Geology topics

J. McCarthy

Publications and source records attributed to J. McCarthy.

11 recordsLinked to original sources

USGS2012

Evidence for an upper mantle low velocity zone beneath the southern Basin and Range-Colorado Plateau transition zone

A 370-km-long seismic refraction/wide-angle reflection profile recorded during the Pacific to Arizona Crustal Experiment (PACE) detected an upper mantle P-wave low-velocity zone (LVZ) in the depth range 40 to 55 km beneath the Basin and Range in southern Arizona. Interpretation of seismic data places constraints on the sub-crustal lithosphere of the southern Basin and Range Province, which is important in light of the active tectonics of the region and the unknown role of the sub-crustal lithosphere in the development of the western United States. Forward travel time and synthetic seismogram techniques are used to model this shallow upper mantle LVZ. Modeling results show that the LVZ is defined by a 5% velocity decrease relative to a P n velocity of 7.95 km s −1 , suggesting either a ∼3–5% mafic partial melt or high-temperature, sub-solidus peridotite.

Geophysical Research Letters↗

USGS2012

Anatomy of a metamorphic core complex: seismic refraction/wide-angle reflection profiling in southeastern California and western Arizona

The metamorphic core complex belt in southeastern California and western Arizona is a NW-SE trending zone of unusually large Tertiary extension and uplift. Midcrustal rocks exposed in this belt raise questions about the crustal thickness, crustal structure, and the tectonic evolution of the region. Three seismic refraction/wide-angle reflection profiles, acquired and analyzed as a part of the U.S. Geological Survey's Pacific to Arizona Crustal Experiment, were collected to address these issues. The results presented here, which focus on the Whipple and Buckskin-Rawhide mountains, yield a consistent three-dimensional image of this part of the metamorphic core complex belt. The seismic refraction/wide-angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles. These phases include refractions from the near-surface and crystalline basement, reflections from boundaries in the middle and lower crust, and reflections and refractions from the upper mantle. The final model consists of a thin veneer (<2 km) of upper plate and fractured lower plate rocks (1.5–5.5 km s−1 ) overlying a fairly homogeneous basement (∼6.0 km s −1 ) and a localized high-velocity (6.4 km s −1 ) body situated beneath the western Whipple Mountains. A prominent midcrustal reflection is identified beneath the Whipple and Buckskin-Rawhide mountains between 10 and 20 km depth. This reflector has an arch-like shape and is centered beneath, or just west of, the metamorphic core complex belt. This event is underlain by a weaker, approximately subhorizontal reflection at 24 km depth. Together, these two discontinuities define a lens-shaped midcrustal layer with a velocity of 6.35–6.5 km s −1 . The apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift (∼10 km) defined by surface geologic mapping and petrologic barometry studies. The layer thins to the northeast and is absent in the Transition Zone. The 6.35–6.5 km s −1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks. This midcrustal layer is underlain by a higher-velocity lower crustal layer that is modeled as only 3–6 km thick beneath the metamorphic core complex belt and regions to the southwest. To the northeast, however, this layer thickens to 8–10 km as the midcrustal layer pinches out above it. The velocity of the lower crust is constrained by traveltime modeling and is 6.6±0.15 kms −1 beneath the western Transition Zone and the metamorphic core complex belt; higher velocities may be present farther to the southwest where the layer is thin. The velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust. Instead, we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle-derived intrusions. The crust-mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26–28 km or, in the case of the Whipple Mountains, actually deepens; a 3-km crustal root is modeled. This lack of upward doming of the Moho, together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension-related structures in the middle and lower crust, suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension, as one would expect if extension occurred along crust-penetrating shear zones (Wernicke, 1981; Wernicke et al., 1985). Instead, domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust.

Journal of Geophysical Research↗

USGS2003

Structure and mechanics of the Hayward-Rodgers Creek Fault step-over, San Francisco Bay, California

A dilatational step - over between the right-lateral Hayward and Rodgers Creek faults lies beneath San Pablo Bay in the San Francisco Bay area. A key seismic hazard issue is whether an earthquake on one of the faults could rupture through the step - over , enhancing its maximum possible magnitude. If ruptures are terminated at the step - over , then another important issue is how strain transfers through the step . We developed a combined seismic reflection and refraction cross section across south San Pablo Bay and found that the Hayward and Rodgers Creek faults converge to within 4 km of one another near the surface, about 2 km closer than previously thought. Interpretation of potential field data from San Pablo Bay indicated a low likelihood of strike-slip transfer faults connecting the Hayward and Rodgers Creek faults. Numerical simulations suggest that it is possible for a rupture to jump across a 4-km fault gap, although special stressing conditions are probably required (e.g., Harris and Day, 1993, 1999). Slip on the Hayward and Rodgers Creek faults is building an extensional pull-apart basin that could contain hazardous normal faults. We investigated strain in the pull-apart using a finite-element model and calculated a ∼0.02-MPa/yr differential stressing rate in the step - over on a least-principal-stress orientation nearly parallel to the strike-slip faults where they overlap. A 1- to 10-MPa stress-drop extensional earthquake is expected on normal faults oriented perpendicular to the strike-slip faults every 50-500 years. The last such earthquake might have been the 1898 M 6.0-6.5 shock in San Pablo Bay that apparently produced a small tsunami. Historical hydrographic surveys gathered before and after 1898 indicate abnormal subsidence of the bay floor within the step - over , possibly related to the earthquake. We used a hydrodynamic model to show that a dip-slip mechanism in north San Pablo Bay is the most likely 1898 rupture scenario to have caused the tsunami. While we find no strike-slip transfer fault between the Hayward and Rodgers Creek faults, a normal- fault link could enable through-going segmented rupture of both strike-slip faults and may pose an independent hazard of M ∼6 earthquakes like the 1898 event.

California↗

USGS1996

Crustal structure of the Colorado Plateau, Arizona: Application of new long-offset seismic data analysis techniques

The Colorado Plateau is a large crustal block in the southwestern United States that has been raised intact nearly 2 km above sea level since Cretaceous marine sediments were deposited on its surface. Controversy exists concerning the thickness of the plateau crust and the source of its buoyancy. Interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick. A thick crust could support the observed topography of the Colorado Plateau isostatically, while a thinner crust would indicate the presence of an underlying low-density mantle. This paper reports results on long-offset seismic data collected during the 1989 segment of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that extended from the Transition Zone into the Colorado Plateau in northwest Arizona. We apply two new methods to analyze long-offset data that employ finite difference travel time calculations: (1) a first-arrival time inverter to find upper crustal velocity structure and (2) a forward-modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals. We find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern Basin and Range province to about 42 km beneath the northern Transition Zone and southern Colorado Plateau margin. We observe some crustal thinning (to ∼37 km thick) and slightly higher lower crustal velocities farther inboard; beneath the Kaibab uplift on the north rim of the Grand Canyon the crust thickens to a maximum of 48 km. We observe a nonuniform crustal thickness beneath the Colorado Plateau that varies by ∼15% and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations. Crustal compositions (as inferred from seismic velocities) appear to be the same beneath the Colorado Plateau as those in the Basin and Range province to the southwest, implying that the plateau crust represents an unextended version of the Basin and Range. Some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the Proterozoic Era.

Journal of Geophysical Research B: Solid Earth↗

USGS1996

Crustal and upper mantle velocity structure of the Salton Trough, southeast California

This paper presents data and modelling results from a crustal and upper mantle wide-angle seismic transect across the Salton Trough region in southeast California. The Salton Trough is a unique part of the Basin and Range province where mid-ocean ridge/transform spreading in the Gulf of California has evolved northward into the continent. In 1992, the U.S. Geological Survey (USGS) conducted the final leg of the Pacific to Arizona Crustal Experiment (PACE). Two perpendicular models of the crust and upper mantle were fit to wide-angle reflection and refraction travel times, seismic amplitudes, and Bouguer gravity anomalies. The first profile crossed the Salton Trough from the southwest to the northeast, and the second was a strike line that paralleled the Salton Sea along its western edge. We found thin crust (∼21–22 km thick) beneath the axis of the Salton Trough (Imperial Valley) and locally thicker crust (∼27 km) beneath the Chocolate Mountains to the northeast. We modelled a slight thinning of the crust further to the northeast beneath the Colorado River (∼24 km) and subsequent thickening beneath the metamorphic core complex belt northeast of the Colorado River. There is a deep, apparently young basin (∼5–6 km unmetamorphosed sediments) beneath the Imperial Valley and a shallower (∼2–3 km) basin beneath the Colorado River. A regional 6.9-km/s layer (between ∼15-km depth and the Moho) underlies the Salton Trough as well as the Chocolate Mountains where it pinches out at the Moho. This lower crustal layer is spatially associated with a low-velocity (7.6–7.7 km/s) upper mantle. We found that our crustal model is locally compatible with the previously suggested notion that the crust of the Salton Trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust. However, we observe an apparently magmatically emplaced lower crust to the northeast, outside of the Salton Trough, and propose that this layer in part predates Salton Trough rifting. It may also in part result from migration of magmatic spreading centers associated with the southern San Andreas fault system. These spreading centers may have existed east of their current locations in the past and may have influenced the lower crust and upper mantle to the east of the current Salton Trough.

California↗

USGS1995

The active southwest margin of the Colorado Plateau: Uplift of mantle origin

During Cenozoic time, the Colorado Plateau was raised about 2 km above sea level. The most-recent and best-documented uplift of the plateau (∼1 km) has been concentrated at its southwest margin between 6 and 1 Ma, whereas the eastern Colorado Plateau may have been at high elevations since Eocene time. To better understand the recent tectonic activity at the southwest margin of the Colorado Plateau, we compile detailed crustal thickness and density information from seismic and gravity data for a region that includes northwest Arizona and the southern tip of Nevada. This information is used to isolate the mantle contribution to uplift. We find that there is relatively low density mantle underlying the southern margin of the plateau in northwest Arizona, which could result from about 60–80 km of thinning of the dense mantle lithosphere combined with about 100 °C of heating through a 100-km-thick mantle layer. The available estimates from earthquake-source seismology in or near the study area are compatible with this estimate of lithospheric thinning. We speculate that uplift may result from subduction-related thinning of the continental lithosphere.

Geological Society of America Bulletin↗

USGS1994

Seismic evidence for a lower-crustal detachment beneath San Francisco Bay, California

Results from the San Francisco Bay area seismic imaging experiment (BASIX) reveal the presence of a prominent lower crustal reflector at a depth of ∼15 kilometers beneath San Francisco and San Pablo bays. Velocity analyses indicate that this reflector marks the base of Franciscan assemblage rocks and the top of a mafic lower crust. Because this compositional contrast would imply a strong rheological contrast, this interface may correspond to a lower crustal detachment surface. If so, it may represent a subhorizontal segment of the North America and Pacific plate boundary proposed by earlier thermo-mechanical and geological models.

California↗

USGS1994

Insights into the kinematic Cenozoic evolution of the Basin and Range-Colorado Plateau transition from coincident seismic refraction and reflection data

Estimates of surface extension in the southern Basin and Range province and transition into the Colorado Plateau range from a few percent to several hundred percent locally, yet the crustal thickness varies perhaps only 10-15 km across these provinces. Within the southern Basin and Range and the metamorphic core complex belt, extremely extended crust is directly juxtaposed against equally thick (or thinner) crust that underwent far milder extension. Unless preextension crustal thickness varied dramatically over a short distance, the crust must have maintained its thickness during extension, through mechanisms that involve crustal flow and magmatism. We employ a 300-km-long profile of seismic refraction and coincident vertical-incidence reflection data to investigate the geophysical signature of these processes from the extended southern Basin and Range province to the unextended Colorado Plateau. By integrating the seismic velocity with the pattern of reflectivity along the profile, we estimate the amounts of Tertiary magmatism and flow that have occurred. We estimate an upper bound of 8 km of mafic material intruded beneath the metamorphic core complex belt and 4 and 5 km of intruded material beneath the Transition Zone and southern Basin and Range province, respectively. We emphasize that this 8-km estimate is strictly an upper bound, and that the actual amount of magmatism was probably less (3 to 4 km). We further speculate that several kilo-meters of silicic rock was added to the metamorphic core complex belt via ductile flow. As suggested by numerous numerical models of crustal extension, we conclude that a mobile, felsic midcrustal layer accommodated most of this crustal flow. This ductile midcrustal layer appears to be thickest beneath the most extended terranes and thinnest beneath the less extended Transition Zone and Colorado Plateau. In contrast, the lowermost crust appears to have thinned passively in an amount that corresponds more directly to the regional surface extension.

Geological Society of America Bulletin↗

USGS1988

Seismic imaging of extended crust with emphasis on the western United States

Understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust. This is particularly true in areas where the last tectonic event was extensional, such as the Basin and Range province of the western United States and much of western Europe. In these regions, a characteristic reflective pattern has emerged, whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective. In the metamorphic-core-complex belt in the western United States, where extension can be as much as an order of magnitude greater than in the more classic continental rift zones, the lower crustal reflectivity thickens and rises, yielding a picture of a crust that is reflective throughout. Synthetic seismic studies have documented that the reflectivity in these regions can be modeled by numerous laminae tens of meters thick and hundreds of meters across, characterized by inter-layered high and low velocities. Two geologic factors are interpreted as contributing to this layered character: ductile strain, responding to stress in the thermally weakened middle and lower crust, and intrusive layering, corresponding to injection of subhorizontal sheets of mantle-derived magmas. These two processes yield a variety of geologic structures, including transposed compositional layering, mylonitic ductile shear zones, and intrusive mafic sheets, all of which occur at the proper scales to cause the prominent reflectivity observed. If metamorphic core complexes are representative of extended continental crust world-wide, then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world.

Geological Society of America Bulletin↗

USGS1988

Relic magma chamber structures preserved within the Mesozoic North Atlantic crust?

The North Atlantic Transect seismic reflection data, collected southwest of Bermuda, have been reinterpreted following post-stack migration and reveal two major intracrustal reflections. The shallower of these two events, located ∼1 s below the igneous basement, is a subhorizontal, undulating surface that in some places is continuous for as much as 10 km. On the basis of its position within the section and its laterally discontinuous nature, we believe that this upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro. A second set of lower crustal reflections, dipping ∼20°-40° eastward, is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection Moho. Several lines of evidence argue against these features being either artifacts or out-of-the-plane events. Instead, their presence may be ascribed either to crustal-penetrating fault zones or to mafic-ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo-spreading center. Because of the laminated character of these events and their typical occurrence within 1.0 to 1.5 s of the reflection Moho, we prefer a compositional versus a structural interpretation for their origin. The gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred; although the interpretation that the crust gradually thins toward fracture zones may still apply in a few localities, significant departures are recognized elsewhere. Similarly, the improved image on the migrated profile documents an increase in complexity across the localized region directly surrounding the Blake Spur fracture zone. An interpretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal-thinning model of Mutter and others.

Geological Society of America Bulletin↗

USGS1984

Speculations on the petroleum geology of the accretionary body: an example from the central Aleutians

In the 300 km wide Adak-Amlia sector of the central Aleutian Trench ??? 36 000 km3 of offscraped trench fill makes up the wedge-shaped mass of the Aleutian accretionary body. Within this wedge, seismic reflection profiles reveal an abundance of potential hydrocarbon-trapping structures. These structures include antiforms, thrust and normal faults, and stratigraphic pinchouts. Maximum closure on these features is 2 km. In addition, the silt and possibly sand size sediment within the offscraped turbidite deposits, and the porous diatomaceous pelagic deposits interbedded with and at the base of the wedge, may define suitable reservoirs for the entrapment of hydrocarbons. Potential seals for these reservoirs include diagenetically-altered and -produced siliceous and carbonate sediment. The organic carbon input into the central Aleutian Trench, based on carbon analyses of DSDP Legs 18 and 19 core samples, suggests that the average organic carbon content within the accretionary body is approximately 0.3-0.6%. Heat flow across the Aleutian Terrace indicates that at present the oil generation window lies at a depth of 3-6.5 km. At depths of 8 km (which corresponds to the maximum depth the offscraped sediment has been seismically resolved beneath the lower trench slope), the probable high (170-180??C) temperatures prohibit all but gas generation. The dewatering of trench sediment and subducted oceanic crust should produce an abundance of fluids circulating within the accretionary body. These fluids and gases can conduct hydrocarbons to any of the abundant trapping geometries or be lost from the system through sea floor seepage. In the Aleutian accretionary body all the conditions necessary for the formation of oil and gas deposits exist. The size and ultimate preservation of these deposits, however, are dependent on the deformational history of the prism both during accretion and after the accretion process has been superceded by subsequent tectonic regimes. ?? 1984.

Marine and Petroleum Geology↗