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George A. Thompson

Publications and source records attributed to George A. Thompson.

16 recordsLinked to original sources
USGS2017

From coseismic offsets to fault-block mountains

In the Basin and Range extensional province of the western United States, coseismic offsets, under the influence of gravity, display predominantly subsidence of the basin side (fault hanging wall), with comparatively little or no uplift of the mountainside (fault footwall). A few decades later, geodetic measurements [GPS and interferometric synthetic aperture radar (InSAR)] show broad (∼100 km) aseismic uplift symmetrically spanning the fault zone. Finally, after millions of years and hundreds of fault offsets, the mountain blocks display large uplift and tilting over a breadth of only about 10 km. These sparse but robust observations pose a problem in that the coesismic uplifts of the footwall are small and inadequate to raise the mountain blocks. To address this paradox we develop finite-element models subjected to extensional and gravitational forces to study time-varying deformation associated with normal faulting. Stretching the model under gravity demonstrates that asymmetric slip via collapse of the hanging wall is a natural consequence of coseismic deformation. Focused flow in the upper mantle imposed by deformation of the lower crust localizes uplift, which is predicted to take place within one to two decades after each large earthquake. Thus, the best-preserved topographic signature of earthquakes is expected to occur early in the postseismic period.

Proceedings of the National Academy of Sciences of↗
USGS2016

Vertical deformation associated with normal fault systems evolved over coseismic, postseismic, and multiseismic periods

Vertical deformation of extensional provinces varies significantly and in seemingly contradictory ways. Sparse but robust geodetic, seismic, and geologic observations in the Basin and Range province of the western United States indicate that immediately after an earthquake, vertical change primarily occurs as subsidence of the normal fault hanging wall. A few decades later, a ±100 km wide zone is symmetrically uplifted. The preserved topography of long-term rifting shows bent and tilted footwall flanks rising high above deep basins. We develop finite element models subjected to extensional and gravitational forces to study time-varying deformation associated with normal faulting. We replicate observations with a model that has a weak upper mantle overlain by a stronger lower crust and a breakable elastic upper crust. A 60° dipping normal fault cuts through the upper crust and extends through the lower crust to simulate an underlying shear zone. Stretching the model under gravity demonstrates that asymmetric slip via collapse of the hanging wall is a natural consequence of coseismic deformation. Focused flow in the upper mantle imposed by deformation of the lower crust localizes uplift under the footwall; the breakable upper crust is a necessary model feature to replicate footwall bending over the observed width ( < 10 km), which is predicted to take place within 1-2 decades after each large earthquake. Thus the best-preserved topographic signature of rifting is expected to occur early in the postseismic period. The relatively stronger lower crust in our models is necessary to replicate broader postseismic uplift that is observed geodetically in subsequent decades.

Journal of Geophysical Research↗
USGS2012

Seismic constraints on the nature of lower crustal reflectors beneath the extending Southern Transition Zone of the Colorado Plateau, Arizona

We determine the reflection polarity and exploit variations in P and S wave reflectivity and P wave amplitude versus offset (AVO) to constrain the origin of lower crustal reflectivity observed on new three-component seismic data recorded across the structural transition of the Colorado Plateau. The near vertical incidence reflection data were collected by Stanford University in 1989 as part of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that traversed the Arizona Transition Zone of the Colorado Plateau. The results of independent waveform modeling methods are consistent with much of the lower crustal reflectivity resulting from thin, high-impedance layers. The reflection polarity of the cleanest lower crustal events is positive, which implies that these reflections result from high-velocity contrasts, and the waveform character indicates that the reflectors are probably layers less than or approximately equal to 200 m thick. The lower crustal events are generally less reflective to incident S waves than to P waves, which agrees with the predicted behavior of high-velocity mafic layering. Analysis of the P wave AVO character of lower crustal reflections demonstrates that the events maintain a constant amplitude with offset, which is most consistent with a mafic-layering model. One exception is a high-amplitude (10 dB above background) event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets. The event has a pronounced S wave response, which along with its negative AVO trend is a possible indication of the presence of fluids in the lower crust. The Arizona Transition Zone is an active but weakly extended province, which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers. Instead, we favor horizontal basaltic intrusions as the primary origin of high-impedance reflectors based on (1) The fact that most xenoliths in eruptive basalts of the Transition Zone are of mafic igneous composition, (2) indications that a pulse of magmatic activity crossed the Transition Zone in the late Tertiary period, and (3) the high regional heat flow observed in the Transition Zone. The apparent presence of fluids near the base of the reflective zone may indicate a partially molten intrusion. We present a mechanism by which magma can be trapped and be induced to intrude horizontally at rheologic contrasts in extending crust.

Arizona↗
USGS2012

Seismic evidence for active magmatic underplating beneath the Basin and Range Province, western United States

Near-vertical and wide-angle seismic reflection data provide evidence for the presence of a magma body at the base of the crust beneath Buena Vista Valley in northwestern Nevada. The seismic response of this hypothesized magma body is characterized by high-amplitude, near-vertical P wave reflections and a comparably strong P -to- S converted phase. The magma body, referred to here as the Buena Vista Magma Body, is probably a single sill with thickness no greater than 200 m and length no greater than 1.8 km. The melt fraction in the sill is undoubtedly greater than 20–30%, and probably exceeds 50%. Melt composition is unconstrained. Although the age of the Buena Vista Magma Body is difficult to determine precisely, it is probably no older than 500,000 years. This suggests that magmatism in the Basin and Range Province is an ongoing process, despite the relative paucity of volcanic rocks erupted at the surface during the last 6 m.y.

Nevada, Idaho↗
USGS2001

Very different crustal response to extreme extension in the southern Basin and Range and Colorado Plateau transition

Clustered about the southwest edge of the Colorado Plateau lie many highly extended terranes. Among these are metamorphic core complexes, distinguished by low-angle normal faults with sufficient offset to expose middle crustal rocks at higher elevation relative to the surrounding areas. About 150 km to the southwest, strong extension in the Salton Trough manifests itself very differently; high-angle normal faults form deep basins, with the strongest extension causing subsidence beneath sea level. We ask why strong extension takes such different forms, and, to help answer the question, we take advantage of seismic and gravity profiles that cross through the Salton Trough, metamorphic core complexes, and the Colorado Plateau. We propose that the relative rate of extension vs. intrusion of crust by magmatism and crustal flow controls the extensional style. We further show that conditions may have been ideal for core complex formation along the edges of the Colorado Plateau because its thick crust and high elevation provided a source of crustal flow and the pressure gradient to drive the flow.

California↗
USGS1998

More than one way to stretch: A tectonic model for extension along the plume track of the Yellowstone hotspot and adjacent Basin and Range Province

The eastern Snake River Plain of southern Idaho poses a paradoxical problem because it is nearly aseismic and unfaulted although it appears to be actively extending in a SW-NE direction continuously with the adjacent block-faulted Basin and Range Province. The plain represents the 100-km-wide track of the Yellowstone hotspot during the last ∼16–17 m.y., and its crust has been heavily intruded by mafic magma, some of which has erupted to the surface as extensive basalt flows. Outside the plain's distinct topographic boundaries is a transition zone 30–100 km wide that has variable expression of normal faulting and magmatic activity as compared with the surrounding Basin and Range Province. Many models for the evolution of the Snake River Plain have as an integral component the suggestion that the crust of the plain became strong enough through basaltic intrusion to resist extensional deformation. However, both the boundaries of the plain and its transition zone lack any evidence of zones of strike slip or other accommodation that would allow the plain to remain intact while the Basin and Range Province extended around it; instead, the plain is coupled to its surroundings and extending with them. We estimate strain rates for the northern Basin and Range Province from various lines of evidence and show that these strains would far exceed the elastic limit of any rocks coupled to the Basin and Range; thus, if the plain is extending along with its surroundings, as the geologic evidence indicates, it must be doing so by a nearly aseismic process. Evidence of the process is provided by volcanic rift zones, indicators of subsurface dikes, which trend across the plain perpendicular to its axis. We suggest that variable magmatic strain accommodation, by emplacement and inflation of dikes perpendicular to the least principal stress in the elastic crust, allows the crust of the plain to extend nearly aseismically. Dike injection releases accumulated elastic strain but generates only the small earthquakes associated with dike propagation. The rate of dike emplacement required to accommodate the estimated longitudinal strain rate of the plain is roughly a composite width of 10 m every 1000 years for the geologically youngest and most active part of the plain. The locus of most rapid intrusion and strain has migrated toward Yellowstone and is now in the northeastern 100–150 km of the plain. Reduced magmatic input in the transition zone of the plain causes the transitional expression of seismicity and faulting there.

Tectonics↗
USGS1993

Does magmatism influence low-angle normal faulting?

Synextensional magmatism has long been recognized as a ubiquitous characteristic of highly extended terranes in the western Cordillera of the United States. Intrusive magmatism can have severe effects on the local stress field of the rocks intruded. Because a lower angle fault undergoes increased normal stress from the weight of the upper plate, it becomes more difficult for such a fault to slide. However, if the principal stress orientations are rotated away from vertical and horizontal, then a low-angle fault plane becomes more favored. We suggest that igneous midcrustal inflation occurring at rates faster than regional extension causes increased horizontal stresses in the crust that alter and rotate the principal stresses. Isostatic forces and continued magmatism can work together to create the antiformal or domed detachment surface commonly observed in the metamorphic core complexes of the western Cordillera. Thermal softening caused by magmatism may allow a more mobile mid-crustal isostatic response to normal faulting.

Geology↗
USGS1990

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.

Special Papers of the Geological Society of Americ↗
USGS1959

Geology and quicksilver deposits of the Terlingua district, Texas

The Terlingua quicksilver district, which has produced more than 150,000 flasks of quicksilver, is in the southern part of the Big Bend region of southwestern Texas. It is a narrow, eastwest area about 20 miles long and lies mainly in southwestern Brewster County. The district is connected by graded road with the nearest railroad, 84 miles north of its center. Quicksilver minerals were first discovered in the district in the latter part of the 19th century, but there was no substantial production until 1900. Although there are about 20 mines and many prospects, more than 90 percent of the quicksilver came from the Chisos-Rainbow, Mariposa, and Study Butte mines. The most productive years were during World War I; since 1946 the district has been idle. Future production depends upon the discovery of new ore bodies-which will be costly-and the working of deposits now considered of too low grade to be profitable.

Texas↗
USGS1944

248 mine, Terlingua quicksilver district, Brewster County, Texas

The 248 quicksilver mine is in Section 248, Block G-4, 2 miles east of Terlingua and 86 miles by dirt road from Alpine, Tex., the nearest railway shipping point (see accompanying maps). Cinnabar, the quicksilver mineral, was discovered before 1902. By 1934 there were only about 700 feet of subsurface workings, but in recent years the Esperado Mining Co., which has the property under lease, has greatly extended exploratory workings and has built a Herreshoff furnace rated at 40 tons per day. At the present time there are 1 1/3 miles of underground workings and 800 feet of surface trenches.

Texas↗
USGS1944

The Mariposa mine, Terlingua quicksilver district, Brewster County, Texas

The Mariposa mine in Brewster County, Tex., ranks second in all-time production of quicksilver in the Terlingua mining district. It is in Section 59, Block G-12, and is about 7 miles by road west of the Terlingua Post Office (see accompanying maps). The nearest railroad shipping point is Alpine, Tex., which. is 93 miles by road to the north. The mine is now controlled by the Esperado Mining Co. of Houston, Tex. Since 1895 the Mariposa mine has produced between 20,000 and 30,000 flasks of quicksilver. The major production was before 1911, but the mine was active during the first World War, and again during the period from 1933 to 1942. In February 1944, the Esperado Mining Co, was reworking some of the old dumps and exploring an ore showing in the east wall of the Perry pit. The productive ground, a rectangular area roughly 3,500 feet long and 1,000 feet wide, centers about California Hill (see plate 1). The principal subsurface workings, about 3 miles of drifts, stapes, and crosscuts, are under California Hill, but much of the ore was taken from numerous pits, trenches, and shafts northeast and west of the hill. Of the 121 shafts over 20 feet deep, 6 are over 100 feet deep and. 2 of these, No. 5 (or Cruz) shaft and the finite shaft, are 300 or more feet deep. The Contratiro winze (see map of workings) extends 200 feet below the 100 level of the mine. Several thousand feet of drifts and crosscuts extending from the deeper shafts explore the mineralized ground through a vertical range of about 300 feet, but almost all the ore that has been produced came from the uppermost workings.

Texas↗