Fault interaction at the junction of the Transverse Ranges and Eastern California shear zone: a case study of intersecting faults James A. Spotila * , Kevin B. Anderson Virginia Polytechnic Institute and State University, Blacksburg, VA, USA Received 21 August 2002; accepted 29 September 2003 Abstract A case study from southern California illustrates the value of detailed geologic data for understanding the kinematics of complex fault systems. The neotectonic behavior of faults at the junction of the Eastern California shear zone (ECSZ) and Transverse Ranges has implications for the interaction of intersecting, segmented fault systems and regional plate boundary evolution. Paleoseismic observations indicate that the North Frontal thrust system (NFTS) has ruptured once in the Holocene with 1.7-m displacement, despite previous speculations of inactivity based on its dissection by younger strike-slip faults. Simple polyphase deformation, in which dextral shear has replaced and overprinted thrusting, is thus not a valid explanation for this system of intersecting faults. This illustrates the limitations of inferring rupture behavior from mapped fault patterns alone. Neotectonic and geomorphic observations along the thrust system also suggest that the thrust segment west of the intersection with the dextral Helendale fault is significantly more active than the segment to the east. This is consistent with a simple block velocity model, in which dextral slip on the Helendale fault is balanced by convergence on the western thrust segment, dextral motion on the poorly studied Pipes Creek fault to the southeast, and inactivity on the eastern thrust segment. This divides the San Bernardino Mountains into domains dominated by thrusting (west) and strike-slip (east), the union of which is a quasi- stable triple junction. We speculate that this union has migrated to the west as the Mojave Desert has been translated southwards along the San Andreas fault. D 2003 Elsevier B.V. All rights reserved. Keywords: Eastern California shear zone; Transverse Ranges; Faults 1. Introduction Complex fault systems are a common mechanism of accommodating plate motion within the continental lithosphere (Molnar, 1988). Suites of intersecting, conjoined, and otherwise spatially associated faults can represent quasi-stable, least-work means of yield- ing to distributed strain (e.g., Jackson and McKenzie, 1994; Nur et al., 1993). As a result, the kinematics of active continental deformation can be perplexing and associated seismicity patterns difficult to forecast. Understanding the interaction of such complex fault systems, with respect to the long-term evolution of plate boundaries and the dynamic behavior of rupture cycles, is an important challenge. 0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2003.09.016 * Corresponding author. Geological Sciences, 4044 Derring Hall, Virginia Tech, Blacksburg, VA 24061, USA; Tel.: +1-540- 231-2109; fax: +1-540-231-3386. E-mail address: spotila@vt.edu (J.A. Spotila). www.elsevier.com/locate/tecto Tectonophysics 379 (2004) 43– 60