northern Part of the Gulf of California

Results from a continuous seismic survey along closely spaced ship tracks in the northern Gulf of California are presented in terms of the tectonics of this region. Apparent vertical offsets of the most recent sediments, ranging in height from several to a few hundred meters, are associated with the central basins (Delfin and Wagner basins), indicating they are the loci of active tectonism. Structural relations inferred from mapping these features are consistent with plate tectonic concepts of the Gulf. Delfin basin represents a single, complex, northeast-southwest–trending, spreading center. Two parallel transform faults, which flank Angel de la Guarda Island and strike northward into Delfin basin from the south, and a complementary transform fault to the north represented by the Wagner basins, end at this spreading center. With the possible exception of the San Jacinto fault, no correlation of active faults was found between the northern Gulf and contiguous land areas. Interpretations of other geophysical and geological data are complicated by the high sedimentation rate in the northern Gulf, yet are generally consistent with our conclusions. Spatial and temporal characteristics of plate boundaries in the northern Gulf are probably influenced by the proximity of continental structures.

Tectonic Elements of the Central Part of the Gulf of California

Geophysical data from closely spaced ship tracks in the central Gulf of California delineate tectonic features associated with the Pacific–North America plate boundary. Three en echelon fracture zones extend southeast from Delfin Basin through Salsipuedes Channel to the northern part of San Pedro Martir Basin, southwest from the southern part of San Pedro Martir Basin to the northeastern flank of Guaymas Basin, and southeast from Guaymas Basin. Large magnetic anomalies and abundant volcanism are associated with segments of these faults. The faults are interpreted to represent a transform fault zone here designated as the “Guaymas fault zone.” Active spreading is taking place within the Guaymas and possibly the San Pedro Martir Basins, although patterns of sediment distribution within these basins preclude a model of simple stationary positions of active spreading centers for more than a few tens of thousands of years at a time.

Aspects of the crustal structure of the western Mojave Desert, California, from seismic reflection and gravity data

Seismic and gravity data taken along line 1 of the 1982 Consortium for Continental Reflection Profiling (COCORP) Mojave Desert Survey (N-S profile, ∼30 km long) have been used to characterize the upper crust north of the San Andreas fault in the western Mojave block of southern California. Consortium for Continental Reflection Profiling seismic reflection data were reprocessed to emphasize the upper 5 seconds (two-way travel time). The resultant common depth point (CDP) sections provided starting models for generating a refined geologic cross-section using a combination of ray tracing (forward modeling) and gravity interpretation. The forward modeling was used to validate the existence of faults and constrain their dips. The gravity data were used to refine the overall model, particularly in poor data areas on the CDP sections. Gravity data, taken along three nearby profiles parallel to primary line of section, were also used to determine the structural trend. Results from the first two seconds indicate the presence of a series of ENE striking reverse faults beneath the late Tertiary and Quaternary sedimentary cover of the western Mojave. The faults dip northward and offset the sediment-basement interface. The largest such feature has an apparent throw of ∼1.8 km and exhibits a subtle scarp at the Earths surface suggesting Holocene displacement. The orientation of these faults, although not an instantaneous representation of the present-day stress field, is consistent with NNW compression across the western Mojave block and WNW striking San Andreas fault, as determined from nearby focal mechanisms and in situ stress measurements. The faults also appear to be closing small sedimentary basins in the Mojave block, which may have formed during an earlier extensional phase, similar to what is happening on a much larger scale in the Los Angeles basin to the south of the San Andreas fault. Reflections between 2 and 5 s, coupled with the local geology and gravity modeling, are consistent with the presence of the Pelona/Rand schist in the subsurface beneath the western Mojave. The upper surface of the schist (i.e., Vincent/Rand thrust equivalent) rises southward toward the San Andreas fault where it is displaced vertically (up to the south) at least 5 km along the E-W trending Hitchbrook fault, such that the schist crops out between the Hitchbrook and subparallel San Andreas to the south. The same structure may exist beneath the Tehachapi mountains, with the roles of the Hitchbrook and San Andreas faults played by the north and south branches of the Garlock fault, respectively. The rising or arching of the basement toward the San Andreas fault (and toward the Garlock) is not only reflected in the geology and topography local to these faults in many places but is also generally observed on seismic reflection profiles in the vicinity of these faults in the western Mojave. Furthermore, the arching is also consistent with a strong component of fault normal compression.

Seismic reflection constraints on the structure of the crust beneath the San Bernardino Mountains, Transverse Ranges, southern California

A 30 km-long N-S seismic reflection line was shot by California Consortium for Crust Studies (CALCRUST) across the southern Mojave Desert and onto the northern flank of the San Bernardino Mountains in southern California. On the northern end of the seismic section, the reflectivity increases markedly in the midcrust at a depth corresponding to a two-way travel time of 4 to 5 s (12–15 km), suggesting a transition between nonreflecting brittle upper crust and reflecting ductile lower crust. The high reflectivity disappears at about 8 s (24 km) and may be correlated with a change in seismic velocity in the lower crust from 6.3 km/s to 6.8 km/s. A band of reflectivity between 9.5 and 10 s (27–30 km) is believed to represent the Moho. The midcrustal relectivity transition and Moho both deflect downward toward the San Bernardino Mountains uplift over the entire length of the profile. The deflection of the midcrustal transition (12°) appears greater than that of the Moho (6°), resulting in a thinning of the lower crust to the south beneath the uplift. In addition, the midcrustal transition coincides with the base of the seismogenic zone (brittle-ductile transition?) which is also dipping southward beneath the San Bernardino Mountains, while the Moho deflection is consistent with elastic flexure resulting from edge loading by the San Bernardino Mountains which have been thrust over the Mojave block. It is suggested that the thinning of the lower crust beneath the San Bernardino Mountains is a result of north directed ductile flow in response to loading by the over thickened upper crust. Since a portion of the load is transmitted through the lower crust to the Moho, the time constant for flow equilibrium must be of the order of or greater than that for the time of uplift (≥2 m.y.).

Paleomagnetic and sedimentological studies at Lake Tahoe, California-Nevada

Abstract Three closely spaced 6-m piston cores were taken in the central part of Lake Tahoe. Cores were split into two complete replicates for paleomagnetic study and the remaining sections were used for stratigraphic and mineralogical analysis. Stratigraphic correlation of the cores is based on two distinctive horizons (volcanic ash and diatomite) and upon three different sedimentological regimes dominated by (1) poorly bedded silts and muds, (2) well bedded graded units, and (3) finely laminated silts. These correlations served as the standards for the evaluation of the paleomagnetic data. Extrapolation of 14 C dates obtained in the upper sections of the Lake Tahoe sediments suggests that the lower sections of the cores may reach ages of 25,000–30,000 years B.P. X-ray, optical, Curie point, and hysteresis measurements show that magnetite is the only important magnetic mineral in the sediments and occurs in the size range of 10 μm. Hematite is essentially absent. Based on large changes in the declination and inclination of the natural remanent magnetism (NRM) within single graded layers the paleomagnetic signature is a post-depositional remanent magnetism (PDRM). This PDRM is believed to be caused by magnetic orientation during compaction. Paleomagnetic measurements show three regimes that are correlated with the stratigraphic regimes. NRM declination and inclination data show good correlation between the three cores and agree well with the correlations based on sediment character. NRM intensity variations are due largely to the variations in magnetite content and its occurrence as either single detrital grains or as inclusions within the larger silicates. Thus the variation in paleo intensity was not determined. Comparisons of Lake Tahoe data with that from Mono Lake show fair correlations of declination and inclination. The occurrence of a short-wavelength, high-amplitude event in the lower section of the Lake Tahoe cores may provide confirmation of the Mono Lake geomagnetic excursion.

Heat flow measurements in the southern portion of the Gulf of California

Abstract Twenty-five new heat flow measurements made in the Gulf of California are presented. All the values except two at the mouth of the Gulf and two in the Sal si Puedes basin are high. The values ranged from 2.0 to greater than 10 μcal/cm 2 sec (82 to > 420 mW/m 2 ) with eight values greater than 5.2 (210 mW/m 2 ). Due to high rates of sedimentation throughout the Gulf, the actual heat flow, in many cases, may be up to 25% greater than that recorded. Most of the heat flow stations are concentrated in the Farallon and Guaymas basins and show a marked increase towards the central deeps, where new crust is believed to be forming. The heat flow values in the Farallon basin show a sharp peak 10–15 km southeast of the central depression while those in the Guaymas basin peak in the depression. The heat flow profiles across the Guaymas and Farallon basins are remarkably similar to those observed on other well sedimented spreading centers such as the northern portion of the Explorer trough. Thus they may provide evidence that the crust is being created by an axially symmetric intrusion process with a major loss of heat due to hydrothermal circulation. The absence of magnetic anomalies in the Gulf has been attributed to the supposed presence of large grains in the intruded basalt. Large grains form by the slow cooling of the basalt under a layer of sediment. Prominent magnetic anomalies have been observed on the northern portion of the Explorer trough. Observational data suggest that the thermal processes at this ridge axis and the center of the Farallon basin are identical. We suggest that further careful study is needed in the Gulf before the slow cooling model is accepted as an explanation for the attenuation of the magnetic anomalies.

Pacific-North American plate interaction and Neogene volcanism in coastal California

Abstract Palinspastic and global plate tectonic reconstructions of the southwestern United States, combined with analysis of the distribution of Cenozoic igneous activity suggest that Neogene and younger volcanic rocks in the Continental Borderland and Coast, Transverse and Peninsular Ranges appear to reflect subduction of the Farallon plate south of the Mendocino Fracture Zone and overriding of the Pacific—Farallon ridge. The igneous activity of Neogene age in coastal California appears to be a continuation of fields of magmatism in adjacent areas of southeastern California, Arizona and Baja California. With the close approach of the Pacific—Farallon ridge to the North American continental margin, progressively younger, hotter and thinner Farallon plate underwent subduction beneath California, resulting in more elevated temperatures at shallower depths and closer to the trench. Consequently, the associated volcanic arc apparently shifted to the west. Volcanic activity in coastal California appears to have been most widespread shortly after overriding of the Pacific—Farallon ridge occurred— that is, at that time at which the oceanic lithosphere was thinnest, and hot, partially molten asthenosphere was closest to the subduction zone. The zone of near-coast igneous activity in California was probably limited on the north by the extension of the Mendocino Fracture Zone in the descending Farallon plate. South of the fracture zone the lithosphere was young and hot; to the north it was older and, therefore, colder. Thus, Neogene igneous activity in the Basin-Range Province was associated with more typical subduction of older lithosphere involving a deeper, down-dip locus of melting, resulting in a wider arc-trench gap. The abrupt narrowing of the Neogene arc-trench gap from north to south occurred near the present-day Garlock Fault. The role of oblique and en echelon strike-slip faulting in permitting and localizing middle Cenozoic magmatism in the areas west of the San Andreas Fault is uncertain. Discrepancies in timing of movement on the northern and southern San Andreas Fault seem to require oblique strike-slip faulting, and by association, igneous activity, in the Borderland and Transverse Ranges prior to 10 m.y. B.P. Oblique, divergent strike-slip faulting in the San Francisco Bay area does seem to be responsible for significant igneous activity between 14 m.y. B.P. and present. The Page Mill, Grizzly peak , Sonoma and Clear Lake volcanic fields occur in zones of divergence between faults of the San Andreas, Calaveras and Hayward systems. Further, the age of these volcanic rocks corresponds with the documented displacement history of the San Andreas Fault.

A simple method for the absolute measurement of thermal conductivity of drill cuttings

We present a method for absolute measurement of thermal conductivity of drill cuttings. The simplicity of the apparatus makes it suitable for nondestructive use of cuttings and for sample sizes too small to be measured with a needle probe. Because the measurement is absolute, no calibration standards are required. Samples are placed in a Plexiglas cup with a lid containing an electric heat source. The base of the cup is placed in good thermal contact with an aluminum‐block heat sink. Upward and radial heat losses are minimized with styrofoam insulation surrounding the cup. The accuracy of the method was estimated by cross‐measurement of selected samples with a well‐calibrated needle probe. Results indicate that errors in measurement are less than 5 percent for sample conductivities greater than 0.8 W/m ⋅ K if the heat loss through the styrofoam insulation is accounted for. Reproducibility is typically within 3 percent. An axisymmetric finite‐element model which simulates the temperature distribution of th...