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Mid-Cretaceous-recent crustal evolution in the central Coast Orogen, British Columbia and Southeastern Alaska

Maria Luisa Crawford, Keith A. Klepeis, George E. Gehrels and Jennifer Lindline
Mid-Cretaceous-recent crustal evolution in the central Coast Orogen, British Columbia and Southeastern Alaska (in Crustal cross sections; from the western North American Cordillera and elsewhere; implications for tectonic and petrologic processes, Robert B. Miller (editor) and Arthur W. Snoke (editor))
Special Paper - Geological Society of America (2009) 456: 97-124

Abstract

The Coast orogen of western coastal British Columbia and southeastern Alaska is one of the largest batholithic belts in the world. This paper addresses the structure and composition of the crust in the central part of this orogen, as well as the history of its development since the mid-Cretaceous. The core of the orogen consists of two belts of metamorphic and plutonic rocks: the western metamorphic and thick-skinned thrust belt comprising 105-90-Ma plutons and their metamorphic country rocks, and the Coast Plutonic Complex on the east, with large volumes of mainly Paleogene magmatic rocks and their high-temperature gneissic host rocks. These two belts are separated by the Coast shear zone, which forms the western boundary of a Paleogene magmatic arc. This shear zone is subvertical, up to 5 km wide, and has been seismically imaged to extend to and offset the Moho. Lithologic units west of the Coast shear zone record contractional deformation and crustal thickening by thrusting and magma emplacement in the mid-Cretaceous. To the east, the Coast Plutonic Complex records regional contraction that evolves to regional extension and coeval uplift and exhumation after ca. 65 Ma. Igneous activity in the Complex formed a Paleogene batholith and gave rise to high crustal temperatures, abundant migmatite and, as a result, considerable strain localization during deformation. In both belts, during each stage of the orogeny, crustal-scale deformation enabled and assisted magma transport and emplacement. In turn, the presence of magma, as well as its thermal effects in the crust, facilitated the deformation. After 50 Ma, the style of crustal evolution changed to one dominated by periods of extension oriented approximately perpendicular to the orogen. The extension resulted in tilting of large and small crustal blocks as well as intra-plate type magmatic activity across the orogen. Seismic-reflection and refraction studies show that the crust of this orogen is unusually thin, probably due to the periods of orogen-perpendicular stretching. Magmatic activity west of the Coast shear zone in the Late Oligocene and Miocene was related to one period of orogen-parallel transtension along the margin. Small-scale, mafic, mantle-derived volcanic activity continues in the region today. The change from convergence to translation and extension is related to a major plate reorganization in the Pacific that led to a change from subduction of an oceanic plate to northwestward translation of the Pacific plate along the northwest coast of North America. Although it has been proposed that this orogen is the site of major (up to 4000 km) pre-Eocene northward terrane translation, there is little evidence for such large-scale displacement or for the kind of discontinuity in the geological record that such displacement would entail.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 456
Title: Mid-Cretaceous-recent crustal evolution in the central Coast Orogen, British Columbia and Southeastern Alaska
Title: Crustal cross sections; from the western North American Cordillera and elsewhere; implications for tectonic and petrologic processes
Author(s): Crawford, Maria LuisaKlepeis, Keith A.Gehrels, George E.Lindline, Jennifer
Author(s): Miller, Robert B.editor
Author(s): Snoke, Arthur W.editor
Affiliation: Bryn Mawr College, Department of Geology, Bryn Mawr, PA, United States
Affiliation: San Jose State University, Department of Geology, San Jose, CA, United States
Pages: 97-124
Published: 2009
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 151
Accession Number: 2010-004835
Categories: Structural geologyIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 1 table, geol. sketch maps
N53°00'00" - N57°00'00", W135°00'00" - W128°00'00"
Secondary Affiliation: University of Wyoming, USA, United StatesUniversity of Vermont, USA, United StatesUniversity of Arizona, USA, United StatesNew Mexico Highlands University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 201003
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