Transtensional analyses of fault patterns and strain provinces of the Eastern California shear zone–Walker Lane on the eastern margin of the Sierra Nevada microplate, California and Nevada

Abstract

Obliquity of the strain velocity field to the deformation zone boundary requires strain to be triaxial. At the plate boundary scale, fault geometries predicted by transtensional theory better explain observed fault patterns in the northern Eastern California shear zone-Walker Lane area than do 2D plane strain predictions. Structural provinces in the Eastern California shear zone -Walker Lane are defined by variations in the orientation of the eastern margin of the Sierra Nevada microplate, and include the Honey LakePyramid Lake, Lake Tahoe, Mono Lake -Long Valley, Owens Valley, and Coso subprovinces. The local geometry of the transtensional zone boundary and the microplate transport direction determines orientations of the instantaneous strain axes for each province. Fault orientations predicted with respect to these axes are consistent with those observed in each structural province. The variation of zone geometry among predominantly coaxially dominated strain provinces over a consistent period of deformation does not result in dramatic changes in shape or orientation among finite strain ellipsoids. Although Quaternary faulting strongly reflects transtensional kinematics, the magnitude of estimated finite strain for each province suggests that present-day transtensional deformation has not accommodated large amounts of extension, vertical thinning, or strike-slip offset. K-values for all structural provinces plot in the k . 1 constrictional field on the logarithmic Flinn (Ramsay) diagram, indicating non-plane, transtensional strain. Collectively, these theoretical applications and structural -tectonic observations have implications for evaluating kinematics and faulting during brittle non-plane strain deformation, and for the overall evolution of the transtensional Sierra Nevada -North America plate boundary.

The Tahoe-Sierra frontal fault zone, Emerald Bay area, Lake Tahoe, California: History, displacements, and rates

“…1B), the maximum instantaneous stretching direction (X i ) should be oriented roughly east-west. As noted by Taylor and Dewey (2009), the N-S-trending WTDPFZ is a normal fault zone formed perpendicular to X i .…”

“…This study, like many previous reports, emphasizes the fact that geodetic strain rates do not necessarily reflect long-term slip rates on discrete faults. Taylor and Dewey (2009) noted that, with the N35-45°W (320° AZ)-trending TSFFZ setting the kinematic boundary condition and a Sierra Nevada transport direction of ~N50-60°W (305° AZ; Fig. 1B), the maximum instantaneous stretching direction (X i ) should be oriented roughly east-west.…”

“…Such a shear zone model involves plane-strain, simple shear. Some modification of that model may be required because northwest-directed transtension involves constrictional strain rather than plane strain (Dewey, 2002;Taylor and Dewey, 2009) and requires a N-S component of shortening. Additionally, in a shear-zone model, all N-S-striking normal faults in the region, which may reflect the east-west X i as in the Lake Tahoe basin (Taylor and Dewey, 2009), should have rotated progressively clockwise.…”

“…1A). The location and orientation of the active structural boundary are important because kinematic models for transtension (Dewey, 2002;Dewey et al, 1998;Taylor and Dewey, 2009) indicate that they set a kinematic boundary condition (e.g., they determine the direction of maximum instantaneous extension, X i ) for transtensional deformation in the Walker Lane belt to the east. The active boundary may also pose a significant seismic hazard.…”

“…1A and 1B), a complex half-graben, is part of the Walker Lane belt, possibly an incipient plate boundary, and a region of dextral transtensional deformation between the internally unfaulted Sierra Nevada and the extensional Basin and Range Province to the east. Numerous studies in the Walker Lane belt have established the following (see, for example, Bormann et al, 2016;Busby, 2013Busby, , 2016Carlson et al, 2013;Dixon et al, 2000;Faulds and Henry, 2008;Hammond et al, 2011;Hammond and Thatcher, 2007;Lifton et al, 2013;Rood et al, 2011a;Schweickert et al, 2004;Surpless et al, 2002;Taylor and Dewey, 2009;Unruh et al, 2003;Wesnousky et al, 2012): The Walker Lane currently takes up about one fifth of the dextral displacement between the Pacific and North American plates. Kinematic complexity characterizes the Walker Lane, with various parts dominated by normal faults, dextral or sinistral strike-slip faults, and/or vertical-axis rotations of crustal blocks.…”