Evaluating thrust belt response to glacial erosion, synorogenic sedimentation, and subduction of a thick plate: Analog modeling insights into the St. Elias Range, Alaska
The goal of this research is to improve our understanding of the structural configuration of convergent margins in response to glacial erosion, synorogenic sedimentation, and subduction of thick crust. Currently, most mechanical development models of convergent margins are disconnected from the role of these processes and their potential coupled tectonic response. To evaluate the role of these processes, we utilized analog sandbox modeling to generate physical insights into the structural growth of wedge shaped thrust belts. We then compared our modeling results to recent, field-based geological and geophysical studies of the St. Elias orogen, located along the convergent margin of southern Alaska. This margin is characterized by large erosive glacial systems, some of the highest recorded depositional rates on earth, flat-slab subduction of ~17 km thick section of an oceanic plateau, and is one of the most tectonically active plate boundaries on earth. The sandbox models in our study simulate the growth and development of an accreting wedge whose deformation is governed primarily by frictional, brittle deformational mechanisms. We used a digital image correlation technique to post-process sequenced photographed images that allow us to calculate velocity vector fields to understand deformational stages and structural configurations of these sandbox analog models. Three models were designed to test the thrust belt response to glacial erosion, synorogenic sedimentation, and subduction of a thick crust. All three models are then compared against an initial baseline model to understand how model parameters such as erosion, sedimentation, and subduction processes independently influence the structural configuration of the orogenic wedge. Major findings from the erosion model are that the wedge responds to erosion in a longitudinal valley by activation of several coeval fore- and back- thrust faults. These coeval structures serve to accommodate shortening and vertical uplift of deeper parts of the wedge in response to progressive erosion. The backthrust faults are located directly beneath the glacial valley or farther back in the wedge. Potential implications for the St. Elias Range are the erosional model is consistent with: a major unexpected structure, the Bagley fault, which is located beneath the Bagley Ice Valley (Bruhn et al., 2012), an important backthrust fault in the development of the thrust belt (Berger et al., 2008), and the exhumation of deeper crustal rocks beneath glacial valley (Enkelmann et al., 2015). The sedimentation model key findings are that the introduction of a thick section of synorogenic strata to the front of the wedge resulted in a geometry change of wider imbricated thrust sheets and broader open hanging wall folds. These changes in thrust sheet geometry in the model results are consistent with similar thrust faults styles interpreted or seen onshore and offshore in the St. Elias orogenic belt (Worthington et al., 2010; Pavlis et al., 2012). Subduction of a thick crust model major findings are the basal décollement fault of the wedge stepped up to a higher stratigraphic level and that the wedge structural configuration adjusted by displacement on coeval out-of-sequence forethrust and backthrust faults, allowing uplift of the entire wedge. Potential implications for the St. Elias orogen with progressive subduction of thick crust are that the model results are consistent with the relocation of the décollement to higher stratigraphic positions (Pavlis et al., 2012; Van Avendonk et al., 2013), activation of out-of-sequence faults are common (Meigs et al., 2008; Pavlis et al., 2012), and exhumation of deeper crustal rocks within the interior of the wedge (Enkelmann et al., 2015). In summary, our findings indicate that glacial erosion, syntectonic sedimentation, and subduction of thick crust may have significant impact on the structural configuration of glaciated convergent margins.
Haq, Purdue University.
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