Brittle deformation along the Gulf of Alaska margin in response to Paleocene-Eocene triple junction migration

Abstract

A spreading center was subducted diachronously along a 2200 km segment of what is now the Gulf of Alaska margin between 61 and 50 Ma, and left in its wake near-trench intrusions and high-T, low-P metamorphic rocks. Gold-quartz veins and dikes, linked to ridge subduction by geochronological and relative timing evidence, provide a record of brittle deformation during and after passage of the ridge. The gold-quartz veins are typically hosted by faults, and their regional extent indicates there was widespread deformation of the forearc above the slab window at the time of ridge subduction. Considerable variability in the strain pattern was associated with the slab window and the trailing plate. A diffuse network of dextral, sinistral, and normal faults hosted small lode-gold deposits (<50,000 oz) in south-central Alaska, whereas crustal-scale dextral faults in southeastern Alaska are spatially associated with large gold deposits (up to 800,000 oz).We interpret the gold-quartz veins as having formed above an eastward-migrating slab window, where the forearc crust responded to the diminishing infl uence of the forward subducting plate, the increasing infl uence of the trailing plate, and the thermal pulse and decreased basal friction from the slab window. In addition, extensional deformation of the forearc resulted from the diverging motions of the two oceanic plates at the margins of the slab window. Factors that complicate interpretations of fault kinematics and near-trench dike orientations include a change in plate motions at ca. 52 Ma, northward translation of the accretionary complex, oroclinal bending of the south-central Alaska margin, and subduction of transform segments. We fi nd the pattern of syn-ridge subduction faulting in southern Alaska is remarkably similar to brittle faults near the Chile triple junction and to earthquake focal mechanisms in the Woodlark basin-the two modern sites of ridge subduction. Therefore, extensional and strike-slip deformation above slab windows may be a common occurrence.

Emplacement of the Kodiak batholith and slab-window migration

“…Marshak and Karig (1977) fi rst proposed that the Sanak-Baranof belt formed by subduction of an oceanic spreading ridge (Fig. 2), and the concept has been supported by many subsequent workers (Hill et al, 1981;Moore et al, 1983;Bradley et al, 1993Bradley et al, , 2003Sisson et al, 1989Sisson et al, , 2003aSisson et al, , 2003bSisson and Pavlis, 1993;Plafker et al, 1989Plafker et al, , 1994Pavlis and Sisson, 1995;Haeussler et al, 1995Haeussler et al, , 2003aHaeussler et al, , 2003bBradley et al, 1998;Young, 1999, Kusky et al, 2003). One exception is Hudson et al (1979) and Hudson (1994), who argued that the forearc plutons formed as a crustal melting event resulting from the subduction of hot, young oceanic crust, but not necessarily the spreading ridge.…”

Three-dimensional mechanics of Yakutat convergence in the southern Alaskan plate corner

“…McKinley/ Denali, has long been recognized as a direct consequence of a restraining bend on the dextral Denali fault system [e.g., Stout and Chase, 1980], and the St. Elias Mountains have long been linked to collisional tectonics of the Yakutat terrane [e.g., Plafker et al, 1978]. Nonetheless, researchers have only recently emphasized the close relationships among the various elements of the southern Alaskan orogen [Fletcher and Freymueller, 1999;Haeussler et al, 2003;Pavlis et al, 2004;Bruhn et al, 2004;Eberhart-Philips et al, 2006;Redfield et al, 2007], although the concept is not new [e.g., Plafker, 1965;von Huene and Scholl, 1991;Scholl et al, 1992]. The modeling discussed here drives home the importance of the close connection between different elements of the southern Alaskan orogen and makes specific predictions that are broadly consistent with known geologic histories in southern Alaska.…”

Eocene volcanism above a depleted mantle slab window in southern Alaska

“…Haeussler et al (2003b), however, outlined plate kinematic criteria against de-coupling of a subducted plate and in support of ridge subduction. Other evidence in support of ridge subduction includes deformation (Haeussler et al, 1995(Haeussler et al, , 2003aPavlis and Sisson, 1995;Kusky et al, 1997Kusky et al, , 2003, mineralization (Haeussler et al, 1995;Weinberger and Sisson, 2003), metamorphism (Sisson et al, 1989;Pavlis and Sisson, 1995;Bowman et al, 2003), sedimentation (Trop et al, 2003), and ophiolite emplacement (Bol et al, 1992;Crowe et al, 1992;Nelson and Nelson, 1993;and Kusky and Young, 1999) in accretionary prism and forearc rocks along southern Alaska. We favor the model of spreading ridge subduction as the mechanism to form a slab window beneath southern Alaska, but suggest that the spreading ridge may have been broken into multiple segments that intersected the subduction zone of southern Alaska at several locations.…”

“…Sets of brittle faults in rocks of the Southern Margin composite terrane, including normal faults and conjugate strike-slip faults that trend perpendicular to regional strike, were coeval with Sanak-Baranof magmatism and indicate that the region above a subducting ridge will have a predominantly extensional and/or strike-slip structural history Bradley et al, 2003;Haeussler et al, 2003a). Accretionary prism and forearc rocks in places of modern ridge subduction (e.g., southern Chile, the Woodlark Basin), have a similar structural history, dominated by normal and strike-slip faulting (Forsythe and Nelson, 1985;Crook and Taylor, 1994).…”