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

An exhumed subduction plate interface recording deep slow slip conditions: observations from the Epidote Amphibolite unit of the Catalina Schist, Santa Catalina Island/Pimu’nga, CA

Paper number: 245-8
Program: Technical Sessions
Time: 9:55 AM – 10:10 AM

Authors

Abstract

Since the formative work by John Platt in his PhD dissertation, the Catalina Schist on Santa Catalina Island/Pimu’nga, CA, has become a top natural laboratory to study feedbacks amongst deformation, metamorphism, slip behaviors, and element cycling in exhumed subduction terranes.

 

We present new work from the Epidote Amphibolite unit of the Catalina Schist. Previous mapping (Platt, 1975) documented an exposure of coherent oceanic lithostratigraphy (meta-pillow basalts, metacherts, metasediments) structurally below an ultramafic-metasedimentary mélange unit. We interpret this as an exhumed paleosubduction interface whose prograde and peak conditions are consistent with those of modern deep slow slip.  

 

Within the metabasalts, dehydration veinlet networks record a prograde pumpellyite to epidote dehydration reaction at 0.5-0.7 GPa, 290-310°C, releasing 2 wt% water over a narrow P-T range. Other epidote textures reveal continued prograde to peak metamorphism and dehydration, reaching conditions of 0.9 GPa and 550°C. In the ultramafic-metasedimentary mélange, microstructures in metasomatic talc-schist matrix record high strain rates consistent with slow slip by frictional processes, which may have been activated by intermittent periods of elevated pore fluid pressures. Detailed geochemistry and petrology on the mélange reveal that metasomatism occurred at prograde to peak conditions of  up to 1.0 GPa and 450-550°C, resulting from transformation of metasedimentary rocks into actinolite-chlorite schists and ultramafic rocks into talc ± actinolite schists. Petrochronology on syn-kinematic rutile cores and their titanite rims reveal both phases grew during Cretaceous subduction, verifying that deformation and metasomatism occurred at prograde to peak conditions during external fluid infiltration. Quartz vein networks in the mélange suggest intermittent elevated pore fluid pressures, caused by fluids that may be sourced from dehydration reactions in underlying metabasalt (i.e., the subducting slab).

 

Together, these data point to a chemically dynamic environment that we infer led to the development of slow slip events hosted in talc schists and facilitated by frictional deformation activated by high pore fluid pressures related to metamorphic dehydration. This field-grounded multi-disciplinary approach is a hallmark of the John Platt workflow, and a key part of his legacy within our community. 

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

doi: 10.1130/abs/2025AM-413896

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