Tectono-Metamorphic Impact of a Subduction-Transform Transition and Implications for Interpretation of Orogenic Belts |
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| Abstract: | Subduction-transform tectonic transitions were common in the geologic past, yet their impact on the evolution of orogenic belts is seldom considered. Evaluation of the tectonic transition in the Coast Ranges of California is used as an example to predict some characteristics of exhumed regions that experienced similar histories worldwide. Elevated thermal gradients accompanied the transition from subduction to transform tectonics in coastal California. Along the axis of the Coast Ranges, peak pressure-temperature (P/T) conditions of 700 to 1000° C at a pressure of ～7 kbar, corresponding to granulite-facies metamorphism, and cooling to 500° C, or amphibolite facies, within 15 million years, are indicated by thermal gradients estimated from the depth to the base of crustal seismicity. Greenschist-facies conditions may occur at depths of 10 km or less. These P/T estimates are consistent with the petrology of crustal xenoliths and thermal models. Preservation of earlier subduction-related metamorphism is possible at depth in the Coast Ranges. Such rocks may record a greenschist or higher-grade overprint over blueschist assemblages, and late growth of metamorphic minerals may reflect dextral shear along the plate margin, with development of orogen-parallel stretching lineations. Thermal overprints of early-formed high-P (HP), low-T (LT) assemblages, in association with orogen-parallel stretching lineations, occur in many orogenic belts of the world, and have been attributed to subduction followed by collision. Alternatively, a subduction-transform transition may have caused the overprints and lineations in some of these orogenic belts. Possible examples are the Sanbagawa belt of Japan and the Haast schists of New Zealand. P/T conditions of inferred granulite-grade metamorphism in the Coast Ranges, and predicted cooling of these rocks through lower thermal gradients, resemble the P/T evolution of many granulite belts, suggesting that some granulite belts may have formed as a result of a subduction-transform transition. Arclike belts of plutons also can form as a consequence of subduction-transform transition. |
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