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10: Geophysical studies and tectonic development of the continental margin off the Western United States, lat 34° to 48°N

By
Eli A. Silver
Eli A. Silver
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Published:
January 01, 1978

The continental margin off the Western United States changes abruptly from the subduction type north of lat 40°38’N to the translational type south of that latitude. Along the Cascadia subduction zone in the northern area a change in structural geometry at about the Columbia River separates the Oregon structural type of fore-arc basin-ridge-slope accretionary prism from the Washington type of low accretionary plateau. This change in gross morphology results from the dominantly landward-dipping thrust faults off Oregon to seaward-dipping thrusts off Washington. Gravity anomalies along the Oregon slope have steep gradients and relatively high amplitudes, whereas off Washington the gradients and amplitudes are low. Magnetic anomalies from the down-going Juan de Fuca plate can be traced inland 100 km from the base of the Washington slope but only 20 km off Oregon.

The central California translational margin is cut by several long fault zones and has five large sedimentary basins and four marginal ridges. The basins all formed in late middle Miocene, probably as a result of a change in direction of Pacific-America plate motion. Basins resting on Franciscan assemblage basement rocks show a greater structural variability than those on granitic basement. Offset of granitic rocks of the Salinian block that underlie the Farallon ridge can be restored by assuming 80 km of right lateral strike-slip along the San Gregorio fault and 600 km along the San Andreas. The San Gregorio fault may join the Hosgri-San Simeon fault zone south of Point Sur, and this fault also shows evidence for 80 to 100 km of right-lateral strike-slip. The San Gregorio-Hosgri fault system may thus be one of the longest in California. The nongranitic offshore ridges are underlain by deformed sedimentary and volcanic rocks, some of which are dated as early to middle Tertiary. A sharp change in structural style between these rocks, which were probably deformed by lower to middle Tertiary subduction, and the gently folded or undisturbed overlying strata of Miocene and younger age may represent the transition from compression associated with subduction to translational shear off central California.

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GSA Memoirs

Cenozoic Tectonics and Regional Geophysics of the Western Cordillera

Robert B. Smith
Robert B. Smith
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Gordon P. Eaton
Gordon P. Eaton
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Geological Society of America
Volume
152
ISBN print:
9780813711522
Publication date:
January 01, 1978

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