Crustal structure of accreted terranes in southern Alaska, Chugach Mountains and Copper River Basin, from seismic refraction results

Abstract

Seismic refraction data were collected along a 320‐km‐long “Transect” line in southern Alaska, crossing the Prince William, Chugach, Peninsular, and Wrangellia terranes, and along several shorter lines within individual terranes. Velocity structure in the upper crust (less than 9‐km depth) differs among the four terranes. In contrast, layers in the middle crust (9‐ to 25‐km depth) in some cases extend across projected terrane boundaries. The following observations can be made: (1) An intermediate‐velocity layer (6.4 km/s) at 9‐km depth extends across the deep projection of the suture between the Chugach and Peninsular terranes, suggesting that the northern Chugach and southern Peninsular terranes are detached and rest on a deeper terrane of unknown origin. (2) The top of a gently north dipping sequence of low‐ and high‐velocity layers (5.7–7.8 km/s), more than 10 km thick, extends from near the surface in the southern Chugach terrane to more than 20‐km depth beneath the southern Peninsular terrane. This sequence, truncated by the suture between the Prince William and Chugach terranes, is interpreted to be an underplated “terrane” made up of fragments of the Kula plate and its sedimentary overburden that were accreted during subduction in the late Mesozoic and/or early Tertiary, during or between times of accretion of the Prince William and Chugach terranes. (3) A thick crustal “root”, with a laminated sequence at its top, extends from a depth of 19 km to as much as 57 km beneath the northern Peninsular and Wrangellia terranes. This root extends across the deep projection of the suture between the Peninsular and Wrangellia terranes, although resolution of this apparent crosscutting relationship is relatively poor. This root may represent tectonically or, possibly, magmatically emplaced rocks. The lower crust beneath the Prince William, Chugach, and southern Peninsular terranes includes a north dipping, 3‐ to 8‐km‐thick section of subducting oceanic crust.

Crustal structure across the coseismic rupture zone of the 1944 Tonankai earthquake, the central Nankai Trough seismogenic zone

“…On the basis of the hypocentral distribution [Doser et al, 1999], they proposed that the downdip limit of the locked zone extends to a depth of 25 -30 km. This extent of the downdip limit is in agreement with the depth of the top of the slab determined from wide-angle seismic survey [Brocher et al, 1994;Fuis et al, 1991]. The crustal model of Brocher et al [1994] shows the updip limit of the coseismic rupture zone is also not shallower than the crustal block with velocities of >5.5 km/s.…”

Synthesis of Crustal Seismic Structure and Implications for the Concept of a Slab Gap beneath Coastal California

“…One implication of the stalled-slab model is that as the Mendocino triple junction moved northward, Gorda oceanic crust would have broken off from the Gorda plate and have become tectonically ac creted to the base of North America. In southern Alaska, there is seismic reflection and refraction evidence that the continental crust has indeed grown by the tectonic emplacement of oceanic crust (Fisher et al, 1989;Fuis et al, 1991;Brocher et al, 1994a). Our view of the models presented by Beaudoin et al (1998) for a seismic line across the landward projection of the Mendo cino Iracture zone is that the Gorda slab is being tectonically emplaced onto the base of North America.…”

Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska

“…In contrast, the Contact fault system within western PWS is composed of subparallel thrust faults that are accommodating postaccretion deformation [e.g., Dumoulin, 1987;Bol and Gibbons, 1992;Plafker et al, 1994]. Fuis et al [1991] showed north-dipping reflectors that project to the surface near the Contact fault system along the land-based TACT profile immediately north of PWS, and EberhartPhillips et al [2006] showed that the Contact fault system can be characterized by a north-dipping low-velocity zone. The Bainbridge fault extends beneath western PWS along a northeast-trending bathymetric lineament that separates the deepest waters of PWS from adjacent islands, with a seafloor elevation change of nearly 1 km.…”