Brian P. Wernicke
Chandler Family Professor of Geology
The research by Wernicke and his group is primarily observational, and is strongly multidisciplinary and collaborative. Current projects include studying large-scale intracontinental extension, mainly in the Basin and Range province of the western United States; studies of high-temperature evolution of the deeper parts of collision zones; and studies of active tectonics of diffuse zones of intracontinental deformation. With field observation and geological mapping as a foundation, students and postdocs in the group address problems using a wide spectrum of methods, including sequence stratigraphy, paleomagnetism, geochronology, petrologic thermobarometry, remote sensing and GPS geodesy.
Large-Scale Intracontinental Extension
The tectonics of horizontal contraction of the earth's continental lithosphere have been well studied for more than a century, in orogens such as the Alps, Caledonides and Appalachians. Equivalent understanding of extension has arisen mainly over the last decade, spurred by geologic field studies in the Basin and Range province of western North America, the earth's best exposed region of large-scale intracontinental extension. Recent work by Wernicke and postdoctoral fellow J.K. Snow in the central Basin and Range (lat 37N) demonstrates about 250 km of west-northwest translation of the Sierra Nevada relative to the Colorado Plateau since 20 Ma, accommodated along both normal and strike-slip fault systems.
Geological reconstructions show the principal zones of extension, now 100-150 km wide, are nearly completely stripped of their upper 12-15 km of crust. The lack of major topographic or gravity anomalies over the extended zones has prompted two major projects, BARGE (Basin and Range Geoscientific Experiment) and the Southern Sierra Nevada Continental Dynamics Project, aimed at understanding how the deep crust responds to extension by imaging the deep structure of the eastern and western boundaries, respectively, of the central Basin and Range. These projects involve explosion seismology, "marine" deep reflection profiling (off barges in Lake Mead), magnetotelluric imaging, xenolith studies, geologic mapping, thermochronology and thermobarometry, and paleomagnetism. One of the main results coming from this work is that the complex patchwork of extended upper crustal blocks are floating on a fluid deep crustal layer, which is appears to have been "pumped" into the region between the separating blocks, completely reconstituting the upper crust. Modeling studies suggest this layer may be as much as four to five orders of magnitude less viscous than the upper mantle.
Cross-sectional evolution of extended upper continental crust, after Wernicke (1992).
Deep Structure of Collision Zones
A largely unsolved problem in geology is the role of deeper crustal layers during mountain building. Physical models of the deep crust have largely outpaced our ability to test them, especially for rock assemblages deformed and metamorphosed beyond the greenschist facies. Wernicke and colleagues are undertaking studies that combine thermobarometric results with high temperature thermochronometers (especially the Sm/Nd system) in the collisional belts of the Pacific Northwest. The Sm/Nd system has proved particularly effective, because major metamorphic phases such as garnet, plagioclase and hornblende close to diffusion of rare earths at upper amphibolite facies conditions (600-700 C). Thus events that may be difficult to date with traditional methods, either because of high diffusivity (closure below 500 C) or restriction to trace phases, may be resolved to within 1-2 million years.
Horizontal normal fault cutting Mississippian Redwall Limestone, Gold Butte area, southern Nevada. Dark layer above fault has been displaced to the left. Cliff face is approximately 250 m high.
Active Tectonics of Diffuse Continental Deformation
The advent of Global Positioning System (GPS) geodesy over the last five years is revolutionizing the study of actively deforming regions of the earth's crust. Understanding the relationship between elastic strain accumulation and release has generally been focused on major plate-boundary faults such as the San Andreas. Less is known, however, about how earthquakes and strain accumulation are related in wide, diffusely deforming belts of continental crust away from the major plate-boundary faults. As recent events such as the Landers and Northridge earthquakes attest, understanding the non-San Andreas components of strain accumulation is both a scientifically and societally relevant problem. Wernicke's group is investigating strain accumulation using GPS along a zone of three widely-spaced Holocene strike-slip fault zones in the southwestern Great Basin. The westernmost zone is by far most active seismically (e.g. 1872 M~7.5 Owens Valley earthquake). This study should resolve whether strain accumulation is focused on the western, seismically active zone or across the entire zone of major Holocene faulting.