Laura Serpa

Cenozoic and Mesozoic structure of the eastern Basin and Range province, Utah, from COCORP seismic-reflection data

COCORP seismic-reflection data collected from the eastern Basin and Range in west-central Utah provide information on Cenozoic extensional tectonics, Mesozoic thrusting, and their interrelationships. Those data show a series of remarkably continuous, low-angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15–20 km. Over that distance, none of these events are significantly cut by any high-angle normal faults. A major detachment beneath the Sevier Desert can be traced from a surface zone of normal faulting to a depth of 12–15 km, with a regional apparent westward dip of 12°. Tentative correlation of upper- and lower-plate events suggests 30–60 km of extensional displacement on this detachment. Whether this structure is a reactivated Mesozoic thrust is uncertain. West-steepening splays off the end of the detachment reach depths of 20 km and may represent a major Mesozoic ramp or zones of distributed ductile shearing during extension. Some events are interpreted to be Mesozoic thrusts, of which at least one (beneath the House Range) has been reactivated during the Cenozoic. The Snake Range decollement dips gently east and has a sense of Cenozoic displacement opposite to that of other Cenozoic detachments farther east. Deep events are most numerous beneath the east side of the Sevier Desert where they occur to depths of 30 km, at the top of or perhaps partly within the anomalously low velocity upper mantle.

Deformation during terrane accretion in the Saint Elias orogen, Alaska

The Saint Elias orogen of southern Alaska and adjacent Canada is a complex belt of mountains formed by collision and accretion of the Yakutat terrane into the transition zone from transform faulting to subduction in the northeast Pacific. The orogen is an active analog for tectonic processes that formed much of the North American Cordillera, and is also an important site to study (1) the relationships between climate and tectonics, and (2) structures that generate large- to great-magnitude earthquakes. The Yakutat terrane is a fragment of the North American plate margin that is partly subducted beneath and partly accreted to the continental margin of southern Alaska. Interaction between the Yakutat terrane and the North American and Pacific plates causes significant differences in the style of deformation within the terrane. Deformation in the eastern part of the terrane is caused by strike-slip faulting along the Fairweather transform fault and by reverse faulting beneath the coastal mountains, but there is little deformation immediately offshore. The central part of the orogen is marked by thrusting of the Yakutat terrane beneath the North American plate along the Chugach–Saint Elias fault and development of a wide, thin-skinned fold-and-thrust belt. Strike-slip faulting in this segment may be localized in the hanging wall of the Chugach–Saint Elias fault, or dissipated by thrust faulting beneath a north-northeast–trending belt of active deformation that cuts obliquely across the eastern end of the fold-and-thrust belt. Superimposed folds with complex shapes and plunging hinge lines accommodate horizontal shortening and extension in the western part of the orogen, where the sedimentary cover of the Yakutat terrane is accreted into the upper plate of the Aleutian subduction zone. These three structural segments are separated by transverse tectonic boundaries that cut across the Yakutat terrane and also coincide with the courses of piedmont glaciers that flow from the topographic backbone of the Saint Elias Mountains onto the coastal plain. The Malaspina fault–Pamplona structural zone separates the eastern and central parts of the orogen and is marked by reverse faulting and folding. Onshore, most of this boundary is buried beneath the western or “Agassiz” lobe of the Malaspina piedmont glacier. The boundary between the central fold-and-thrust belt and western zone of superimposed folding lies beneath the middle and lower course of the Bering piedmont glacier.

Tectonic processes during oblique collision: Insights from the St. Elias orogen, northern North American Cordillera

[1]xa0Oblique convergence in the St. Elias orogen of southern Alaska and northwestern Canada has constructed the worlds highest coastal mountain range and is the principal driver constructing all of the high topography in northern North America. The orogen originated when the Yakutat terrane was excised from the Cordilleran margin and was transported along margin-parallel strike-slip faults into the subduction-transform transition at the eastern end of the Aleutian trench. We examine the last 3 m.y. of this collision through an analysis of Euler poles for motion of the Yakutat microplate with respect to North America and the Pacific. This analysis indicates a Yakutat-Pacific pole near the present southern triple junction of the microplate and predicts convergence to dextral-oblique convergence across the offshore Transition fault, onland structures adjacent to the Yakutat foreland, or both, with plate speeds increasing from 10 to 30 mm/yr from southeast to northwest. Reconstructions based on these poles show that NNW transport of the collided block into the NE trending subduction zone forced contraction of EW line elements as the collided block was driven into the subduction-transform transition. This suggests the collided block was constricted as it was driven into the transition. Constriction provides an explanation for observed vertical axis refolding of both earlier formed fold-thrust systems and the collisional suture at the top of the fold-thrust stack. We also suggest that this motion was partially accommodated by lateral extrusion of the western portion of the orogen toward the Aleutian trench. Important questions remain regarding which structures accommodated parts of this motion. The Transition fault may have accommodated much of the Yakutat-Pacific convergence on the basis of our analysis and previous interpretations of GPS-based geodetic data. Nonetheless, it is locally overlapped by up to 800 m of undeformed sediment, yet elsewhere shows evidence of young deformation. This contradiction could be produced if the overlapping sediments are too young to have accumulated significant deformation, or GPS motions may be deflected by transient strains or strains from poorly understood fault interactions. In either case, more data are needed to resolve the paradox.

Crustal extension and magmatic processes: COCORP profiles from Death Valley and the Rio Grande rift

New crustal-scale information on the interaction between normal faulting and magmatic activity is provided by recent COCORP deep seismic reflection profiles in Death Valley, California, and by reprocessing of the COCORP data from the Socorro area of the Rio Grande rift, New Mexico. The most striking feature on the seismic sections from these areas is a prominent, subhorizontal series of reflectors at mid-crustal depth. Previous studies have suggested that thin tabular magma bodies lie within the mid-crustal reflective zones. In addition, because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the COCORP data, these mid-crustal horizons are here inferred to be detachments or zones of tectonic decoupling. Upper-crustal Cenozoic faults do not appear to penetrate deeper than 15 km in Death Valley and 13 km in the Rio Grande rift. In Death Valley, these faults are relatively planar, with moderate dips (20° to 35°), and appear to bound large basement rocks. One such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central Death Valley to the surface location of a 690,000-yr-old basaltic cinder cone. Listric and low- to moderate-angle normal faults are evident on the reprocessed New Mexico data and constitute the structural component of upper-crustal extension. In particular, a listric master fault traceable to depths as great as 13 km is inferred to underlie the Albuquerque basin. Unlike the Death Valley data, no faults are observed to merge with the Socorro magma body per se. Rather, subhorizontal to moderately west-dipping packages of reflections are imaged between the base of the faulted upper crust (13 km depth) and the mid-crustal magma body (about 20 km depth). The middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion. Events seen in the middle and lower crust are generally subhorizontal, and prominent layering is observed. A band of reflections attributed to the crust-mantle boundary is evident on most seismic sections. The upper mantle appears seismically transparent. On some of the profiles, the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections, at least one of which is a layer of magma. This association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes.

Death Valley bright spot: A midcrustal magma body in the southern Great Basin, California?

A previously unrecognized midcrustal magma body may have been detected by COCORP deep seismic reflection profiles in the Death Valley region of the southern Great Basin. High-amplitude, relatively broad-band reflections at 6 s (15 km) are attributed to partially molten material within a subhorizontal intrusion. This “bright spot” extends laterally at least 15 km beneath central Death Valley. A moderately dipping normal fault can be traced from the inferred magma chamber upward to a 690 000-yr-old basaltic cinder cone. The fault zone is inferred to have been a magma conduit during the formation of the cinder cone. Vertical variations in crustal reflection character suggest that the Death Valley magma body may have been emplaced along a zone of decoupling that separates a faulted brittle upper crust from a more ductile and/or intruded lower crust. The Death Valley bright spot is similar to reflections recorded by COCORP in 1977 in the Rio Grande rift, where both geophysical and geodetic evidence support the inference of a tabular magma chamber at 20-km depth.

Exhumation rates in the St. Elias Mountains, Alaska

Abstract Reprocessing and interpretation of seismic profiles from the Gulf of Alaska in the vicinity of the St. Elias Mountains indicate that over 3000 km 3 of sediment were deposited on the ocean shelf since the end of the last glacial epoch, approximately 10,000 years ago. This estimate of the sediment volume does not include material moved out of the region by long shore currents, transportation from the shelf to deeper waters, or material trapped in fiords and basins on and near land. Thus, an estimated sedimentation rate offshore of 7.9 mm/year and the average erosion rate of 5.1 mm/year from the St. Elias Mountains, derived from the estimated sedimentation rate, represent minimum estimates. We infer that these estimated rates are not typical of the approximately 5 m.y. history of mountain building in the area, but rather, they represent the result of efficient erosion by glaciers during this interglacial period.

Structure of the central Death Valley pull-apart basin and vicinity from COCORP profiles in the southern Great Basin

COCORP deep seismic reflection profiles in the vicinity of the central Death Valley pull-apart basin in southeastern California provide three-dimensional information on the subsurface of an active extensional terrane. Variations in the orientation and density of reflectors indicate that the crust and upper mantle of the region is divisible into three seismic zones which may represent regions of differing lithology, rheology, or both. The reflections in the upper ∼5 s (15 km) of the data have gentle to moderate dips; between 5 and 10 s, reflections are predominantly subhorizontal; and below ∼ 10 s (30 km), there are no notable reflections. The boundaries between the above reflecting zones are marked by prominent reflecting horizons which are continuous throughout the survey region. The observed reflection geometries resemble those predicted by the crustal model of the region proposed by Wright and Troxel (1973) on the basis of geological studies. In addition, many of the upper-crustal reflectors can be traced directly to mapped features. Based on those correlations, the upper reflecting zone (0-5 s) is interpreted to be a region of brittle deformation with the various upper-crustal reflectors interpreted as faults and basin sediments. The reflecting horizon at the base of the upper zone appears to be the lower boundary of the faulted upper crustal blocks and, it has been suggested that it locally includes partially molten rock. The observed geometries and amplitudes of reflections from the lower crust (15-30 km depth or 5-10 s two-way traveltime) are consistent with the model of Wright and Troxel for a ductilely deformed and intruded lower crust. The prominent reflecting horizon at the base of that zone is designated the reflection Moho and the seismically transparent lowest zone appears to correspond to the upper mantle. The seismic data define a zone of faults (referred to here as the Wingate Wash fault zone) which appears to form the southern boundary of the central Death Valley basin and also may have provided a conduit for the migration of magma from a mid-crustal magma body to the surface. The Wingate Wash fault zone appears to intersect the southern Death Valley fault zone and the frontal faults of the Black Mountains in the subsurface beneath the youngest volcanic edifice in the region. Those three fault zones appear to separate the Panamint, Owlshead, and Black Mountain upper-crustal fault blocks. From the available data, a reconstruction of the possible fault-block movements during the time of basin subsidence is presented. That reconstruction suggests that the central Death Valley basin formed as a result of the combined down-to-the-east rotation and northwest translation of the fault blocks in manner similar to that proposed by Reches for other parts of the Basin and Range.

Three‐dimensional model of the late Cenozoic history of the Death Valley region, southeastern California

The accumulation of a large database on the timing and kinematics of late Cenozoic deformation in the Death Valley region of southeastern California indicates a complex three-dimensional history. On the basis of paleogeographic reconstructions we suggest the system was initiated as a localized pull-apart between two conjugate strike-slip faults, the Garlock and Furnace Creek faults, and evolved into a system characterized by distributed transtension related to the eastern California shear zone. Our reconstructions differ from previous models in the incorporation of significant vertical axis rotations of a number of crustal blocks to explain paleomagnetic data from the region. The model may resolve (1) a long-standing problem of the eastern termination of the Garlock fault which is explained here as a complex system of splays that initially terminated in the pull-apart between the Furnace Creek and Garlock systems; and (2) the complex architecture of the Black Mountains which is explained here in terms of initial extreme attenuation between the Garlock and Furnace Creek systems with overprinting by a fold and normal fault system that operated simultaneously as a result of distributed transtension. This model suggests much of the displacement field is taken up in rotations and translations, and the actual crustal thinning in our model is relatively small (50–66% of original thickness).

Intracrustal complexity in the United States midcontinent: Preliminary results from COCORP surveys in northeastern Kansas

Unusually clear indications of complex structure in the mid-to-lower crust is revealed by seismic reflection surveys in northeastern Kansas. This complexity contrasts markedly with the layer-cake simplicity of both the overlying sedimentary cover and most previous crustal models for the central United States. Seismic sections collected by COCORP (Consortium for Continental Reflection Profiling) as part of a major east-west traverse across the Neniaha Ridge and Midcontinent Geophysical Anomaly indicate that below a thin, relatively flat layered Paleozoic sedimentary section, the deep crust is characterized by numerous dipping and arcuate reflections and diffractions. In many places layered and crosscutting, these reflections suggest convoluted three-dimensional folded, faulted, and intruded structures. Specific identification of these deep features may be possible if future surveys can trace them to accessible depths. The basement above these reflection complexes contains significantly fewer reflections—consistent with, but not necessarily diagnostic of, the granitic terrane that dominates basement drill-hole samples in the region. Among the events at these shallower basement depths are several east-dipping reflections, some of which may be major faults. Travel times corresponding to expected Moho depths (about 36 km) are characterized less by specific reflections than by an apparent decrease in the density and number of reflections. While evidence of crustal heterogeneity is common among deep reflection studies, the Kansas seismic results outlined in this brief report stand out as being unusually clear representations of such.

Role of seismogenic processes in fault-rock development: An example from Death Valley, California

Fault rocks developed along the Mormon Point turtleback of southern Death Valley suggest that a jog in the oblique-slip Death Valley fault zone served as an ancient seismic barrier, where dominantly strike-slip ruptures were terminated at a dilatant jog. Dramatic spatial variations in fault-rock thickness and type within the bend are interpreted as the products of: (1) fault overshoot, in which planar ruptures bypass the intersection of the two faults composing the bend and slice into the underlying footwall; and (2) implosion brecciation, in which coseismic ruptures arrested at a releasing bend in the fault lead to catastrophic collapse brecciation, fluid influx, and mineralization.