Plate margin deformation and active tectonics along the northern edge of the Yakutat Terrane in the Saint Elias Orogen, Alaska, and Yukon, Canada

Ronald Bruhn

doi:10.1130/ges00807.1

Abstract

The northwest directed motion of the Pacific plate is accompanied by migration and collision of the Yakutat terrane into the cusp of southern Alaska. The nature and magnitude of accretion and translation on upper crustal faults and folds is poorly constrained, however, due to pervasive glaciation. In this study we used high-resolution topography, geodetic imaging, seismic, and geologic data to advance understanding of the transition from strike-slip motion on the Fairweather fault to plate margin deformation on the Bagley fault, which cuts through the upper plate of the collisional suture above the subduction megathrust. The Fairweather fault terminates by oblique-extensional splay faulting within a structural syntaxis, allowing rapid tectonic upwelling of rocks driven by thrust faulting and crustal contraction. Plate motion is partly transferred from the Fairweather to the Bagley fault, which extends 125 km farther west as a dextral shear zone that is partly reactivated by reverse faulting. The Bagley fault dips steeply through the upper plate to intersect the subduction megathrust at depth, forming a narrow fault-bounded crustal sliver in the obliquely convergent plate margin. Since . 20 Ma the Bagley fault has accommodated more than 50 km of dextral shearing and several kilometers of reverse motion along its southern flank during terrane accretion. The fault is considered capable of generating earthquakes because it is linked to faults that generated large historic earthquakes, suitably oriented for reactivation in the contemporary stress field, and locally marked by seismicity. The fault may generate earthquakes of Mw <= 7.5.

Highlights

The Saint Elias and eastern Chugach Mountains of Alaska, USA, and the Yukon, Canada provide a classic locality to study relationships between glaciation, tectonics, and landscape evolution (Fig. 1; Worthington et al, 2010; Enkelmann et al, 2010; Berger and Spotila, 2008; Meigs et al, 2008; Jaeger et al, 2001; Meigs and Sauber, 2000)
The structure and tectonics of the collisional plate boundary is discussed proceeding from the syntaxis in the east to the terminus of the Bagley fault in the west
The structural geology of the syntaxis surrounding the Fairweather fault provides insight into how deformation was transferred onto the Bagley fault and the underlying megathrust

Summary

INTRODUCTION

The Saint Elias and eastern Chugach Mountains of Alaska, USA, and the Yukon, Canada provide a classic locality to study relationships between glaciation, tectonics, and landscape evolution (Fig. 1; Worthington et al, 2010; Enkelmann et al, 2010; Berger and Spotila, 2008; Meigs et al, 2008; Jaeger et al, 2001; Meigs and Sauber, 2000). The plate boundary bends abruptly westward at the northern end of the Fairweather fault creating a structural syntaxis that is a locus of rapid tectonic uplift and exhumation (Plafker, 1987; Bruhn et al, 2004; Spotila and Berger, 2010; Enkelmann et al, 2008, 2009; Koons et al, 2010; Chapman et al, 2012) West of this syntaxis, the Chugach–Saint Elias fault is the north-dipping suture between tectonically accreted rocks of the Yakutat terrane and the overlying metamorphic and igneous rocks of the Early Tertiary plate margin of southern Alaska (Fig. 2; Plafker, 1987). The light yellow areas marked Barkley Ridge and Steller Ridge are prominent geomorphic features noted in the text

Gulf of Alaska

6: Hope Creek thrust fault 7

7: Pamplona zone thrust faults 8: Cape Suckling–Bering Glacier fault 9

5: Miller Creek thrust fault

DISCUSSION

CONCLUSIONS