245 Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt, Part I

We welcome contributions inspired by the diverse research of John Platt in structural geology and tectonics showing how the rock record can be used to illuminate the fundamental processes of crustal deformation and mountain building.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 217D

Were amphibolite facies rocks within the Catalina Schist metamorphosed beneath the leading edge of the western Peninsular Ranges batholith prior to juxtaposition with lower grade blueschists?

Paper number: 245-10
Program: Technical Sessions
Time: 10:25 AM – 10:40 AM

Authors

Abstract

Platt (1975) proposed a nascent subduction model for the Franciscan Complex of northern California based upon his mapping and analysis of an inverted metamorphic sequence of fault-bounded, amphibolite to blueschist facies units of the Catalina Schist in offshore southern California.  His model, which invoked mantle heat transfer from the hanging-wall downward into accreted rocks within the newly initiated subduction complex, successfully explains the high-T, high-P/T metamorphism of blocks entrained within subduction mélange and supports models that link their age to mid-Jurassic emplacement of the Coast Range Ophiolite.  While Middle Jurassic subduction initiation is widely accepted for the Franciscan complex, it cannot explain the history of the Catalina Schist: (1) 120-105 Ma, amphibolite facies metamorphism and anatexis in the Catalina Schist post-dates emplacement of local mid-Jurassic forearc basement by >50 m.y.; (2) slow-cooling of the amphibolite unit until 100 Ma; (3) 20 m.y. difference between amphibolite facies and blueschist facies metamorphism; and (4) no blueschist overprint of amphibolite facies rocks. We have performed thermal calculations to evaluate an alternative model in which amphibolite facies metamorphism in the Catalina Schist occurred beneath the leading edge of the western 130-100 Ma Peninsular Ranges batholith (PRB) due to heat conduction from the arc.  Proximity to the arc is supported by evidence that the Catalina Schist was exhumed from beneath PRB during Neogene rifting. The modeled subduction zone initially dipped 45° with the trench 100 km west of the PRB western margin. We propose a forearc thrust emplaced the protolith of the Catalina Schist at 35-40 km depth prior to metamorphism and link it to the intra-arc Matir thrust of northern Baja.  Shallowing of the subduction zone to 27° at 105-100 Ma accounts for eastward relocation of the PRB and juxtaposes high-grade Catalina Schist with sub-accreted blueschists. Using a sequence of pluton emplacement consistent with observations from the western PRB, calculations indicate that arc-heating can produce syn-subduction, 120-105 Ma, amphibolite facies metamorphism at 35-40 km depth up to 10 km west of the PRB. Results are insensitive to intrusive history and reasonable rates of subduction. About 15 km of syn-subduction exhumation of the forearc between 105-85 Ma is required for the amphibolite facies rocks avoid blueschist facies overprinting related to shallowed subduction that generated the eastern PRB between 100-88 Ma.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414192

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