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Plutonism at different crustal levels: Insights from the ~5–40 km (paleodepth) North Cascades crustal section, Washington

By
Robert B. Miller
Robert B. Miller
Department of Geology, San José State University, San José, California 95192-0102, USA
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Scott R. Paterson
Scott R. Paterson
Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA
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Jennifer P. Matzel
Jennifer P. Matzel
Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA
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Published:
January 01, 2009

The crystalline core of the North Cascades preserves a Cretaceous crustal section that facilitates evaluation of pluton construction, emplacement, geometry, composition, and deformation at widely variable crustal levels (~5–40-km paleodepth) in a thick (≥55 km) continental magmatic arc. The oldest and largest pulse of plutonism was focused between 96 and 89 Ma when fluxes were a minimum of 3.9 × 10‒6 km3/yr/km of arc length, but the coincidence with regional crustal thickening and underthrusting of a cool outboard terrane resulted in relatively low mid- to deep-crustal temperatures for an arc. A second, smaller peak of magmatism at 78–71 Ma (minimum of 8.2 × 10‒7 km3/yr/km of arc length) occurred during regional transpression. Tonalite dominates at all levels of the section. Intrusions range from large plutons to thin (<50 m) dispersed sheets encased in metamorphic rocks that record less focused magmatism. The percentage of igneous rocks increases systematically from shallow to middle to deep levels, from ~37% to 55% to 65% of the total rock volume. Unfocused magmas comprise much higher percentages (~19%) of the total plutonic rock at deep- and mid-crustal depths, but only ~1% at shallower levels, whereas the largest intrusions were emplaced into shallow crust. Plutons have a range of shapes, including: asymmetric wedges to funnels; subhorizontal tabular sheets; steep-sided, blade-shaped bodies with high aspect ratios in map view; and steep-sided, vertically extensive (≥8 km) bodies shaped like thick disks and/or hockey pucks. Sheeted intrusions and gently dipping tabular bodies are more common with depth. Some of these plutons fit the model that most intrusions are subhorizontal and tabular, but many do not, reflecting the complex changes in rock type and rheology in arc crust undergoing regional shortening. The steep-sheeted plutons partly represent magma transfer zones that fed the large shallow plutons, which were sites of intermittent magma accumulation for up to 5.5 m.y. Downward movement of host rocks by multiple processes occurred at all crustal levels during pluton emplacement. Ductile flow and accompanying rigid rotation were the dominant processes; stoping played an important secondary role, and magma wedging and regional deformation also aided emplacement. Overall, there are some striking changes with increasing depth, but many features and processes in the arc are similar throughout the crustal section, probably reflecting the relatively small differences in peak temperatures between the middle and deep crust. Such patterns may be representative of thick continental magmatic arcs constructed during regional shortening.

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GSA Special Papers

Crustal Cross Sections from the Western North American Cordillera and Elsewhere: Implications for Tectonic and Petrologic Processes

Robert B. Miller
Robert B. Miller
San José State University, San José, California, USA
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Arthur W. Snoke
Arthur W. Snoke
University of Wyoming, Laramie, Wyoming, USA
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Geological Society of America
Volume
456
ISBN print:
9780813724560
Publication date:
January 01, 2009

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