HR: 09:05h
AN: T51A-05 INVITED     [PDF]
TI: What can we Learn From Small Non-Recoverable Strains at Plate Boundaries?
AU: * Lewis, J C
EM: lewis@geo.umass.edu
AF: Department of Geosciences University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003 United States
AU: Pluhar, C J
EM: cpluhar@emerald.ucsc.edu
AF: Department of Earth Sciences, University of California, Santa Cruz, CA 95064 United States
AB: Background seismicity carries often overlooked information about how the crust responds to plate motions. Integrating focal mechanisms for background seismicity with (1) geologic observations, and (2) geodetic constraints, is critical to establishing a better understanding of both the rock record and contemporary deformation. Treating the crust as a micropolar continuum it is possible to constrain not only the orientations and relative magnitudes of the principal strains but also the vorticity of crustal blocks with respect to the large-scale continuum. We show the utility of this approach with examples from the Cascadia margin and the Coso Range (within the Eastern California shear zone). In the upper crust of the Cascadia margin, seismogenic strain appears to be dominated by accommodation of motion of the Oregon forearc block. This suggests that the shallow crust is responding to long-term motion of the Oregon forearc rather than the interseismic locking of the subduction megathrust. In the area west of Mt. Rainier, this response is marked by non-zero relative vorticity in a regime of N-S shortening and crustal thickening. To date, geologic studies necessary to evaluate the significance of this vorticity have not been completed. In contrast within the Coso Range of California, seismogenic strain at Wild Horse Mesa indicates a component of relative vorticity that is broadly consistent with paleomagnetically constrained finite rotations of the ca. 3 Ma lava flows that compose the mesa. This area is centered at a right-releasing step in the Eastern California shear zone and thus is experiencing active transtension. Stratigraphic constraints have been used to suggest that significant dextral shearing in this region initiated ca. 3.5-2 Ma. The seismogenic response to transtension is depth-dependent plane strain with crustal thinning above 5 km and horizontal dextral shearing from 5-8 km. Both structural levels indicate subhorizontal E-W maximum stretching. Relative vorticity at deeper levels is consistent with down-viewed clockwise rotation, in accord with paleomagnetic results. Relative vorticity at shallower levels is consistent with E tilting of crustal blocks, which has not been resolved paleomagnetically. In addition, micropolar modeling of outcrop-scale brittle faults exposed in the eastern part of Wild Horse Mesa shows subhorizontal maximum stretching directions that are in general agreement with those determined for contemporary deformation. At 90% confidence, bootstrap models suggest a sense of block rotation that is in accord with that evident from the paleomagnetic data, as well as prolate strain, consistent with transtension. The latter finding suggests that the time-integrated record of shearing captures the contemporary, depth dependent plane strains as a 3-dimensional deformation. These rocks, in fact, record evidence for partitioning of strain at fine spatial scales with faults that record crustal thinning and crustal thickening intimately mixed with the more dominant strike-slip faults. In total, the neotectonic record of non-recoverable strain at Wild Horse Mesa is in accord with (1) expectations based on the current boundary conditions, and (2) models for the formation of the youthful Eastern California shear zone. The faults that provide this record are invariably limited to the outcrop scale, and are interpreted to be akin to the structures that accommodate contemporary background seismicity. These small structures therefore appear to provide an important link between understanding the rock record and contemporary non-recoverable deformation.
DE: 1208 Crustal movements--intraplate (8110)
DE: 7230 Seismicity and seismotectonics
DE: 8123 Dynamics, seismotectonics
DE: 8150 Plate boundary--general (3040)
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting