HR: 17:15h
AN: T14B-06    [Abstracts]
TI: Magmatism and Crustal Structure of the North Cascades Arc, NW U.S.A.
AU: * Miller, R B
EM: rmiller@geosun.sjsu.edu
AF: Dept. of Geology, San Jose State University, Washington Square, San Jose, CA 95192- 0102, United States
AU: Bowring, S
EM: sbowring@mit.edu
AF: Dept. of Earth, Atmospheric, and Planetary Sciences, MIT, Green Bldg., Cambridge, MA 02139, United States
AU: Gordon, S
EM: gordo204@umn.edu
AF: Dept. of Geology & Geophysics, Univ. of Minnesota, 310 Pillsbury Drive, Minneapolis, MN 55455, United States
AU: Matzel, J
EM: jmatzel@gmail.com
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, United States
AU: Michels, Z
EM: drumasaurusrex@yahoo.com
AF: Dept. of Geology, San Jose State University, Washington Square, San Jose, CA 95192- 0102, United States
AU: McLean, N
EM: nmclean@MIT.EDU
AF: Dept. of Earth, Atmospheric, and Planetary Sciences, MIT, Green Bldg., Cambridge, MA 02139, United States
AU: Paterson, S R
EM: paterson@usc.edu
AF: Dept. of Earth Sciences, Univ. of Southern California, Earth Sciences, Los Angeles, CA 90089, United States
AU: Whitney, D L
EM: dwhitney@umn.edu
AF: Dept. of Geology & Geophysics, Univ. of Minnesota, 310 Pillsbury Drive, Minneapolis, MN 55455, United States
AB: The North Cascades crystalline core (Cascades core) is part of an episodic, and at times high flux continental arc system that records a close interplay between magmatism and regional deformation. This thick (> 55 km), 96- to 45 Ma arc is the southern continuation of the Coast Batholith. The magmatic peak was at 96 to 89 Ma, when average fluxes were a minimum of 1.2x10-5 km3/yr/km of arc length. This magmatism was recorded throughout the arc, but is most voluminous in the southern part of the core, which preserves a crustal section extending from 5 to 40 km paleodepth. Tonalites dominate at all levels of the section, and plutonic rocks increase systematically from shallow to middle to deep levels; from ~ 37% to 55% to 65% of the total rock volume. The magmatic peak was coincident with major regional shortening and burial of large tracts of supracrustal rocks to depths of 8 to 11 kbar. Shortening was driven in part by underthrusting of a cool outboard terrane (Wrangellia), which resulted in a low geothermal gradient for an arc and may have led to near cessation of magmatism at 88 Ma. After a magmatic lull, plutonism was restricted to the NE half of the arc. A small peak (2.6x10-6 km3/yr/km of arc length) in magmatism occurred at ca. 78 to 71 Ma. The end of this interval broadly coincided with underthrusting of Late Cretaceous sediments to depths of 30-40 km beneath the arc, and the probable translation of deep- crustal magmatic roots. Plutonism, amphibolite-facies metamorphism, and migmatization continued from ca. 68 to 45 Ma, and spanned the transition from transpression to dextral transtension. Latest arc magmatism in the Eocene was broadly coeval with ridge subduction and rapid exhumation, including nearly isothermal decompression from 10 to < 5 kbar of the Skagit Gneiss Complex. Magmatism was also associated with sub- horizontal, arc-sub-parallel, southward-directed deep-crustal flow from the region of peak metamorphism. Eocene magmatism was extremely variable, ranging from mafic dike swarms and primitive mafic layered complexes, to voluminous leucotonalites and trondhjemitic pegmatites, to calc-alkaline granodiorites, to A-type granites. Nd isotopic values are also more variable than for older components of the arc. The Skagit Gneiss Complex was the focus of Paleogene magmatism and metamorphism, and the volume of leucocratic tonalite orthogneiss suggests that much of the crust in the NE half of the arc was reconstituted at this time. Overall, magmatism and deformation in the arc are markedly varied in space and time. Major structural events, including initial underthrusting of Wrangellia and then Late Cretaceous sediments, later deep-crustal flow associated with exhumation, and ridge subduction significantly modified the deep crust of the arc. These events are probably in part reflected by episodes of higher magmatic fluxes that coincided with periods of major deformation during different tectonic regimes and were separated by lulls during which there is little recorded deformation.
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8035 Pluton emplacement
DE: 8038 Regional crustal structure
DE: 8104 Continental margins: convergent
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting