Provenance of the upper Miocene Modelo Formation and subsidence analysis of the Los Angeles basin, southern California: Implications for paleotectonic and paleogeographic reconstructions Peter E. Rumelhart Department of Earth and Space Sciences, University of California, Raymond V. Ingersoll* } Los Angeles, California 90095-1567 ABSTRACT tion was transferred to the San Andreas fault; transpression has dom- inated the Los Angeles basin since 6 Ma, including rapid uplift, flexural The Los Angeles basin has undergone three stages of development subsidence due to tectonic loading, and rapid sedimentary filling. The related to complex plate interactions within the evolving San Andreas rapid subsidence and filling and the sudden switch between transten- transform system: transrotation (16–12 Ma), transtension (12–6 Ma), sion and transpression in the Los Angeles basin are typical of strike-slip and transpression (6–0 Ma). Timing of these stages correlates with basins in general. However, initiation of the Los Angeles basin by tran- microplate-capture events along the continental margin, and is ex- srotation reflects the uncommon process of microplate capture along pressed in changes in subsidence rates and provenance within the Los the rapidly evolving California margin. Angeles basin. The Modelo Formation and related units were deposited in the INTRODUCTION northern part of the Los Angeles basin at bathyal depths during late Miocene time. The northern Los Angeles basin was segmented into The compositional evolution of siliciclastic sediment of strike-slip basins three subbasins, in each of which coarse sediment was deposited as sub- is not well understood. Other than studies by Link (1982) of the Ridge basin marine fans (Puente, Tarzana, and Simi). A fourth fan system (Piru) in southern California, by Ridgeway and DeCelles (1993) of the Burwash formed in the Ventura basin, just north of the Los Angeles basin. The basin in the Yukon, and by Critelli et al. (1995) of the Puente Formation of Puente, Tarzana, and Piru fans were derived from the San Gabriel the Los Angeles basin in southern California, there is a dearth of detailed block, which consists primarily of crystalline basement and lesser vol- petrologic studies of strike-slip basins. canic and sedimentary components. Sandstone within the Puente fan The Los Angeles basin is recognized as an excellent example of sedi- reflects unroofing of the central and eastern San Gabriel block. The mentation in a strike-slip setting (Biddle, 1991). It has been proposed that Tarzana fan was derived primarily from the central San Gabriel block, the basin formed as the result of clockwise crustal rotations during middle and the Piru fan was derived primarily from the western San Gabriel and late Miocene time (Luyendyk and Hornafius, 1987). As such, it is one block, which is distinctly characterized by Ca-rich plagioclase derived of the only known examples of a transrotational basin (Ingersoll, 1988). from Proterozoic anorthosite and related bodies. The lack of Ca-rich Furthermore, because of its prolific hydrocarbon production, the Los Ange- plagioclase in the other fans eliminates the western San Gabriel block les basin has been the subject of intense industry interest over the past cen- as a possible source area, and confirms differentiation of the Ventura tury (Biddle, 1991). Despite its eminence, many of the important lithologic basin from the Los Angeles basin by late Miocene time. The Simi fan units have not been petrographically investigated, nor have petrographic was derived from locally uplifted Cretaceous and Paleogene strata; data been incorporated into stratigraphic and tectonic interpretations of the sandstone composition reflects the recycling of these sediments. basin. In this paper we summarize an investigation of sandstone within the Subsidence and provenance analyses are consistent with the follow- upper Miocene Modelo Formation of the Santa Monica Mountains. Using ing paleogeographic and paleotectonic reconstruction. Beginning at ap- petrologic data, as well as microprobe compositional data from detrital pla- proximately 16 Ma, transrotation of the Western Transverse Ranges in- gioclase grains, we suggest a possible provenance within the San Gabriel duced extension and thermal subsidence of the Los Angeles basin area. Mountains. Similarly, very few subsidence studies of the Los Angeles basin A second pulse of extension and thermal subsidence occurred when have been published (e.g., Mayer, 1987, 1991; Sawyer et al., 1987). On the motion began along the San Gabriel fault at 12 Ma. Right slip of basis of our provenance and subsidence data, we propose a paleotectonic 60–70 km occurred along the San Gabriel fault, which produced and paleogeographic reconstruction for the northern Los Angeles and east- transtension in the Los Angeles basin area and deposition of the ern Ventura basin regions during middle to late Miocene time. Puente, Tarzana, Simi, and Piru fan systems. At 6 Ma, transform mo- The Los Angeles basin is one of many Neogene basins along the western margin of California (e.g., Blake et al., 1978; Crowell, 1974, 1987; Dickin- *Corresponding author. E-mail: [email protected] son et al., 1987; Luyendyk and Hornafius, 1987; Mayer, 1987, 1991; Yeats, Data Repository item 9733 contains additional material related to this article. GSA Bulletin; July 1997; v. 109; no. 7; p. 885–899; 8 figures; 3 tables. 885 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/109/7/885/3382736/i0016-7606-109-7-885.pdf by guest on 29 September 2021 RUMELHART AND INGERSOLL Figure 1. Generalized geologic map of Los Angeles basin and San Gabriel Mountains and locations of Modelo Formation outcrops. C—Cal- abasas area; EVB—eastern Ventura basin; MCF—Malibu Coast fault; SMS—Santa Monica Slate; TA—Topanga anticline; L—location of Laubacher #1 well (actual location is the western edge of the Santa Monica Mountains (SMM), approximately 15 km west of figure margin); R— location of Rancho #1 well; S—location of Simi #33 well. Inset: LA—Los Angeles basin; SB—Santa Barbara basin. 1987; Biddle, 1991; Wright, 1991). It is located at the northern end of the Pre–Los Angeles basin strata consist mostly of material associated with Peninsular Ranges, and is bounded on the north by the San Gabriel fault, on the Mesozoic-Paleogene convergent margin. The basement includes Juras- the east by the San Andreas fault, and on the west by the southern Califor- sic to Cretaceous metasedimentary rocks of the subduction complex (e.g., nia continental borderland (Fig. 1). The Los Angeles basin is a small (30 km Catalina schist, Pelona schist), as well as granodiorite and tonalite associ- wide) rhombohedral basin southwest of the San Andreas fault zone, which ated with the Mesozoic magmatic arc (Fig. 2; Yerkes et al., 1965). Locally, forms the present boundary between the Pacific and North American plates Precambrian metamorphic rocks (gneiss of the San Gabriel Mountains) are (Atwater, 1970, 1989; Bohannon and Parsons, 1995). The Los Angeles also part of the basement (Ehlig, 1981). Pre-basin rocks also include sedi- basin formed in a former forearc area, which encompassed the length of the mentary and volcanic rocks of the forearc basin (e.g., Tuna Canyon, Coal west coast of North America during the Mesozoic and Paleogene (Dickin- Canyon, Trabuco, Ladd, Williams, Silverado, Santiago, Santiago Peak, son, 1976, 1981; Crouch and Suppe, 1993). Transform tectonism initiated Sespe and Vaqueros formations; Fig. 2). approximately 25 Ma, when the Pacific-Farallon ridge reached the conti- The oldest sedimentary unit in the Los Angeles basin proper is the lower– nental margin (Atwater, 1970, 1989; Dickinson, 1981; Bohannon and Par- middle Miocene Topanga Group. The Topanga Group consists of conglom- sons, 1995). The basin began subsiding approximately 18–16 Ma (Horna- erate, sandstone, and mudstone deposited at middle-bathyal to nonmarine fius et al., 1986; Mayer, 1991; Rumelhart, 1994a), probably in association depths (Yerkes et al., 1965; Lane, 1987; Blake, 1991). Interfingering with with the onset of clockwise rotation of the western Transverse Ranges and included within the Topanga Group are several volcanic units (e.g., (Luyendyk and Hornafius, 1987; Crouch and Suppe, 1993), and continued Conejo and El Modeno volcanic rocks) that reflect crustal extension (Yerkes until about 3 Ma, when north-south shortening resulted in regional uplift et al., 1965). The middle-bathyal to nonmarine San Onofre Breccia both in- (Atwater, 1970; Mayer, 1991; Rumelhart, 1994a). terfingers with and lies unconformably on the Topanga Group (Vedder and 886 Geological Society of America Bulletin, July 1997 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/109/7/885/3382736/i0016-7606-109-7-885.pdf by guest on 29 September 2021 SUBSIDENCE ANALYSIS OF THE LOS ANGELES BASIN Figure 2. Generalized strati- graphic chart for Los Angeles basin (modified from Blake, 1991). Note nonlinear vertical scale. Howell, 1976; Stuart, 1979; Blake, 1991). The San Onofre Breccia contains Formation and below the Pico Formation in the northeastern Ventura basin. clasts, derived from the west, of Catalina Schist, indicating that the metamor- Kew (1924) redefined the Modelo Formation to include parts of the Vaque- phic basement was exposed and shedding sediment into the Los Angeles ros, resulting in the unit being about 2745 m thick in the type area. In 1931, basin by early middle Miocene time. The middle to upper Miocene Mon- Hoots defined middle to upper Miocene strata, which unconformably over- terey Formation lies unconformably on the Topanga Group and locally the lie the Topanga Group (i.e., Calabasas Formation) and are overlain by the San Onofre Breccia (Blake, 1991). The Monterey Formation consists of Pico Formation, as the Modelo Formation (about 1750 m thick). Dibblee siliceous hemipelagic shale deposited at bathyal depths (Pisciotto and Garri- (e.g., 1991, 1992) defined middle and upper Miocene strata of the Santa son, 1981). Coeval with Monterey deposition, the Modelo and Puente for- Monica Mountains as the Monterey Formation.