Crustal Structure Along The ACCRETE Corridor
Hollister, L.S. (1) , Morozov, I. (2), Smithson, S. (2), Das, T. (1),Diebold, J. (3), Andronicos, C.(1), and Chardon, D. (1).
(1) Department of Geosciences, Princeton University, Princeton, NJ 08544(2) Department of Geology, University of Wyoming, Laramie, Wyoming 82071(3) Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964
We present the crustal structure of the entire 260 km long ACCRETE corridor along the border between SE Alaska and British Columbia. Multichannel seismic reflection profiles, acquired in Dixon Entrance and Portland Canal by R/V EWING in 1994, imaged crustal reflectors and Moho across the fundamental structures separating the Alexander terrane and Stikinia. We also imaged P-wave velocity structure and reflectivity using travel-time tomography, forward ray tracing and mid-point stacking of the wide-angle reflection/refraction seismic dataset. The seismic data sets are integrated with surface geology providing constraints on crustal structure from the surface to the Moho.The stacked Moho reflector correlates well with the ray tracing model and multi-channel seismic section. The eastern margin of the Alexander terrane features strong crustal reflectors, mostly dipping gently WSW. Moho reflections within this zone are stronger and more continuous than those seen in the terranes to the east. West of the Coast shear zone (CSZ; it is one of the fundamental structures separating terranes), Moho is about 25 km in depth. NE of the CSZ, within a lateral distance of 30 km or less, the Moho deepens from about 25 km to 32 km. It shallows gently between 40 km to 70 km further NE of the CSZ, after which it deepens gradually, reaching a depth of about 36 km under the Skeena fold and thrust belt.
Several reflectors, which dip about 30-40 degrees NE, are imagedwithin the crust west of the CSZ. One of these reflectors is reliablytraced from the near surface to the Moho under the surface exposure of theCSZ. These reflectors probably correspond to the west vergingmid-Cretaceous thrust system. In the upper crust of the Coast PlutonicComplex (CPC), several reflectors dipping at about 20-25 degrees NE maycorrespond to a deformation fabric related to 50-53 Ma extension of theCPC. The termination of a number of prominent mid- and lower crustalreflections within the CPC reveals a discontinuity of the reflectivity inthe vicinity of the CSZ. These reflectors may correspond to mafic tointermediate composition, shallow north dipping sills mapped at the surfacethat project to the line of seismic sectionThe Eocene Kasiks sill complex, consisting of a minimum 7 km thickness of shallow dipping sills, produced ~1/5 the total thickness of the present continental crust in the CPC. Detailed structural data show that the sills intruded during subvertical shortening partitioned into two directions of extensional shearing. Two kinematic domains record this partitioning. The first, along the eastern edge of the sills, records top to the northeast normal shearing. A second domain dominates the central and western parts of the complex and records top to the north-northwest normal shearing. This domain projects to the zone of lower crustal reflectors and relatively high P-wave velocities seen in the line of seismic section. Northeast directed greenschist facies shear zones overprint magmatic structures suggesting northeast directed shearing outlasted north-northwest directed shearing. Pb/U ages (Andronicos et al, 1997) fix the sill emplacement and exhumation to between 50 and 53 Ma. The geochronologic data combined with the localization of extensional strain during decreasing temperatures suggests intrusion of the sills was synchronous with and may have triggered exhumation of the high-grade root of the CPC. Intrusion of these syn-kinematic sills suggests magmatic accretion during extension was a significant process for crustal formation of the CPC.
We conclude from the combined geologic and seismic datasets that the crust of the CPC was thickened between 53 and 50 Ma by magmatic inflation of the lower crust; the thickening was accompanied by substantial removal of the upper crust by tectonic exhumation.
Reference:
Andronicos, C.L., Chardon, D., Hollister, L.S., Gehrels, G., and Isachsen, C., 1997, Formation of Continental Crust by Magmatic Accretion During Late Orogenic Extension. EOS, v. 78, no. 46, p. F629.
