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GSA TODAY • Electronic Publications, P Vol. 10, No. 11 November 2000 INSIDE • Rocky Mountain, South-Central Section Meeting, p. 10 GSA TODAY • Electronic Publications, p. 12 A Publication of the Geological Society of America • GeoVentures, p. 28 Dynamics of Plate Boundary Fault Systems from Basin and Range Geodetic Network (BARGEN) and Geologic Data Brian Wernicke, [email protected], Anke M. Friedrich, Nathan A. Niemi, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA Richard A. Bennett, James L. Davis, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA ABSTRACT Continuously recorded Global Posi- tioning System (GPS) data from the northern Basin and Range suggest that contemporary deformation is quite slow and broadly distributed, rather than being concentrated in the relatively nar- row zones of historical earthquakes. Sur- prisingly, however, in north-central Nevada, the data indicate rapid, range- normal crustal shortening at a rate of 2–3 mm/yr in an area where the geol- ogy indicates crustal extension via Holocene normal faulting. A possible explanation for the conflicting geodetic and geologic data is that the region of shortening represents the contractile side of a slowly east-propagating defor- mation pulse generated by the 1915 Pleasant Valley and 1954 Dixie Valley and Fairview Peak earthquakes. Such pulses, which are transient effects not recorded by faulting, are predicted by a broad class of physical models, but have only been observed within a few years after very large earthquakes, when the signal is much larger than the long-term deformation rate. The Basin and Range, and similar areas with a combination of low long-term deformation rates and large earthquakes, may therefore have Figure 1. Relief map of Great Basin region showing locations of BARGEN (Basin and Range Geodetic the best potential by combining mod- Network) continuous GPS sites. IGS—International GPS Services. ern geologic and geodetic data to eluci- these kinematic phenomena have been are spaced about 30 km apart. This part of date fault system behavior, in particular segregated by frequency band or time the Basin and Range reaches a maximum how transient effects from an earth- scale, each requiring different approaches, width of 750 km at latitude 40°N, and quake on one fault may influence pat- such as seismology, geodesy, structural includes some 20–25 basin-range pairs. terns of stress and seismic strain release geology, and isotope geochemistry. Major The ranges began forming between 10 and on others. These types of data are essen- progress is likely to transcend temporal 15 Ma as the Sierra Nevada–Great Valley tial in developing realistic models of and disciplinary boundaries, challenging block moved westward relative to the inte- seismic hazard, and in linking us to bridge them with coordinated, rior, first at rates near 20 mm/yr almost short–time scale observations with broadband research. due west (Wernicke and Snow, 1998), then longer term geologic processes. In this paper, we describe a prototype slowing to the current overall rate near 12 of such an effort focused on the northern mm/yr northwest (Hearn and Humphreys, INTRODUCTION Basin and Range province of western 1998; Bennett et al., 1999; Thatcher et al., The relative motion of rock masses is North America. The area lies between the 1999; Dixon et al., 2000). Total displace- the prime observable for competing the- Sierra Nevada–Great Valley block to the ment of the block since 16 Ma is ~250 km ories of earth deformation, at time scales east and the Colorado Plateau to the west, (Wernicke and Snow, 1998). Most of the ranging from a fraction of a second (e.g., and is characterized by north-trending ranges are delimited on one side by a seismic waves) to hundreds of millions of mountain ranges and basins of curiously major normal fault with significant years (breakup and aggregation of super- uniform size and spacing (Fig. 1). Ranges continents). Traditionally, the study of rise ~1500 m above adjacent basins, and Fault Systems continued on p. 2 IN THIS ISSUE GSA TODAY November Vol. 10, No. 11 2000 Dynamics of Plate Boundary Fault Student Travel Grants ................................17 Systems from Basin and Range GSA TODAY (ISSN 1052-5173) is published monthly Geodetic Network (BARGEN) and Toward a Stewardship of the Global by The Geological Society of America, Inc., with offices at 3300 Commons, Part XI .....................................19 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box Geologic Data...........................................1 9140, Boulder, CO 80301-9140, U.S.A. Periodicals postage paid at Boulder, Colorado, and at additional mailing offices. Dialogue......................................................3 Position Announcements ...........................20 Postmaster: Send address changes to GSA Today, Member Service Center, P.O. Box 9140, Boulder, CO 80301-9140. 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