Research Paper GEOSPHERE Morphology, structure, and kinematics of the San Clemente and Catalina faults based on high-resolution marine geophysical data, GEOSPHERE, v. 16, no. 5 southern California Inner Continental Borderland (USA) https://doi.org/10.1130/GES02187.1 Maureen A.L. Walton1, Daniel S. Brothers1, James E. Conrad1, Katherine L. Maier1,*, Emily C. Roland2, Jared W. Kluesner1, and Peter Dartnell1 1Pacific Coastal and Marine Science Center, U.S. Geological Survey, Santa Cruz, California 95060, USA 15 figures; 1 set of supplemental files 2School of Oceanography, University of Washington, 1501 NE Boat Street, Seattle, Washington 98195, USA CORRESPONDENCE: [email protected] ABSTRACT fault of southeastern Alaska [USA], North Anatolian fault of Turkey, Alpine fault of New Zealand), and are capable of generating large (M6+) earthquakes (e.g., CITATION: Walton, M.A.L., Brothers, D.S., Conrad, J.E., Maier, K.L., Roland, E.C., Kluesner, J.W., and Catalina Basin, located within the southern California Inner Continental Stein et al., 1997; Hauksson et al., 2012; Howarth et al., 2012; Yue et al., 2013). Dartnell, P., 2020, Morphology, structure, and kine- Borderland (ICB), United States, is traversed by two active submerged fault Despite their proximity to large population centers, particularly near coasts, there matics of the San Clemente and Catalina faults based systems that are part of the broader North America–Pacific plate boundary: is much we do not yet understand about the way strike-slip systems form and on high-resolution marine geophysical data, southern California Inner Continental Borderland (USA): Geo- the San Clemente fault (along with a prominent splay, the Kimki fault) and the deform. For example, the recent 2016 Mw 7.8 Kaikoura earthquake of New Zea- sphere, v. 16, no. 5, p. 1312– 1335, https://doi .org /10 Catalina fault. Previous studies have suggested that the San Clemente fault land (e.g., Hollingsworth et al., 2017) highlights the complexity involved during .1130 /GES02187.1. (SCF) may be accommodating up to half of the ~8 mm/yr right-lateral slip a major strike-slip fault rupture and the need for improved understanding of distributed across the ICB between San Clemente Island and the mainland strike-slip systems. Ongoing research aims to better characterize how, why, and Science Editor: Andrea Hampel coast, and that the Catalina fault (CF) acts as a significant restraining bend in where strike-slip faults form, generate earthquakes, partition slip in oblique envi- Associate Editor: James A. Spotila the larger transform system. Here, we provide new high-resolution geophysical ronments, and behave at stepovers and endpoints, and ultimately, how to use Received 6 August 2019 constraints on the seabed morphology, deformation history, and kinematics tectonic geomorphology to quantify deformation and potential geohazards. Revision received 1 April 2020 of the active faults in and on the margins of Catalina Basin. We significantly High-resolution constraints on fault geometry are particularly important for under- Accepted 27 May 2020 revise SCF mapping and describe a discrete releasing bend that corresponds standing Quaternary fault deformation history (e.g., Brothers et al., 2015) and with lows in gravity and magnetic anomalies, as well as a connection between for characterizing active fault systems, because even a subtle geometry change Published online 10 July 2020 the SCF and the Santa Cruz fault to the north. Subsurface seismic-reflection can inhibit or promote earthquake rupture propagation (e.g., Wesnousky, 2006). data show evidence for a vertical SCF with significant lateral offsets, while The California Inner Continental Borderland (ICB) offshore of southern the CF exhibits lesser cumulative deformation with a vertical component California (United States) and northern Baja California (Mexico) (Fig. 1) offers indicated by folding adjacent to the CF. Geodetic data are consistent with SCF an opportunity to examine a set of active strike-slip faults that accommodate right-lateral slip rates as high as ~3.6 mm/yr and transpressional convergence as much as 8 mm/yr of right-lateral shear, or ~15% of the total Pacific–North of <1.5 mm/yr accommodated along the CF. The Quaternary strands of the SCF America plate boundary slip budget of 48–50 mm/yr (Platt and Becker, 2010; and CF consistently cut across Miocene and Pliocene structures, suggesting DeMets and Merkouriev, 2016). Several significant earthquakes have occurred generation of basin and ridge morphology in a previous tectonic environment along offshore faults in the ICB, including the 1981 Mw 6.0 Santa Barbara that has been overprinted by Quaternary transpression. Some inherited crustal Island earthquake, the 1986 Mw 5.8 Oceanside sequence, the recent 2018 Mw fabrics, especially thinned crust and localized, relatively hard crustal blocks, 5.3 Santa Cruz Island event, the 1951 Ms 5.9 San Clemente Island earthquake, appear to have had a strong influence on the geometry of the main trace of and the largest recorded ICB earthquake to date, the Ms 6.2 offshore Ensenada the SCF, whereas inherited faults and other structures (e.g., the Catalina Ridge) earthquake of 1964 (Richter, 1958; Allen et al., 1960; Hauksson and Jones, 1988; appear to have minimal influence on the geometry of active faults in the ICB. Pacheco and Nábělek, 1988; Bent and Helmberger, 1991; Astiz and Shearer, 2000, Legg et al., 2015) (Fig. 1). Shaking from earthquake ruptures can also enhance the risk of local tsunamis via uplift at restraining bends or coseismic ■ INTRODUCTION slope failure (e.g., Legg and Borrero, 2001; Legg et al., 2004a). Several large submarine landslides have been documented in the offshore California border- Strike-slip faults are characteristic of continental transform plate boundaries land (Bohannon and Gardner, 2004; Locat et al., 2004; Normark et al., 2004a; worldwide (e.g., San Andreas fault of southern California [USA], Queen Charlotte Lee et al., 2009; Legg and Kamerling, 2003; Brothers et al., 2018). This paper is published under the terms of the CC-BY-NC license. *Now at National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Hataitai, Wellington 6021, New Zealand © 2020 The Authors GEOSPHERE | Volume 16 | Number 5 Walton et al. | Morphology, structure, and kinematics of the San Clemente and Catalina faults Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/5/1312/5151191/1312.pdf 1312 by guest on 02 October 2021 Research Paper Understanding how one structure can lead to the formation of another is N 120º W 119º W 118º W 117º W critical for accurate interpretation of tectonic geomorphology and geohazard 35º 0 30 60 N assessments. It is well understood that pre-existing crustal fabrics can influ- km ence strike-slip fault propagation or fault reactivation (e.g., Christie-Blick and SAF Biddle, 1985; Scholz, 2002; Cunningham and Mann, 2007). Numerous studies bathy elev. (m) have documented reactivation of pre-existing fault structures, including in our SBB WTR 0 N study area offshore of southern California (e.g., Crouch and Suppe, 1993; Fisher −2500 −5000 et al., 2009; Sorlien et al., 2015), and others have analyzed the effects of crustal 34º LA SMB structures on strike-slip fault geometry (e.g., Johnson and Watt, 2012; Johnson 2018 Fig. 6B et al., 2018). Christie-Blick and Biddle (1985) differentiated between “essential” SCB SPB 1981 and “incidental” pre-existing structures, i.e., structures that significantly influ- CF ence strike-slip fault geometry and propagation and those that are inherited and OCB N SCFCB do not affect strike-slip deformation, respectively. The study of essential versus Fig. 14 Fig. 6A ~8 mm/yr1986 incidental pre-existing structures and their relative influence on the development 33º SNB ICB SDT of active fault systems is an important topic in crustal deformation research. SD Fig. 2, Fig. 13 1951 This study focuses on characterizing active structures and deformation U.S. in and on the margins of Catalina Basin, an understudied region of the ICB MEX (Figs. 1, 2A), which we interpret to contain two Holocene-active offshore fault N zones—the San Clemente and Catalina fault zones. Four of the aforemen- tioned >M5 earthquakes in the ICB occurred near Catalina Basin (Astiz and 32º Fig. 12 Shearer, 2000), yet neither the San Clemente fault (SCF) nor the Catalina fault (CF) have been examined systematically with modern high-resolution marine Figure 1. Inner Continental Borderland (ICB) location map (southern California, geophysical data within Catalina Basin. Additionally, the SCF alone may be USA) showing the National Centers for Environmental Information Southern accommodating as much as 4–6 mm/yr of right-lateral slip based on geologic California Coastal Relief Model (version 2) bathymetry (Calsbeek et al., 2013), ESRI topography, approximate regional geologic boundaries (bold black dashed lines data and GPS models, about half of the total slip taken up within the ICB (Legg, with bold labels), and southern California faults (red lines) from the U.S. Geological 1985, 1991a, 2005; Larson, 1993; Bennett et al., 1996; Humphreys and Weldon, Survey and California Geological Survey Quaternary Fault and Fold Database for 1994; Goldfinger et al., 2000), and the CF has been thought to be convergent the United States (https://www.usgs.gov /natural -hazards /earthquake -hazards / faults). Approximate epicenters of the 1951 M 5.9 San Clemente Island earthquake, and thus tsunamigenic (e.g., Legg and Borrero, 2001). s 1981 Mw 6.0 Santa Barbara Island earthquake, 1986 Mw 5.8 Oceanside sequence, Numerous important studies over the past few decades have described and 2018 Mw 5.3 Santa Cruz Island earthquake are highlighted with yellow stars. a first-order tectonic and geologic framework for the ICB and Catalina Basin; The 8 mm/yr value is GPS-modeled slip accommodated within the ICB from Platt and Becker (2010). Locations of Figures 2, 12, and 13 are outlined in dashed white.