Structural and Thermochronological Constraints on the Role of Partial Melting During Late-Orogenic Extension

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; R o o t E n t r y F 4p C o m p O b j b W o r d D o c u m e n t O b j e c t P o o l 4p 4p 4 @ - . / 0 1 2 3 4 5 6 7 F Microsoft Word 6.0 Document MSWordDoc Word.Document.6 ; 9526915 Teyssier The goal of the proposed project is to investigate the role that partial melting of crust plays during late-orogenic extension. Orogenic evolution typically involves crustal shortening and thickening which have important thermal consequences. In particular, thermal relaxation is expected to lead to partial melting of the mid-to lower crust after a characteristic time of approximately 20-30 million years. The late stages of orogenic evolution are commonly characterized by crustal extension. Therefore, it is proposed that a fundamental link exists between partial melting of crust, which results in mechanical weakening, and late-orogenic extension produced by gravitational collapse of the thickened crust under its own weight. The proposed working hypothesis involves extension of the rigid upper crust accommodated by ductile flow and migmatite-dome development in the lower crust. The test will be conducted in the Shuswap metamorphic core complex, British Columbia, which displays, at present exposure level, a cross section of various levels of the crust, from high-grade metamorph ic rocks and migmatites to unmetamorphosed sedimentary rocks. Preliminary work as well as published data suggest that partial melting in the core of this complex was coeval with development of Paleogene extensional fabric in the overlying crustal layers. In addition, existing isotopic data suggest that terrain cooled rapidly (30-50(C/.yr) during Paleogene time. Therefore, it is proposed that rapid cooling and exhumation of the terrain was coeval with partial melting of the mid-to lower crust, and that the mechanical weakening associated with partial melting was responsible for gravitational collapse of the orogen. An integrated approach based upon the methods of structural and metamorphic geology as well as isotopic dating will test the proposed hypothesis. ; Oh +' 0 $ H l D h R:\WWUSER\TEMPLATE\NORMAL.DOT S u m m a r y I n f o r m a t i o n ( , 9526915 Felicia Smith Felicia Smith @ @ @ @ F # Microsoft Word 6.0 1 ; e = e ` ` j j j j j j j S 1 5 T 5 S j S j j j j ~ j j j j @ 9526915 Teyssier The goal of the proposed project is to investigate the role that partial melting of crust plays during late-orogenic extension. Orogenic evolution typically involves crustal shortening and thickening which have important thermal consequences. In particular, thermal relaxation is expected to lead to partial melting of the mid-to lower crust after a characteristic time of approximately 20-30 million years. The late stages of orogenic evolution are commonly characterized by crustal extension. Therefore, it is proposed that a fundamental link exists between partial melting of crust, which results in mechanical weakening, and late-orogenic extension produced by gravitational collapse of the thickened crust under its own weight. The proposed working hypothesis involves extension of the rigid upper crust accommodated by ductile flow and migmatite-dome development in the lower crust. The test will be conducted in the Shuswap metamorphic core complex, British Columbia, which displays, at present exposure level, a cross section of various levels of the c