Mountain Meadows Dacite: Oligocene intrusive complex that welds together the Los Angeles Basin, northwestern Peninsular Ranges, and central Transverse Ranges, California

Abstract

Dikes and irregular intrusive bodies of distinctive Oligocene biotite dacite and serially related hornblende latite and felsite occur widely in the central and eastern San Gabriel Mountains, southern California, and are related to the Telegraph Peak granodiorite pluton. Identical dacite is locally present beneath Middle Miocene Topanga Group Glendora Volcanics at the northeastern edge of the Los Angeles Basin, where it is termed Mountain Meadows Dacite. This study mapped the western and southwestern limits of the dacite distribution to understand the provenance of derived redeposited clasts, to perceive Neogene offsets on several large strikeslip faults, to test published palinspastic reconstructions, and to better understand the tectonic boundaries that separate contrasting pre-Tertiary rock terranes where the Peninsular Ranges meet the central and western Transverse Ranges and the Los Angeles Basin. Transported and redeposited clasts of dacite-latite occur in deformed lower Miocene and lower middle Miocene sandy conglomerates (nonmarine, nearshore, and infrequent upper bathyal) close to the northern and northeastern margins of the Los Angeles Basin for a distance of nearly 60 km. Tie-lines between distinctive source suites and clast occurrences indicate that large tracts of the ancestral San Gabriel Mountains were elevated along range-bounding faults as early as 16-15 Ma. The tie-lines prohibit very large strike-slip offsets on those faults. Transport of eroded dacite began south of the range as early as 18 Ma. Published and unpublished data about rocks adjacent to the active Santa Monica-Hollywood-Raymond oblique reverse left-lateral fault indicate that cumulative left slip totals 13-14 km and total offset postdates 7 Ma. This cumulative slip, with assembly of stratigraphic and paleogeographic data, invalidates prior estimates of 60 to 90 km of left slip on these faults beginning about 17-16 Ma. A new and different palinspastic reconstruction of a region southwest of the San Andreas Fault Zone is proposed. Our reconstruction incorporates 20° of clockwise rotation of tracts north of the Raymond Fault from the easternmost Santa Monica Mountains to the Vasquez Creek Fault (San Gabriel south branch). We interpret the Vasquez Creek Fault as a reverse and right-lateral tear fault. Right slip on the tear becomes reverse dip slip on the northeast-striking Clamshell-Sawpit fault complex, interpreted as an offset part of the Mount Lukens Fault. This explains the absence of evidence for lateral offset of the Glendora Volcanics and associated younger marine strata where those are broken farther east by the eastern Sierra Madre reverse fault system. About 34 km of right slip is suggested for all breaks of the San Gabriel fault system. New paleogeographic maps of the Paleogene basin margin and of a Middle Miocene marine embayment and strandline derive in part from our palinspastic reconstruction. These appealingly simple maps Þ t well with data from the central Los Angeles Basin to the south and southwest.

Tectonics of Ridge Basin region, Southern California

“…The southern branch, referred to also as the Vasquez Creek fault (Powell, 1993), joins the Sierra Madre fault zone that forms the steep southern face of the San Gabriel Mountains. This scarp extends on eastward to the Cajon Pass region and is the result of displacements on the Sierra Madre, Cucamonga, and other faults (Ehlig, 1981;Bortugno and Spittler , 1986;Morton and Matti, 1987;Powell, 1993;Yeats et al, 1994;Ingersoll and Rumelhart, 1999;Dolan et al, 2000;McCulloh et al, 2001). Powell (1993) concluded tentatively that three faults make up the San Gabriel system, in age from oldest to youngest: Canton, northern branch of San Gabriel, and Vasquez Creek.…”

“…Southeastward from the Castaic Lowland, I propose that the Canton fault, as described below, extended into the San Fernando Valley. This fault is now abandoned and buried beneath Miocene Modelo beds and probably follows along the southwest side of a subsurface basement ridge (Yeats et al, 1994;McCulloh et al, 2001;Yeats and Stitt, this volume, Chapter 5). Northwest of the Castaic Lowland, however, I assign ~45 km of displacement to the San Gabriel fault, approximately along the stretch of the older Canton fault.…”

“…The Canton fault was active previous to Modelo deposition; the width and severe deformation within its shear zone beneath the Modelo indicates that it was already a major fault in pre-Modelo time. I suggest below that the Canton fault extends southward from the Castaic Lowland and into the complex region at the plunging western end of the San Gabriel Mountains where it is overprinted by the Santa Susana thrust (Yeats et al, 1999;McCulloh et al, 2001). The San Gabriel fault, younger than the Canton, followed along the course of the Canton fault in the Ridge Basin area.…”

“…The northern branch is offset by cross-cutting faults in the eastern San Gabriel Mountains, and farther to the east, enters the complex Cajon Pass region (Morton and Miller, 1975;Matti et al, 1985;Bortugno and Spittler, 1986;Meisling and Weldon, 1989;Matti and Morton, 1993;Morton and Matti, 1993;Weldon et al, 1993;Powell, 1993;Ingersoll and Rumelhart, 1999;Dolan et al, 1997Dolan et al, , 2000McCulloh et al, 2001). The southern branch, referred to also as the Vasquez Creek fault (Powell, 1993), joins the Sierra Madre fault zone that forms the steep southern face of the San Gabriel Mountains.…”

Ridge Basin and San Gabriel Fault in the Castaic Lowland, Southern California

“…Key piercing points: Basal Caliente Formation against Mint Canyon Formation, granitic rocks (GR) and Paleogene sequence (PA) from upper Piru and Canton creeks to subsurface near Placerita Oil Field; offset of Lowe Granodiorite (L-L), which is similar to offset of western edge of Mountain Meadows Dacite (not shown), fan heads of Devil Canyon and Hasley conglomerates (open arrow), breccia derived from San Gabriel fault, including Violin Breccia and breccia in upper Mint Canyon Formation (Tmc-br) (open teeth in direction of downthrow). Numbers in parentheses are right separation: 31 ± 5 km on Canton fault, this chapter; 42-75 km on San Gabriel fault north of intersection with Canton fault (larger number from Crowell, this volume, Chapter 6; smaller number from Powell, 1993); 25+ km on San Gabriel fault for offset of Devil Canyon and Hasley conglomerates from San Gabriel Mountains; 20-22 km on San Gabriel fault east of intersection with Vasquez Creek fault (fromEhlig, 1981, andMcCulloh et al, 2001); 5-13 km on Vasquez Creek fault (fromPowell, 1993, andMcCulloh et al, 2001). WCF-Whitney Canyon fault.…”