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Geodetic constraints on areal changes in the Pacific–North America plate boundary zone: What controls Basin and Range extension?

¹ Nevada Bureau of Mines and Geology, and Seismological Laboratory, University of Nevada, Reno, Nevada 89557, USA

Using 1500 geodetic velocities we model the present-day spatial patterns of areal changes inside the Pacific–North America plate boundary zone. From this model we show that between the central Gulf of California and the Queen Charlotte Islands there is no significant net change in surface area. This zero net areal-change result allows us to relate regions of areal growth to areas of equivalent contraction elsewhere within the plate boundary zone. We find that areal growth of the Basin and Range province (BRP) and its eastern margin (5.2 ± 0.1 x 10³ m²/yr) is balanced by areal reduction near northwestern California between 38°N and 42°N. The San Andreas fault system south of 38°N and the plate boundary zone north of 42°N (including the Juan de Fuca and Gorda Ridge systems) each have no significant net areal change. Our results suggest a kinematic relationship between extension in the BRP and contraction near the northern California Coast Ranges and Klamath Mountains. From these observations we propose that, although BRP extension may be caused by internal forces, the southernmost Cascadia subduction zone provides a "window of escape" that acts as a stress guide to BRP extension as well as northwestward Sierra Nevada motion. Such a dynamic model is consistent with independent findings that (1) the least principal horizontal stress orientations in the BRP are toward northern California, (2) extension directions in the BRP have changed orientation to track the northward migration of the Mendocino triple junction, and (3) the southernmost Cascadia subduction zone is a relatively weak plate boundary.

Key Words: geodesy • dilatation • contraction • extension • plate boundary zone • Basin • Range • western United States • tectonics • lithospheric dynamics

This article has been cited by other articles:

				L. M. Flesch A possible "window of escape" in the southern Cascadia subduction zone Geology, October 1, 2007; 35(10): 959 - 960. [Full Text] [PDF]