Controls on pseudotachylyte formation during tectonic exhumation in the South Mountains metamorphic core complex, Arizona

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Abstract Pseudotachylyte formed during the later stages of extension in the South Mountains metamorphic core complex. Microstructural and microchemical evidence of pseudotachylyte formation processes is well preserved in granitoid mylonite host rocks because of rapid cooling during tectonic exhumation. Petrographic studies indicate a cataclasite precursor to pseudotachylyte. Transmission electron microscope analysis shows glass in pseudotachylyte, providing evidence of melting. Electron microprobe analyses demonstrate variations in glass, microlite, and crystallite composition with proximity to quartz clasts, indicating reaction between clasts and melt. Amphibole crystallites exhibit sieve textures, interpreted as recording rapid crystallization and subsequent decompression. Decompression is postulated to result from drops in fluid pressure along jogs in the slip surface or following formation of injection veins. Both pseudotachylyte and cataclasite are typically subparallel to C-surfaces in the host mylonite. Cataclasite not associated with pseudotachylyte is composed of quartz + feldspars ± biotite. Pseudotachylyte everywhere contains biotite. The model proposed for formation of pseudotachylyte and cataclasite in foliated granitoid protoliths includes seismic failure along C-surfaces. Where biotite is absent, slip produces cataclasite. Where biotite is present, initial cataclasis may be followed by frictional melting, facilitated by biotite’s low melting temperature and lowering of the melting temperatures of other phases by release of water during biotite breakdown.