George A. Thompson

Cainozoic evolution of the state of stress and style of tectonism of the Basin and Range province of the western United States

Cainozoic evolution of the modern plate boundary along the western United States from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western United States. Extensions tectonism responsible for the modern Basin and Range province appears to represent a unique late-stage episode of a much longer period of extension initiated in an ‘in tra -arc ’ setting contemporaneously with calc-alkaline magmatism. Basin-range extension is distinguished from early extension on the basis of angular unconformities, differences in fault trends and spacing, and associated magmatism (basaltic). Prebasin-range extension (i.e. extension preceding the break-up of the region into ranges resembling the modern ones) was under way locally by at least 30 Ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks, by large regions of the crust underlain by passively emplaced subvolcanic batholiths, and by the thickness and distribution of stratigraphic units. Locally, high strain rates that accompanied early extensions of as much as 50-100 % are implied. Data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform WSW -ENE least principal stress orientation in the period ca. 20-10 Ma, during this early extension. The change from early extension to basin-range style faulting of the upper 15 km of crust, which resulted in broadly spaced ranges (25-35 km crest-crest spacing), was time-transgressive and probably not abrupt; locally both types occurred concurrently. Southern Basin and Range block faulting occurred largely in the period 13-10 Ma, in response to a stress field orientated similarly to that responsible for the early extension. In contrast, northern Basin and Range block faulting developed after 10 Ma and continues to the present in response to a stress field orientated approximately 45° clockwise to the earlier stress field. This modern stress field, with a WNW -ESE to E-W directed least principal stress, characterizes the entire modern Basin and Range province and Rio Grande rift region. The 45° change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the San Andreas transform fault. Inclusion of pre-basin-range extension may help resolve the discrepancy between estimates of 15-30% for basin-range block faulting and total extension estimates of 100-300 % for the Basin and Range province.

Regional geophysics of the Colorado Plateau

Abstract The Colorado Plateau (CP) is a relatively coherent block surrounded on three sides by the extensional block faulted regime of the Basin and Range Province (BRP) and the Rio Grande Rift (RGR). The CP appears to be part of an inter-related system including the Sierra Nevada, BRP, and RGR which has undergone major uplift and extension during the last 20 m.y. The final elevation in any area probably depends upon which processes dominate there. In most geophysical properties the CP is intermediate between the BRP/RGR and the stable platform of the southern Great Plains. However, many BRP/RGR geophysical characteristics appear to extend well inward of the classical Plateau physiographic boundary. Geologically these 50–100 km wide zones of transition are marked by normal faulting and late Tertiary and Quaternary volcanism. The interior of the Plateau is characterized by a 40 km-thick crust, a Pn velocity of about 7.85 km/sec, and an average heat flow of 1.5–1.6 HFU. Available data on the modern stress field in the Plateau interior indicate high horizontal stresses and a stress field oriented differently from that in the surrounding BRP/RGR, inconsistent with the theory that the Plateau is merely an inherited, more coherent subplate subjected to the same stresses as its surroundings. Free-air gravity anomalies on the CP average near zero and imply isostatic equilibrium; however, crustal thickness is insufficient to explain all the elevation, and low density mantle material must be involved. P wave velocity, gravity, and electrical data are compatible with a depth of approximately 80 km to low density, low velocity, hot conductive mantle (= asthenopshere?). These data also suggest a lithospheric thickness of ~120 km for the southern Great Plains, while surface wave data for the BRP suggest an approximately 60 km-thick lithosphere. Low angle subduction in mid-Cenozoic time has been inferred from calc-alkalic magmatic activity throughout a broad region of the western U.S. Subsequent restriction of this activity to the Sierras and westernmost BRP suggest an abrupt switch to steep subduction which occurred by about 20 m.y. ago, probably with rupturing of the low angle slab. Gradual warming and expansion (with phase changes) of the cutoff stagnant shallow slab is suggested as a possible mechanism for regional uplift.

Basin and Range rifting in northern Nevada: Clues from a mid-Miocene rift and its subsequent offsets

A linear rift composed of dike swarms and graben-filling volcanic rocks marks the inception of Basin and Range rifting in northern Nevada 17 to 14 m.y. ago. This mid-Miocene rift, with its associated aeromagnetic anomaly, indicates S68/sup 0/W-N68/sup 0/E (+-5/sup 0/) extension. A small strike-slip (transform) fault at Sawtooth dike, one short element of the rift, confirms the mid-Miocene extension direction. The present extension, based on data throughout the Basin and Range province, is N65/sup 0/W-S65/sup 0/E (+-20/sup 0/), which is consistent with younger fault offsets of the rift. In the Sawtooth dike region this 45/sup 0/ change in direction of extension took place between 15 and 6 m.y. B.P. The Nevada rift is thought to be a part of a linear rift zone roughly 700 km long, which includes feeder dikes of Columbia River basalts and the mid-Miocene location of the Yellowstone hot spot. Extension in this zone was probably perpendicular to a then-active trench along the western plate margin. The clockwise change in extension direction is consistent with a superposition of right-lateral shear along the plate margin when the San Andreas fault was activated.

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.

OVERVIEW: Late Cenozoic tectonics of the central and southern Coast Ranges of California

The central and southern Coast Ranges of California coincide with the broad Pacific‐North American plate boundary. The ranges formed during the transform regime, but show little direct mechanical relation to strike-slip faulting. After late Miocene deformation, two recent generations of range building occurred: (1) folding and thrusting, beginning ca. 3.5 Ma and increasing at 0.4 Ma, and (2) subsequent late Quaternary uplift of the ranges. The ranges rose synchronously along the central California margin and are still rising; their long axes are quasiparallel to the plate boundary and strike-slip faults. The upper crustal internal and marginal structures of the ranges are contractional, dominated by folds and thrusts resulting from the convergent component of plate motion. Newly constructed transects using seismic reflection and refraction, plus gravity and magnetic studies, reveal lower crustal basement(s) at depths of 10‐22 km. The upper surface of the basement and Moho show no effect of the folding and thrusting observed in the upper crust. We conclude that horizontal shortening is accommodated at depth by slip on subhorizontal detachments, and by ductile shear and thickening. The ranges are marked by high heat flow; weak rocks of the Franciscan subduction complex; high fluid pressure; bounding high-angle reverse, strike-slip, or thrust faults; and uplift at a rate of 1 mm/yr beginning about 0.40 Ma. Transverse compression manifested in folding within the Coast Ranges is ascribed in large part to the well-established change in plate motions at about 3.5 Ma.

The northern Nevada rift: Regional tectono-magmatic relations and middle Miocene stress direction

As defined by the most recent aeromagnetic surveys, the north-northwest-trending northern Nevada rift zone extends for at least 500 km from southern Nevada to the Oregon Nevada border. At several places along the rift, the magnetic anomaly is clearly related to north-northwest-trending dikes and flows that, based on new radiometric dating, erupted between 17 and 14 Ma and probably during an even shorter time interval. The tectonic significance of the rift is dramatized by its length, its coincidence in time and space (at its northern terminus) with the oldest silicic caldera complex along the Yellowstone hot-spot trend, and its parallelism with the subduction zone along the North American coast prior to the establishment of the San Andreas fault. The northern Nevada rift is also equivalent in age, trend, and composition to feeder dikes that fed the main eruptive pulse (∼95% volumetrically) off the Columbia River flood basalts in northern Oregon ∼15.5-16.5 Ma. Because of these similarities, both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south-southeast and north-northwest, respectively, from a central mantle plume. The site of the initial breaching of the North America plate by this plume is probably the McDermitt volcanic center at the north end off the rift near the Oregon-Nevada border. The present north-northwest trend of the rift and its internal elements, such as dikes and lava-filled grabens, record the orientation of the arc-normal extensional stress in this back-arc region at the time of emplacement. Paleomagnetic evidence presented by others and interpreted to indicate block rotations at three sample localities is not consistent with either a rotation of dikes within the rift or with a regional rotation of the entire rift. The present north-northwest trend of the rift reflects the state of stress in the Basin and Range during middle Miocene time and is consistent with stress indicators of similar age throughout the Basin and Range and Rio Grande rift provinces.

Mantle plume influence on the Neogene uplift and extension of the U.S. western Cordillera

espite its highly extended and thinned crust, much of the western Cordillera in the United States is elevated more than 1 km above sea level. Therefore, this region cannot be thought of as thick crust floating isostatically in a uniform mantle; rather, the lithospheric mantle and/or the upper asthenosphere must vary in thickness or density across the region. Utilizing crustal thickness and density constraints, we modeled the residual mass deficit that must occur in the mantle lithosphere and asthenosphere beneath the western Cordillera. A major hot spot broke out during a complex series of Cenozoic tectonic events that included lithospheric thickening, backarc extension, and transition from a subduction to a transform plate boundary. We suggest that many of the characteristics that make the western Cordillera unique among extensional provinces can be ttributed to the mantle plume that created the Yellowstone hot spot.

The Role of Magma Overpressure in Suppressing Earthquakes and Topography: Worldwide Examples

In an extending terrane basaltic magma supplied at a pressure greater than the least principal stress (overpressure) may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting. As vertical dikes intrude, they press against their walls in the direction opposite the least principal stress and increase its magnitude. The emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses. This process tends to equalize the stresses and thus diminishes the deviatoric stress (difference between maximum and minimum stresses) that creates faults and causes earthquakes. Observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension. Recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks.

Host rock rheology controls on the emplacement of tabular intrusions: Implications for underplating of extending crust

The pooling, ponding, and horizontal intrusion of basaltic magma at various depths into the crystalline crust is paradoxical because the stress conditions favoring such intrusions do not favor the opening of vertical feeder conduits necessary for their formation. The most rigid zones of the crust and upper mantle tend to behave elastically and store stress when subjected to tectonic forces, while the more ductile zones flow under stress. These rheological differences within the crust and upper mantle allow variation in the magnitude of deviatoric stress, and such variation has a profound effect on tabular intrusions. A vertical dike intruding into extending crust increases the horizontal least principal stress of the host rock when it is emplaced, and that effect is magnified in rheologically ductile zones where the pre-existing deviatoric stress has been partially relaxed. Subsequent dikes intruding into the ductile zone may encounter stress conditions that have been altered to the extent that the local least principal stress has become vertical, and horizontal intrusion initiates. Multiple geological and geophysical observations of horizontal intrusions in extending crust indicate that such principal stress interchanges occur commonly.

Oceanic ridges and transform faults: Their intersection angles and resistance to plate motion

Abstract The persistent near-orthogonal pattern formed by oceanic ridges and transform faults defies explanation in terms of rigid plates because it probably depends on the energy associated with deformation. For passive spreading, it is likely that the ridges and transforms adjust to a configuration offering minimum resistance to plate separation. This leads to a simple geometric model which yields conditions for the occurrence of transform faults and an aid to interpretation of structural patterns in the sea floor. Under reasonable assumptions, it is much more difficult for diverging plates to spread a kilometer of ridge than to slip a kilometer of transform fault, and the patterns observed at spreading centers might extend to lithospheric depths. Under these conditions, the resisting force at spreading centers could play a significant role in the dynamics of plate-tectonic systems.