Rivero, Carlos, and John H. Shaw, 2011, Active folding and blind thrust faulting induced by basin inversion processes, inner California borderlands, in K. McClay, J. Shaw, and J. Suppe, eds., Thrust fault-related folding: AAPG Memoir 94, 9 p. 187 – 214. Active Folding and Blind Thrust Faulting Induced by Basin Inversion Processes, Inner California Borderlands Carlos Rivero1 Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. John H. Shaw Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. ABSTRACT The present bathymetry, basin geometries, and spatial earthquake distribution in the inner California borderlands reflect complex basin inversion processes that reactivated two low- angle Miocene extensional detachments as blind thrust faults during the Pliocene to Holocene. The Oceanside and the Thirtymile Bank detachments comprise the inner California blind thrust system. These low-angle detachments originated during Neogene crustal extension that opened the inner California borderlands, creating a rift system that controlled the deposition of early to late Miocene sedimentary units and the exhumation of the metamorphic Catalina schist. During the Pliocene, a transpressional regime induced by oblique convergence between the Pacific and the North American plates reactivated the Oceanside and the Thirtymile Bank detachments as blind thrust faults. This reactivation generated regional structural wedges cored by faulted basement blocks that inverted the sedimentary basins in the hanging wall of the Miocene extensional detachments and induced contractional fold trends along the coastal plain of Orange and San Diego counties. Favorably oriented high-angle normal faults were also reactivated, creating zones of oblique and strike-slip faulting and folding such as the offshore segments of the Rose Canyon, San Diego, and the Newport-Inglewood fault zones. We eval- uate several different styles of geometric and kinematic interactions between these high-angle strike-slip faults and the low-angle detachments, and favor interpretations where deep oblique slip is partitioned at shallow crustal levels into thrusting and right-lateral strike-slip faulting. Analyses of seismic reflection profiles, well data, earthquake information, and sea-floor geology indicate that the Oceanside and the Thirtymile Bank blind thrust faults are active and represent important sources of earthquakes in this region. Restored balanced cross sections provide a minimum southwest-directed slip of 2.2–2.7 km (1.4–1.8 mi) on the Oceanside thrust and illustrate the function of this detachment in controlling the processes of basin inversion and the development of the overlying fold and thrust belt. 1Present address: Structural Geology Team, Chevron Exploration Technology Company, Houston, Texas. Copyright n2011 by The American Association of Petroleum Geologists. DOI:10.1306/13251338M943432 187 188 Rivero and Shaw INTRODUCTION translation of the Transverse Ranges from the south to its present location (Yeats, 1976; Crouch and Suppe, The southern margin of the inner California border- 1993; Nicholson et al., 1993; Bohannon and Geist, 1998; land province (sICB) comprises the San Pedro shelf and Ingersoll and Rumelhart, 1999). Rivero et al. (2000) sug- the Oceanside-Capistrano Basin (Figure 1). This region gested that the seismogenic inner borderland blind is limited to the north by the Los Angeles Basin, to the thrust system originated when two of these detachments east by the coastal plain of the Orange and San Diego were reactivated by basin inversion processes initiated counties, to the west by the Santa Catalina Island, and to in the late Pliocene, during the onset of the modern the south by the United States-Mexico maritime bound- transpressional regime (Figure 2). Here we confirm and ary (Vedder, 1976; Drewry and Victor, 1997). elaborate on these results, showing that large-scale thrust Geologic data from nearby islands, sea-floor sam- faulting and folding are driven by structural wedging ples, and oil industry wells indicate that the sedimen- (Medwedeff, 1992) and argue that basin inversion pro- tary rocks deposited in the sICB are mainly Quaternary cesses are the primary tectonic mechanisms controlling to lower Miocene turbiditic and fluvial sand-shale de- the physiography and geology of the southern margin posits overlying Cretaceous to Paleogene marine con- of the inner borderlands. glomerate, sandstone to siltstone formations, and Me- To document these basin inversion mechanisms, we sozoic Catalina schist (Legg, 1980; Vedder et al., 1986; analyze more than 10,000 km (6214 mi) of industry seismic Clarke et al., 1987, among others). reflection profiles, well data, seismicity, and sea-floor The main physiographic characteristic of the sICB is geologic maps. We perform a structural analysis that a series of elongated basins and ridges trending sub- involves kinematic and forward modeling techniques parallel to the coast and to the relative slip vector between based on quantitative structural relationships between the Pacific and the North American plates (Luyendyk, fold and fault shapes (Suppe, 1983; Mount et al., 1990; 1991; Atwater and Stock, 1998). Based on the presence Erslev, 1991; Suppe and Medwedeff, 1992; Allmendinger, of these physiographic features, geophysical data, and 1998). We also use advanced three-dimensional (3-D) coastal geology, several authors have attributed the modeling techniques to generate precise representa- earthquake activity in this part of the inner California tions of fault surfaces and key stratigraphic markers borderlands to a regional system of active strike-slip (Plesch et al., 2007). The lateral extent and geometry of faults similar to that in the onshore region around the the active blind thrust ramps and fold trends are de- Peninsular Ranges (e.g., Legg and Ortega, 1978; Clarke termined by mapping of direct fault-plane reflections et al., 1987; Legg, 1989). However, the inner California and folded reflections throughout the basin areas cov- borderlands do not display the apparent spatial corre- ered by the seismic grid (Figures 2, 3). The 3-D mod- lation between earthquake activity and regional strike- eling was also used to quantify the distribution of dip slip fault zones that is observed around the onshore slip on the active fault system and to further constrain region of the Peninsular Ranges (Figure 1). In contrast, the geometric analysis. seismicity in this area is diffuse and scattered (Ziony and To illustrate our findings, we present new regional Jones, 1989; Astiz and Shearer, 2000; Richards-Dinger structural interpretations of the active submarine fold and Shearer, 2000). Poorer offshore earthquake locations and thrust belt located offshore Dana Point, and new because of limited station coverage presumably con- geometric representations oftheoffshoreNewport- tribute to this pattern; however, they alone are insuf- Inglewood, Rose Canyon, and San Diego Trough fault ficient to explain why seismicity is not localized along a zones. These new structural interpretations are consis- few major offshore strike-slip faults. Moreover, the focal tent with basin inversion processes and the presence mechanism of the 1986 (5.6 local magnitude scale) Ocean- of both active blind thrust and strike-slip faults in the side earthquake, the largest recorded event in the re- southern inner California borderlands. gion, indicates faulting dominated by thrust motion (Hauksson and Jones, 1988; Pacheco and Na´beˆlek, 1988). Thus, we interpret these observations to reflect a com- BASIN INVERSION plex mixture of strike-slip and blind thrust faulting in the inner borderlands that is similar to the style of defor- Formation of sedimentary basins commonly involves mation in the onshore Los Angeles Basin (Hauksson, extensional deformation of the crust, with concomi- 1990; Wright, 1991; Shaw and Suppe, 1996). tant development of rifting and normal faulting (Bally During the past decade, several authors have de- and Snelson, 1980; Allen and Allen, 1990). Subsequent scribed the function of a Miocene system of low-angle compressional-transpressional tectonic phases gener- normal faults in the Neogene opening of the inner ally induce the contraction of the basins in a process borderland province and in the clockwise rotation and called basin inversion or inversion tectonics (Gleinner Active Folding and Blind Thrust Faulting Induced by Basin Inversion Processes 189 Figure 1. Map of the inner California borderlands and the study area. Major surface fault traces and earthquake locations (1977 to 2000) are shown (from Richards-Dinger and Shearer, 2000). The earthquake focal mechanism represents the 1986 (5.3 local magnitude scale) Oceanside earthquake location from Astiz and Shearer (2000). Pacific NOAM slip vector (PACI) from McCaffrey (2005). L. A. = Los Angeles Basin; SCI = Santa Catalina Island; SCL = San Clemente Island. The bathymetric contour interval is 200 m (656 ft). Digital southern California topography was generated from digital elevation data provided by the U.S. Geological Survey. and Boegner, 1981; Bally, 1984), characterized by up- deforms shallower strata and older structures and may lifting, flexure, and folding of the basin floor and the induce deep-seated structural wedging and back thrust- sedimentary infill (Ziegler, 1983; Williams et al., 1989; ing (Roure et al., 1990). At shallow levels, the propaga- Letouzey, 1990) (Figure 4). tion of the deformation