Kinematics and Evolution of the Southern Eastern California Shear Zone, Based on Analysis of Fault Strike, Distribution, Activity, Roughness, and Secondary Deformation

Abstract

The Eastern California shear zone (ECSZ) is a major element of the Pacific-North America plate boundary that has been extensively studied, yet displays unexplained kinematic traits. The shear zone is wide and consists of discontinuous, irregular strike-slip faults, rather than a well-integrated fault zone. Individual faults in the ESCZ are more closely spaced and geometrically complex than other comparable secondary zones of strike-slip, such as to the west of the southern San Andreas fault (SAF), to the east of the SAF in northern California, and in the Marlborough fault system of New Zealand. The geometric complexity of faults in the ECSZ also results in complex stepovers and strain transfer zones, which impact rupture pattern and strain partitioning in unknown ways. Kinematic models of the spatiotemporal evolution and rotation of the ECSZ exist, yet these have not been thoroughly tested (Dixon & Xie, 2018;Dokka & Travis, 1990a, 1990bNur et al., 1993). There is thus need for systematic study of the kinematics of faulting in the ECSZ.The ECSZ is also an ideal setting to investigate the influence of geometry on fault processes. The geometry and kinematics of faults and fault networks are relevant for both our understanding of their mechanical behavior and for seismic hazards. Geometric complexity on faults is a form of roughness, which can influence rupture dimension and style, fault strength and stress field, slip distribution and heterogeneity during rupture, and earthquake recurrence (

The Mojave Section of the San Andreas Fault (California), 2: Pleistocene Records of Near‐Field Transpression Illuminate the Atypical Evolution of the Restraining “Big Bend”

“…The energetic cost of this extrusion is likely minimal along its northwestern and southwestern boundaries as it involves mechanically efficient conjugate faulting along mature low‐roughness strike‐slip faults (the MSAF and the western segment of the Garlock Fault) (Hatem & Dolan, 2018; McGill et al., 2009). At the eastern boundary (i.e., the Eastern California Shear Zone), extrusion might be facilitated by the decoupling of crustal deformation from a fixed deeper shear zone, as possibly recorded by a progressive westward migration of the activity of the ECSZ through time (Dixon & Xie, 2018; Spotila & Garvue, 2021). It remains however unclear how the ECSZ absorbs the extrusion: even if assuming the model of Dixon and Xie (2018) is correct, we need a mechanism to absorb the ∼E‐W shortening required by the convergence between Western Mojave and the North America plate (see further discussion below).…”

What Controls Early Restraining Bend Growth? Structural, Morphometric, and Numerical Modeling Analyses From the Eastern California Shear Zone

“…The restraining bends in the SECSZ are distinct, selfsimilar zones of warped primary fault geometry, secondary faulting, and entirely basement-cored (granitic, dioritic, and metamorphic bedrock) uplifts of low to moderate relief. We have identified 22 restraining bends with clear expressions, although there are less obvious examples with lower relief (or masked inherited relief) and more obscure secondary deformation (Spotila & Garvue, 2021; their Figure 13). By mapping the form, modeling Tectonics 10.1029/2023TC008148 the growth, and describing the kinematic evolution of SECSZ restraining bends, we seek to relate deformation character to variable fault boundary conditions, answer whether bends simplify or complexify with increasing deformation and fault interaction, and test what role transpression plays in the early stage of strike-slip fault evolution.…”

“…A second proposed cause of transpression is progressive counterclockwise rotation of right‐lateral faults via bookshelf faulting or due to local rotation of the direction of maximum compression, which would currently favor shear on N‐trending faults and transpression on older NW‐trending faults (Nur et al., 1993). While significant clockwise fault block rotations are supported for the left‐lateral domains of the ECSZ, including the Eastern Transverse Ranges and the northeast quadrant of the Mojave block (e.g., Carter et al., 1987; Schermer et al., 1996), significant counterclockwise block rotation following the formation of the right‐lateral faults of the Mojave block (<10 Ma) is neither supported by paleomagnetic studies (e.g., Golombek & Brown, 1988; Valentine et al., 1993; Wells & Hillhouse, 1989) nor geomorphic expression (Spotila & Garvue, 2021). A third and final explanation for initiation of transpression in the SECSZ is that warping of the closely spaced primary faults occurs due to growth of secondary structures, differential loading, penetrative strain, and repeated multi‐fault surface ruptures (Spotila & Garvue, 2021).…”

“…While significant clockwise fault block rotations are supported for the left‐lateral domains of the ECSZ, including the Eastern Transverse Ranges and the northeast quadrant of the Mojave block (e.g., Carter et al., 1987; Schermer et al., 1996), significant counterclockwise block rotation following the formation of the right‐lateral faults of the Mojave block (<10 Ma) is neither supported by paleomagnetic studies (e.g., Golombek & Brown, 1988; Valentine et al., 1993; Wells & Hillhouse, 1989) nor geomorphic expression (Spotila & Garvue, 2021). A third and final explanation for initiation of transpression in the SECSZ is that warping of the closely spaced primary faults occurs due to growth of secondary structures, differential loading, penetrative strain, and repeated multi‐fault surface ruptures (Spotila & Garvue, 2021). This explanation is supported by (a) a power‐law relationship between fault strike variability, a metric of fault roughness, and fault maturity, where the SECSZ faults have relatively sinuous and variable trends when compared to faults with greater net‐slip and slip‐rate and (b) a greater number and degree of transpressional than transtensional features (i.e., numerous restraining bends and basin inversion between faults) (Spotila & Garvue, 2021).…”

“…A third and final explanation for initiation of transpression in the SECSZ is that warping of the closely spaced primary faults occurs due to growth of secondary structures, differential loading, penetrative strain, and repeated multi‐fault surface ruptures (Spotila & Garvue, 2021). This explanation is supported by (a) a power‐law relationship between fault strike variability, a metric of fault roughness, and fault maturity, where the SECSZ faults have relatively sinuous and variable trends when compared to faults with greater net‐slip and slip‐rate and (b) a greater number and degree of transpressional than transtensional features (i.e., numerous restraining bends and basin inversion between faults) (Spotila & Garvue, 2021). This suggests that the SECSZ faults are immature and have yet to smoothen via strain‐weakening where faults eventually integrate into a narrow through‐going zone of shear.…”