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A new reconstruction of the Paleozoic continental margin of southwestern North America: Implications for the nature and timing of continental truncation and the possible role of the Mojave-Sonora megashear

By
Calvin H. Stevens
Calvin H. Stevens
1
San Jose State University, Department of Geology, San Jose, California 95192, USA
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Paul Stone
Paul Stone
2
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
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Jonathan S. Miller
Jonathan S. Miller
3
San Jose State University, Department of Geology, San Jose, California 95192, USA
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Published:
January 01, 2005

Data bearing on interpretations of the Paleozoic and Mesozoic paleogeography of southwestern North America are important for testing the hypothesis that the Paleozoic miogeocline in this region has been tectonically truncated, and if so, for ascertaining the time of the event and the possible role of the Mojave-Sonora megashear. Here, we present an analysis of existing and new data permitting reconstruction of the Paleozoic continental margin of southwestern North America. Significant new and recent information incorporated into this reconstruction includes (1) spatial distribution of Middle to Upper Devonian continental-margin facies belts, (2) positions of other paleogeographically significant sedimentary boundaries on the Paleozoic continental shelf, (3) distribution of Upper Permian through Upper Triassic plutonic rocks, and (4) evidence that the southern Sierra Nevada and western Mojave Desert are underlain by continental crust.

After restoring the geology of western Nevada and California along known and inferred strike-slip faults, we find that the Devonian facies belts and pre-Pennsylvanian sedimentary boundaries define an arcuate, generally south-trending continental margin that appears to be truncated on the southwest. A Pennsylvanian basin, a Permian coral belt, and a belt of Upper Permian to Upper Triassic plutons stretching from Sonora, Mexico, into westernmost central Nevada, cut across the older facies belts, suggesting that truncation of the continental margin occurred in the Pennsylvanian. We postulate that the main truncating structure was a left-lateral transform fault zone that extended from the Mojave-Sonora megashear in northwestern Mexico to the Foothills Suture in California. The Caborca block of northwestern Mexico, where Devonian facies belts and pre-Pennsylvanian sedimentary boundaries like those in California have been identified, is interpreted to represent a missing fragment of the continental margin that underwent ∼400 km of left-lateral displacement along this fault zone. If this model is correct, the Mojave-Sonora megashear played a direct role in the Pennsylvanian truncation of the continental margin, and any younger displacement on this fault has been relatively small.

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GSA Special Papers

The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives

Thomas H. Anderson
Thomas H. Anderson
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Jonathan A. Nourse
Jonathan A. Nourse
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James W. McKee
James W. McKee
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Maureen B. Steiner
Maureen B. Steiner
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Geological Society of America
Volume
393
ISBN print:
9780813723938
Publication date:
January 01, 2005

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