| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.341.106.2014 Site U14201J.M. Jaeger, S.P.S. Gulick, L.J. LeVay, H. Asahi, H. Bahlburg, C.L. Belanger, G.B.B. Berbel, L.B. Childress, E.A. Cowan, L. Drab, M. Forwick, A. Fukumura, S. Ge, S.M. Gupta, A. Kioka, S. Konno, C.E. März, K.M. Matsuzaki, E.L. McClymont, A.C. Mix, C.M. Moy, J. Müller, A. Nakamura, T. Ojima, K.D. Ridgway, F. Rodrigues Ribeiro, O.E. Romero, A.L. Slagle, J.S. Stoner, G. St-Onge, I. Suto, M.H. Walczak, and L.L. Worthington2Background and objectivesA fundamental hypothesis to be tested with results from Expedition 341 is that the St. Elias orogen has undergone a perturbation that has markedly changed the spatial patterns and rates of deformation and exhumation in the orogenic wedge. It is proposed that enhanced glacial erosion associated with the mid-Pleistocene transition and the establishment of highly erosive ice streams leads to substantial mass redistribution in the wedge, shutting down existing regions of active deformation and refocusing the deformation and exhumation patterns of the orogen (Berger et al., 2008; Worthington et al., 2008, 2010; Chapman et al., 2008). Testing the hypothesis that the onset of ice streams with correspondingly high erosion rates has markedly altered orogenesis (Berger et al., 2008) can be accomplished by developing depositional ages and sediment accumulation rates and acquiring provenance records from near the Bering Glacier, where tectonic deformation patterns have been shown to evolve with sedimentation (Worthington et al., 2010). Site U1420 lies within the Bering Trough (Fig. F1B), a shelf-crossing trough formed by the Bering Glacier advancing across the shelf during glacial maxima (Carlson and Bruns, 1997). The drill site penetrates through an angular unconformity at shallow subseafloor depths (180–200 m expected) that has been suggested to be the first occurrence of grounded glaciers reaching extents equivalent to the modern shelf edge (Berger et al., 2008). In high-resolution seismic Line GOA2505 and coincident crustal scale Line STEEP09 (Figs. F1A, F2), a series of reflectors is imaged between the seafloor and the angular unconformity (Horizon H1). These reflectors are the signature of glacial advance–retreat cycles with a range of interpretations presented in Berger et al. (2008), Powell and Cooper (2002), Willems (2009), and Worthington et al. (2008). Seismic sequences stratigraphically above this angular unconformity were expected to represent proximal glacial deposits and many deeper sequences thought to represent shelf to slope glacimarine and paraglacial strata deposited during a time of a prograding margin with varying proximity to temperate ice. Lithofacies were expected to vary from coarse-grained diamict and conglomerate (ice proximal) to massive mud (ice-distal/withdrawn) (Fig. F3) (Powell and Cooper, 2002). Beneath this angular unconformity (Fig. F2) are the waning folds of a portion of the Pamplona zone, which is a fold-and-thrust belt that has accommodated some of the tectonic shortening caused by the underthrusting of the Yakutat Terrane beneath North America to form the St. Elias orogen (Worthington et al., 2010). Previous mapping suggests that folding within the Pamplona Zone beneath the Bering Trough waned by the time of seismic Horizon H2 (Fig. F2) (Berger et al., 2008; Worthington et al., 2010). These seismic profiles also cross active faults on the slope (BT1 and BT2) and abandoned faults beneath the current shelf (BT3, BT4, and BT5) (Fig. F1). Structures BT3, BT4, and BT5 are currently buried by more than ~1500 ms two-way traveltime (TWT) of undeformed sediments and have gradually been rendered inactive starting before deposition of Horizon 2 (Zellers, 1995; Berger et al., 2008; Worthington et al., 2008). Site U1420 is located adjacent to Structure BT4 with the goal of determining the age of cessation of deformation as indicated by the absence of growth strata above Horizon H2 (Worthington et al., 2010). Lack of significant deformation in the sequences above Horizon H2 indicates that the underlying faults were abandoned prior to the angular unconformity (Horizon H1), possibly because of loading by sediments (Berger et al., 2008; Worthington et al., 2010). On the forelimb of the fold associated with Fault BT5, seismic facies interpreted as a continental shelf break are present between 1.0 and 1.5 s TWT. This interpretation suggests that this location was a continental shelf edge prior to the formation of the angular unconformity associated with a glacial advance to near the modern shelf edge (Fig. F2). Truncations of seismic strata, the presence of growth stratal packages on the backlimb, and the overall geometry of Fault BT5 provide evidence that this structure accommodated Yakutat-North America convergence as a growth fold in addition to acting as the former shelf edge. The overall architecture of the continental margin is thus the product of coupled depositional and tectonic processes. |
|||
