Roy A. Johnson

Tectonic evolution of the Jurassic–Cretaceous Great Valley forearc, California: Implications for the Franciscan thrust-wedge hypothesis

Interpretation of seismic reflection data and restoration of depositional geometries of Cretaceous forearc basin strata in the northwest Great Valley of California provide important controls on structural reconstructions of the western margin of the Sacramento Valley and northern Coast Ranges. Monoclinal eastward dips of Great Valley Group strata and fault systems striking northwest-southeast, which are features proposed as evidence for a west-dipping blind Great Valley–Franciscan sole thrust and related backthrusts, instead are expressions of bedding geometry that resulted from folding of the Paskenta and related synsedimentary normal faults, depositional onlap, and a major structural-stratigraphic discontinuity. The discontinuity separates east-dipping Aptian and younger Great Valley Group strata from beds of lower Great Valley Group and Coast Range ophiolite that were deformed and erosionally or structurally truncated by mid-Cretaceous time. Dip divergence imaged between the supracrop and subcrop of the discontinuity is not unique to the ancient Great Valley forearc, but is also observed in modern forearc basins. Advocates of the Franciscan thrust-wedge model have also proposed that west-dipping, shingled patterns of seismic events imaged beneath the Sacramento Valley are imbricate thrust slices of the Great Valley Group. This hypothesis, however, is incompatible with borehole, potential-field, and seismic-refraction data that characterize the Sacramento Valley basement as ophiolitic. Seaward-dipping reflections in the ophiolitic basement of the Sacramento Valley are analogous to layering developed in the oceanic crust of volcanic rifted margins or generated along midocean ridges. Thus, late-stage tectonic mechanisms are not required to interpret a forearc that owes much of its present-day bedding architecture to processes coeval with deposition. Thickening of the Great Valley Group stratigraphic section (Valanginian–Turonian) in the hanging walls of the Paskenta, Elder Creek, and Cold Fork fault zones, combined with attenuation or complete omission of preextensional units (including the Coast Range ophiolite) and geometric evidence based on seismic reconstructions, suggest that these faults are Jurassic–Cretaceous normal faults that developed in a submarine setting. Down-structure views of the Great Valley outcrop belt simplify otherwise complex map relations and portray the Paskenta and related faults as half-graben bounding faults that accommodated significant northwestward tectonic transport of hanging-wall rocks. It is significant that these faults sole into the Coast Range fault, an enigmatic forearc structure that juxtaposes rocks of the Franciscan Complex (blueschists) with rocks of the Coast Range ophiolite and Great Valley Group that have sustained only zeolite-grade metamorphism. Discovery of Jurassic–Cretaceous crustal-scale extension in the Great Valley forearc suggests that a significant part of Coast Range fault-related attenuation developed early in the history of the subduction complex.

Mean crustal velocity: a critical parameter for interpreting crustal structure and crustal growth

Abstract Mean crustal velocity is a critical parameter for genesis of continental crystalline crust because it is a function of mean crustal composition and therefore may be used to resolve continental crustal growth in space and time. Although the best values of mean crustal velocity are determined from wide-angle reflection measurements, most studied here necessarily come from vertical averages in crustal refraction determinations. The mode of 158 values of mean crustal velocity is 6.3 km/s, a velocity which corresponds to a mean crustal composition of granodiorite to felsic quartz diorite; Archean crust may be slightly more mafic. Mean crustal velocities range from 5.8 to 7.0 km/s. The lowest values invariably are found in thermally disturbed rift zones and the highest values correspond to velocities in gabbro. Velocities in island arcs may be as low as 6.0 km/s but are typically 6.5–6.9 km/s which corresponds to andesitic composition; estimates of island arc composition are andesitic. If values of mean crustal velocity are not biased, this observation suggests that continental crust did not grow simply by addition of island arc material. Possibilities are that crust formed from fusion of island arcs and was later changed to more felsic composition by addition of material from the mantle or that the late Archean episode of major crustal growth did not involve processes similar to younger island arcs. Some crustal blocks might be changed in composition and thickness by such processes as underplating, interthrusting, necking and sub-crustal erosion. Specially designed experiments are suggested to determine this parameter so critical for understanding genesis of continental crust.

Seismic reflection evidence for seismogenic low-angle faulting in southeastern Arizona

Focal mechanisms for large (M > 6) earthquakes in extensional terranes suggest that seismogenic normal faults have dips that range from {approximately}30{degree} to {approximately}70{degree}. Geologic relations suggest that low-angle faults have accommodated large-scale upper-crustal extension. These disparate observations are often reconciled by arguments that low-angle faults move aseismically or rotate to low angles from initially high angles. Seismic reflection data from the Tucson basin in southeast Arizona image a low-angle normal fault (the Santa Rita fault) that crops out along the trend of late Quaternary fault scarps caused by large-magnitude (M {approximately} 6.7-7.6) earthquakes. Velocity-independent dip analysis from shot records of the Santa Rita fault indicates that it has a true dip of {approximately}20{degree} to a depth of at least 6km. This observation suggests that low-angle extensional faults indeed may be seismogenic and that actual mechanisms for accommodation of upper-crustal extension depend on local conditions of stress and preexisting geologic structure.

Gravity profiles across the Cheyenne Belt, a precambrian crustal suture in southeastern Wyoming

Abstract Geologic discontinuities across the Cheyenne Belt of southeastern Wyoming have led to interpretations that this boundary is a major crustal suture separating the Archaean Wyoming Province to the north from accreted Proterozoic island arc terrains to the south. Gravity profiles across the Cheyenne Belt in three Precambrian-cored Laramide uplifts show a north to south decrease in gravity values of 50–100 mgal. These data indicate that the Proterozoic crust is more felsic (less dense) and/or thicker than Archaean crust. Seismic refraction data show thicker crust (48–54 km) in Colorado than in Wyoming (37–41 km). We model the gravity profiles in two ways: 1) thicker crust to the south and a south-dipping ramp in the Moho beneath and just south of the Cheyenne Belt; 2) thicker crust to the south combined with a mid-crustal density decrease of about 0.05 g/cm 3 . Differences in crustal thickness may have originated 1700 Ma ago because: 1) the gravity gradient is spatially related to the Cheyenne Belt which has been immobile since about 1650 Ma ago; 2) the N-S gradient is perpendicular to the trend of gravity gradients associated with local Laramide uplifs and sub-perpendicular to regional long-wavelength Laramide gradients and is therefore probably not a Laramide feature. Thus, gravity data support the interpretation that the Cheyenne Belt is a Proterozoic suture zone separating terrains of different crustal structure. The gravity “signature” of the Cheyenne Belt is different from “S”-shaped gravity anomalies associated with Proterozoic sutures of the Canadian Shield which suggests fundamental differences between continent-continent and island arc-continent collisional processes.

Crustal structure and tectonic evolution of the anza rift, northern Kenya

Abstract The Anza trough is a Mesozoic rift located in northern Kenya that appears to be the failed third arm of a paleo-triple junction which allowed the separation of Madagascar from Africa during the Jurassic. The rift is oriented NW-SE and its tectonic evolution is related to that of the Mesozoic southern Sudan rift system. We analyzed seismic and gravity data from the southwestern side of the Anza rift including the Chalbi Desert to gain a better understanding of rift structure. Gravity data delineate the main rift basins as well as a small sub-basin on the southwest side of the main rift. Normal faulting evident on the NW end of a 42-km-long, NW-SE oriented Vibroseis® profile, marks the western boundary of the sub-basin. This sub-basin is offset from the trend of the main Anza trough; the western boundary may be a complex fault zone accommodating a change in direction of the main rift trend. Gravity values increase to the NW in the faulted area, suggesting shallowing of basement. A strong NW-dipping reflection from 0.5 s to almost 3 s is interpreted as a pre- to mid-Cretaceous unconformity. The configuration of the unconformity and the normal faulting strongly resembles the half-graben geometry imaged in the East African Rift. Numerous discontinuous reflections can be seen deeper in the section between 6 and 9 s, but a distinct reflection Moho cannot be interpreted with certainty. In addition to seismic and gravity data, regional geologic and well data lead us to conclude that there are probably Jurassic marine sediments in the bottom of the Anza rift.

Cenozoic tectonic evolution of the Ruby Mountains metamorphic core complex and adjacent valleys, northeastern Nevada

Seismic-reflection and borehole data along with crustal-scale refraction/reflection data provide new evidence for the Cenozoic tectonic evolution of the Ruby Mountains metamorphic core complex and Huntington, Ruby, and Lamoille valleys. Analyses of these data suggest: (1) along the western flank of the Ruby Mountains an early stage of upper-crustal extension provided accommodation space for deposition of apparently synextensional strata; (2) the oldest sedimentary rocks in the developing basin along the western flank of the Ruby Mountains are middle Eocene in age, suggesting that active upper-crustal extension and early basin formation in northeastern Nevada began at least by that time; (3) Ruby Valley is bounded by high-angle, east-dipping normal faults on the west and a relatively low-angle, west-dipping normal fault on the east; (4) crustal thicknesses beneath the eastern flank of the Ruby Mountains do not reflect local topographic relief and estimated amounts of extension; and (5) adjacent to the range, maximum thicknesses of basin-fill sedimentary rocks do not directly reflect maximum amounts of exhumation of the Ruby Mountains. Together, these observations suggest that either preexisting crustal roots (subsequently dissipated), or middle- or lower-crustal flow prior to and during extension, were involved in evolution of the core complex.

Localization of listric faults at thrust fault ramps beneath the Great Salt Lake Basin, Utah: Evidence from seismic imaging and finite element modeling

Reflection seismic data from the Great Salt Lake Basin, Utah, show that the major basin-bounding normal faults decrease in dip from ∼60° at the surface to ∼10°–20° at depths as shallow as 4–6 km. This rapid decrease in fault dip at depths shallower than the brittle-ductile transition zone in the Basin and Range Province suggests an explanation other than a gradual change of rheology and stress orientations with depth. Using a dense grid of seismic data, gravity data, borehole data, and published geologic information from islands in the lake, we constrain the position of the Sevier age Willard thrust and a footwall imbricate and show their reactivation as normal faults during Tertiary extension. In the absence of surface geologic information, we use available subsurface information from the lake to draw an analogy with the Ogden duplex in the Wasatch Front, where Cenozoic normal faulting was superimposed on an earlier Sevier age thrust regime to give rise to listric normal faults. Our interpretations are consistent with finite element modeling results, which demonstrate that extensional slip on preexisting thrust ramps leads to the formation of energetically favored synthetic normal faults, some of which may merge with the thrust ramp and obtain listric geometries. Further slip on these listric faults gives rise to secondary synthetic and antithetic faults resulting in hanging wall grabens.

Raft model of crustal extension: Evidence from seismic reflection data in southeast Arizona

An archlike zone of seismic reflectivity, interpreted as an uplifted zone of ductilely deformed middle and lower crust, is imaged below the Pinaleno Mountains core complex in southeast Arizona. The top of the reflective zone coincides with the base of an inferred mid Tertiary detachment fault beneath the Safford basin but diverges from the detachment fault as an apparent mylonite front to form a culmination at ∼1.9 s (∼4 km) beneath the Pinalenlo Mountains. From this culmination, the zone of reflectivity dips to the southwest below the Eagle Pass detachment fault and flattens at ∼4.8 s (∼13.5 km) beneath the relatively unextended upper crust of the Galiuro Mountains. Most of the reflective fabric probably formed during mid-Tertiary extension, although some of it may be older. These data suggest that mylonite zones form not only as the continuation of detachment faults into the brittle-ductile transition, but also along a regional zone of decoupling between the middle and upper crust. Highly extended and relatively unextended domains in the Basin and Range may be separated by zones of discrete (simple) shear in the upper crust, but both are rafted above regional bulk pure shear in the middle and lower crust.

An axial view of a metamorphic core complex: Crustal structure of the Whipple and Chemehuevi Mountains, southeastern California

A 135-km-long, NW-SE trending, seismic refraction/wide-angle reflection profile provides a unique along-strike view of the crustal structure of a belt of metamorphic core complexes in southeastern California: the Whipple, Chemehuevi, and Sacramento mountains metamorphic core complexes. Interpretation of the seismic data was done by two-dimensional forward modeling of travel times and amplitudes. The final model consists of (1) a thin (< 1.5 km) veneer of upper plate and fractured lower plate rocks (velocities of 1.5–5.3 km s−1) overlying a fairly homogeneous basement with velocities of 6.0 km s−1; (2) a localized, high-velocity (6.4 km s−1) body, situated directly beneath the Whipple Mountains; (3) a 6.3–6.4 km s−1 middle crust that is thickest beneath the core complexes; (4) a 6.65±0.15 km s−1 lower crust; (5) crustal thickness of 27 km with a deeper crustal root (3 km) beneath the Whipple Mountains metamorphic core complex; and (6) a Pn velocity of 8.0±0.10 km s−1. The crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust. Upper crustal velocities are higher beneath the Whipple Mountains (where velocities increase to 6.4 km s−1 at ∼5 km depth) than beneath the Chemehuevi and Sacramento mountains. In addition, midcrustal discontinuities rise 2–5 km beneath the Whipple complex compared to the other complexes. These observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the Whipple Mountains compared to rocks in the Chemehuevi and Sacramento mountains. Despite the enhanced uplift and extension in the Whipple Mountains, the crust is thicker here (30 km) than anywhere else along the Colorado River extensional corridor. This may be in part a relic of compressional and magmatic thickening during the Mesozoic. However, we suggest that inflation of the crust during Tertiary extension was the dominant mechanism. Both mantle-derived magmatism and lateral ductile inflow in the crust are proposed.

Understanding paleoclimate and human evolution through the hominin sites and paleolakes drilling project

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