Crustal structure across the central Alaska Range: Anatomy of a Mesozoic collisional zone

Abstract

[1] A first-order process in the growth of continents is the collision and accretion of terranes against continental margins. Collision leads to the formation of a suture zone between the accreted terrane and the former continental margin. New insights on the suturing process are observed from two receiver function transects across the Mesozoic Alaska Range suture zone. Three distinct crustal sections are identified from observations of crustal thickness, intracrustal discontinuities, and Vp/Vs: a northern section with ∼27 km thick crust of felsic to intermediate composition, a central section that is ∼37 km thick that exhibits intracrustal discontinuities and has felsic to intermediate composition, and a southern section that is ∼30 km thick and has a more mafic composition. We interpret these sections to correspond with the former continental margin (Yukon composite terrane), the suture zone proper, and the allochthonous oceanic terrane (Wrangellia composite terrane). The boundary between the Yukon composite terrane and the suture zone appears to be a subhorizontal discontinuity that accommodated underthrusting of crust from the suture zone beneath the former continental margin. The boundary between the suture zone and the Wrangellia composite terrane, in contrast, appears to be a relatively discrete, vertical boundary. The observed variability in the crust across the Alaska Range suture zone is likely controlled by the differing compositions of the terranes involved, which influences how each section responds to precollisional, syncollisional, and postcollisional deformation.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

“…Sometimes the entire crustal thickness is preserved in accreted terranes, as in the Triassic Wrangellia terrane of North America, or only truncated units from all crustal layers are found, as in the accreted Gorgona and Columbia oceanic plateaus of South America. Seismic refraction studies indicate that the total thickness of the Wrangellia composite terrane crust is about 25 + km in Vancouver (Ramachandran et al, 2006;Clowes et al, 1995) and 30 km in Alaska (Brennan et al, 2011). Approximately 6 km of exposed stratigraphic thickness, correlated to the sedimentary and upper crustal layers of the Wrangellia oceanic plateau, is found in Vancouver Island (Greene et al, 2010).…”

“…Results from seismic anisotropy studies and crystal fractionation modeling of arc crustal magma development support the theory that the ultramafic high velocity layer under island arcs is often delaminated before or during accretion. In the accreted Wrangellia oceanic plateau, seismic refraction studies of the crust do not show any high P wave velocities (Brennan et al, 2011;Ramachandran et al, 2006), which can be interpreted as loss of the ultramafic subcrustal layer. However, interestingly enough, combined gravity and seismic studies of modern island arcs, oceanic plateaus, and submarine ridges do not involve a high density unit between the crust and mantle Grow, 1973;Magnani et al, 2009;Christeson et al, 2008;Gohl and UenzelmannNeben, 2001;Sallares et al, 2003;Recq et al, 1998;Walther, 2003;Sinha et al, 1981;Peirce and Barton, 1991;Hampel et al, 2004;Shulgin et al, 2011;Patriat et al, 2002), contrary to the laboratory-derived densities of the arc CMTL rocks.…”

Ten kilometer vertical Moho offset and shallow velocity contrast along the Denali fault zone from double-difference tomography, receiver functions, and fault zone head waves

“…These studies reveal a sharp contrast in the Moho across the Hines Creek fault, a structure about 25km north of the Denali fault (e.g., Brennan et al, 2011;Wang and Tape, 2014, Fig. S3).…”

“…Previous studies estimating Moho depth (Ferris et al, 2003;Brennan et al, 2011;Wang and Tape, 2014) suggest that the offset of the Moho is located 30km north of the main trace of the Denali fault, perhaps along the Hines Creek fault which has been interpreted as the northernmost boundary of the WCT-YCT suture zone (Wahrhaftig et al, 1975). To test the relative importance of the Hines Creek fault in terms of velocity contrast, we repeat the 1D model analysis (grid points shown in Fig.…”

“…West of this junction, the suture zone consists of a complicated assemblage of Paleozoic to Cretaceous sedimentary strata (Ridgway et al, 1997;Brennan et al, 2011) intruded by Quaternary plutons (Plafker et al, 1992). The complex deformation and uplift patterns associated with the Denali fault in this area -especially the McKinley Strand ( Fig.…”