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Abstract

Structural syntaxes, tectonic aneurysms, and fault-bounded fore-arc slivers are important tectonic elements of orogenic belts worldwide. In this study we used high-resolution topography, geodetic imaging, seismic, and geologic data to advance understanding of how these features evolved during accretion of the Yakutat Terrane to North America. Because glaciers extend over much of the orogen, the topography and dynamics of the glaciers were analyzed to infer the location and nature of faults and shear zones that lie buried beneath the ice. The Fairweather transform fault system terminates by obliqueextensional splay faulting within a structural syntaxis, where thrust faulting and contractional strain drive rapid tectonic uplift and rock exhumation beneath the upper Seward Glacier. West of the syntaxis, oblique plate convergence created a dextral shear zone beneath the Bagley Ice Valley that may have been reactivated by reverse faulting when the subduction megathrust stepped eastward during the last 5-6 Ma. The Bagley fault zone dips steeply through the upper plate to intersect the subduction megathrust at depth, forming a fault-bounded crustal sliver capable of partitioning oblique convergence into strike-slip and thrust motion. Since ca. 20 Ma the Bagley fault accommodated more than 50 km of dextral shearing and several kilometers of reverse motion along its southern fl ank during terrane accretion. The fault is considered capable of generating earthquakes because it is suitably oriented for reactivation in the contemporary stress fi eld, links to faults that

Introduction: Neogene tectonics and climate-tectonic interactions in the southern Alaskan orogen themed issue

“…Their analysis of glacial fl ow in the upper Seward Glacier leads to their suggestion of a releasing bend along the strike-slip system that they use to explain the unusual, very young, low-temperature cooling ages recognized in this area by Enkelmann et al (2009). Bruhn et al (2012) also use glacial fl ow and ice surface morphology to evaluate active structure across the Bagley Ice Field valley, and conclude that the valley is underlain by a longlived, dextral-oblique structure that remains active. Their use of glacial valley morphologies and comparison of the geology across the Bagley Ice Field valley leads to a provocative interpretation of ~50 km of dextral slip along with several kilometers of south-side-up displacement across the buried structure.…”

“…The third paper in this series of contributions on the geology of the St. Elias orogen, that of Bruhn et al (2012), uses a novel approach to examine the bedrock geology in this heavily glaciated orogen. The authors use remote sensing data of exposed bedrock and glacier surfaces that integrate several years of observation for ice fl ow.…”

“…Two papers Pavlis et al, 2012) are companion articles on different aspects of the geology of the coastal fold and thrust belt of the St. Elias orogen, where sedimentary cover from the Yakutat microplate is stripped from basement within the actively deforming convergent and transpressional boundaries of the microplate. Bruhn et al (2012) considered a broader synthesis of the regional tectonics of the orogen, emphasizing active structures recognized by indirect observations from geomorphic features, glacial geology, and glacial fl ow characteristics. At an even larger scale, Bauer et al (2014) analyze the entire orogen from passive seismic imaging.…”

Active tectonics of the St. Elias orogen, Alaska, observed with GPS measurements

“…[37] Some studies have proposed that a dextral shear zone in the Bagley Ice Valley ( Figure S4) accommodates part of the relative plate motion within the orogen. Bruhn et al [2012] suggested that a Bagley fault might connect the thrust faults around Mount St. Elias with faults in the Bering Glacier area. Based on surveys of a trilateration network at the western end of the ice valley, Savage and Lisowski [1988] found significant dextral shear of~8 mm/yr across the Bagley.…”

“…Several studies [e.g., Lahr and Plafker, 1980;Perez and Jacob, 1980;Savage and Lisowski, 1988;Estabrook et al, 1992] suggested models for the basic regional tectonic framework or focused on segments of the orogen, but available data were sparse, and major questions remained about the distribution of relative motion and the location of active structures, the present-day deformation front between the Yakutat block and southern Alaska, how far the effects of the collision extended, and the large-scale geodynamics of the collision. Work done in the past decade in geology [e.g., Chapman et al, 2008Chapman et al, , 2012Bruhn et al, 2004Bruhn et al, , 2012Pavlis et al, 2012], thermochronology [e.g., Enkelmann et al, 2009;Berger et al, 2008], and geodynamic modeling [Koons et al, 2010] has shed new light on the structural framework and temporal evolution of the orogen, but detailed estimates of present-day motions were still missing.…”