Stratigraphy, provenance, and tectonic significance of the Punchbowl block, San Gabriel Mountains, California, USA

Abstract

The Punchbowl block is a fault-bounded crustal sliver in the eastern San Gabriel Mountains of southern California with important implications for conflicting reconstructions of the San Andreas fault system. Detailed mapping, determination of conglomerate-clast and sandstone compositions, and dating of detrital and igneous zircon of Oligocene-Miocene strata define two distinct subbasins and document initiation of extension and volcanism ca. 25-24 Ma, followed by local exhumation of the Pelona Schist, and transition from alluvial-fan to braided-fluvial deposition. Strata of the Punchbowl block correlate with those of other regions in southern California, confirming 40-50 km of dextral slip on the Punchbowl fault, and supporting reconstructions with 60-70 km of dextral slip on the San Gabriel/ Canton fault and ~240 km of dextral slip on the southern San Andreas fault. Provenance and probable correlations of Punchbowl-block strata argue against 80-110 km of dextral slip on the San Francisquito-Fenner-Clemens Well fault and limit the time interval during which such slip could have occurred. Synthesis of these findings with previous work produces paleogeographic reconstructions of the Punchbowl block and its probable correlatives through time.

Kinematics and paleogeology of the western United States and northern Mexico computed from geologic and paleomagnetic data: 0 to 48 Ma

“…At 6 Ma, we successfully restore the Pelona Schist in the San Gabriel Mountains block opposite the Orocopia Schist in the Orocopia Mountains (Fig. 9); these exposures had a common origin in the Chocolate Mountains anticlinorium (Ingersoll et al, 2014;Coffey et al, 2019). Thus, we accept 6 Ma as the likely origination time for this branch of the San Andreas system.…”

A Multi‐Proxy Approach Using Zircon (U‐Th)/He Thermochronometry and Biomarker Thermal Maturity to Robustly Capture Earthquake Temperature Rise Along the Punchbowl Fault, California

“…Model results are also shown as % He loss as a function of peak temperature for specific slip zone half-widths (Figure 7c). Analogous to prior work (e.g., Coffey, Ingersoll, & Schmitt, 2019;Lachenbruch, 1986;, modeled temperature rise is sensitive to both slip zone half-width and coefficient of friction. For example, for a given displacement, a thinner deforming zone yields higher temperatures than a wider deforming zone (Figure 7b).…”

“…Detrital zircon U‐Pb age spectra from the Punchbowl Fm include peaks at ∼70–100 Ma, ∼150–160 Ma, ∼210–260 Ma, ∼1,400 Ma, and ∼1,700 Ma (Hoyt et al., 2018; Ingersoll et al., 2013). Prior work suggests the provenance for zircon grains in the Punchbowl Fm is the distal Mojave region located northeast of the present location of the Punchbowl Fm (Barth et al., 1997; Coffey, Ingersoll, & Schmitt, 2019; Meisling & Alexander, 1993; Woodburne, 1975).…”

“…Faulting may have occurred in two phases, with half of the displacement before deposition of the Punchbowl Fm and the remaining displacement in the Plio‐Pleistocene (∼6‐1 Ma) following deposition of the entire Punchbowl Fm (Chester & Chester, 1998; Meisling & Alexander, 1993; Woodburne, 1975). Alternatively, all of the slip may have occurred in the last ∼5 Ma (Coffey, 2015; Coffey, Ingersoll, & Schmitt, 2019; Schulz & Evans, 1998, 2000).…”

“…Here, the PF juxtaposes Miocene‐Pliocene Punchbowl Fm to the northeast against Mesozoic and older gneissic crystalline basement to the southwest (Figure 1). Basement rocks are variably mapped as Precambrian crystalline basement, Mesozoic and older crystalline rock, or Mesozoic Wilson Diorite (Barth, 1990; Chester & Chester, 1998; Coffey, 2015; Coffey, Ingersoll, & Schmitt, 2019). Although some crystalline basement rocks in the San Gabriel Mountains have well‐constrained crystallization ages, the age of the gneiss at our specific sample sites is unknown.…”