Evolution of a Cretaceous Subduction Model: Insights from the Catalina Schist

April 27th, 2013

The Catalina Schist crops out on Santa Catalina Island and underlies the southwestern California borderland, preserving a Cretaceous subduction zone environment. The Catalina Schist consists of amphibolite-facies rocks atop a suite of rocks metamorphosed at progressively lower temperatures with increasing depth.

This inverted metamorphic sequence has been explained to be the result of tectonic juxtaposition of the subduction zone sequence with rocks formed in a slab-parallel thrust in the forearc. Subsequent subduction erosion and faulting has juxtaposed the lower grade blueschist-facies rocks with the higher grade units of the Catalina Schist.

The epidote-blueschist unit of the Catalina Schist exists between the high-grade epidote-amphibolite unit and the low-grade lawsonite-blueschist unit and was originally mapped together with the epidote-amphibolite unit as the Catalina greenschist unit. Significant differences in metamorphic grade separate these units and bring into question whether the epidote-blueschist facies metamorphism occurred within a slab-parallel megathrust in the forearc region or within the accretionary complex of the subduction zone proper.

Last summer I collected a series of samples along Cottonwood Canyon, starting from apparent epidote-blueschist at the bottom to amphibolite at the top. Thin-section analysis of these samples has shown a complete lack of gradation between units, strongly implying unique histories of formation and perhaps the existence of a second metamorphic thrust zone. Pseudosection analysis of XRF data for some samples, as well as electron microprobe data, can refine the tectonic model of the Catalina Schist to include specifics regarding the formation of the epidote-blueschist.

Michael Barthelmes, ’13
Dodgeville, WI
Major: Geology

Sponsor: Emily Walsh

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