HR: 09:00h
AN: V41A-05    [PDF]
TI: Crustal Architecture and Structural Patterns of the Cretaceous to Paleogene Cascades Arc
AU: * Miller, R B
EM: rmiller@geosun.sjsu.edu
AF: Dept. of Geology, San Jose State Univ., San Jose, CA 95192-0102 United States
AU: Paterson, S R
EM: paterson@usc.edu
AF: Dept. of Earth Sciences, Univ. of Southern California, Los Angeles, CA 90089 United States
AU: Matzel, J P
EM: jpmatzel@mit.edu
AF: Dept. of EAPS, MIT, Cambridge, MA 02139 United States
AB: The crystalline core of the North Cascades (Cascades core) preserves a crustal section of ca. 10- to 40 km paleodepths that facilitates evaluation of the crustal architecture and partitioning of strain during major shortening of this 96- to 45 Ma continental magmatic arc. The dominantly tonalitic plutons in the arc are steep-sided, vertically extensive bodies that commonly are sheeted. They intrude oceanic and arc terranes juxtaposed along initially gently dipping thrusts before 92 Ma. Shortening, crustal thickening, and amphibolite-facies metamorphism of the Cascades core were synchronous with large ($>$ 100 km) thrust displacements at shallow levels in the ca. 100-80 Ma NW Cascades thrust system. Deformation in the core was dominated by early tight to isoclinal, gently inclined to recumbent folds that were followed by open to tight, more upright folds. SW-vergent, moderate to steeply dipping reverse shear zones, which postdate terrane juxtaposition, were localized next to plutons. They have $<$ 10 km displacement and were responsible for less shortening than the folding. Metamorphic and geochronologic data indicate rapid burial of plutons and host rocks during the mid-Cretaceous deformation. Shortening in sub-greenschist-facies, ophiolitic rocks above the Cascades core was manifested by upright folds and steep reverse shear zones that reactivated Jurasssic faults inferred to have formed in an oceanic fracture zone. Folding and reverse shear persisted in the Cascades core until at least 65 Ma, well after major thrusting ceased in the NW Cascades system. The younger shortening structures are confined to deeper, more internal parts of the arc, and were probably localized in part by magmatism and thermal weakening. This shortening was also broadly coincident with cryptic thrust burial of sedimentary protoliths of the Swakane Gneiss from the surface to 40 km between 73 to 68 Ma. This underthrusting may have removed the roots of the arc plutons. Similar deformation patterns throughout the crustal section imply structural coupling of large parts of the arc. This coupling may reflect the consistently tonalitic composition (rather than more mafic composition with depth) of plutonic rocks and low paleogeothermal gradients of the section. Structural partitioning was more strongly controlled by relatively localized rheological contrasts, particularly the presence of rigid plutons and other mechanical anisotropy.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8035 Pluton emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting