Mojave-Yavapai boundary zone, southwestern United States: A rifting model for the formation of an isotopically mixed crustal boundary zone
- 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA
- 2Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA
- 3Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA
Abstract
The boundary between the Paleoproterozoic Mojave and Yavapai crustal provinces in the southwestern United States is a 75-km-wide zone characterized by complex, isotopically mixed (Pb) crust, similar to other transitional crustal and lithospheric boundaries worldwide. Mojave crust is more isotopically evolved than the juvenile crust of the Yavapai province, indicating an older crustal component. Several areas of isotopically juvenile rocks within and near the boundary zone correspond to exposures of coeval >1.73 Ga supracrustal rocks that contain pillow basalts and bimodal metavolcanic sequences. We suggest that regions of juvenile crust within the Mojave province and its boundary zone represent rift basins, produced by extension or transtension near the eastern margin of the province, into which primitive basaltic lavas and bimodal volcanic sequences were erupted. The largest region of juvenile material within the boundary zone is bound to the west by a series of coincident, N-trending isotopic, geochemical, metamorphic, and geophysical discontinuities that may represent a fundamental, buried crustal structure, perhaps a rift margin. We propose that the distinct isotopic signature of the boundary zone persisted through subsequent deformational events, including the one that juxtaposed the two provinces. If our model for the formation of the boundary zone is correct, its isotopically mixed character is at least partly an artifact of the precollision history of the eastern Mojave province, and may not solely reflect the processes that juxtaposed the two provinces. Formation of new crust by rifting may represent an underappreciated form of crustal growth during the Paleoproterozoic that is distinct from arc accretion. Models for the formation of complex crustal boundary zones must consider the early, prejuxtaposition history of terranes in interpreting their geochemical and geophysical characteristics.
- Proterozoic tectonics
- southwestern United States
- crustal boundaries
- Mojave province
- Yavapai province
- isotopic boundaries
Footnotes
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↵GSA Data Repository item 2006137, lead isotopic data from northwestern Arizona, is available online at www.geosociety.org/pubs/ft2006.htm, or on request from editing{at}geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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↵*Currently at the Institute for Geophysics, Seismology, and Geodynamics, Eidgenössische Tech nische Hochshule, Zürich, Switzerland
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↵A pseudogravity map is created by transforming aeromagnetic signals into the hypothetical gravity signals generated by a body with proportional density and magnetization using Poisson's relationship (Baranov, 1957; Blakely, 1995). This transformation enhances bulk rock properties of long- wavelength bodies, down to approximately 20 km depth, by filtering out short-wavelength magnetic signals (dominantly generated in the upper crust) and the obliquity of the crustal magnetization to the ambient field. Pseudogravity maps are useful for determining the edges of old long-wavelength bodies (e.g., Rajaram et al., 2000), as they are less dominated by either modern tectonic settings or present-day density distributions than true gravity maps.
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- Accepted 31 March 2006.
- Received 5 January 2006.
- Revision received 29 March 2006.
- Geological Society of America
