Kenneth Ridgway, PhD , Purdue University
The combination of field-based geologic studies and numerical modeling provides a robust tool for evaluating the geodynamics of convergent margins. Analysis of crustal scale geologic features in the field provide building blocks and realistic constraints for numerical models that help us understand the present state of strain at the Earth’s surface. We are using this conceptual approach to interpret the modern basin dynamics and those recorded by Cenozoic strata in southern Alaska. The Cook Inlet and Bristol Bay sedimentary basins contain a rich stratigraphic database that helps us understand key stages in the Cenozoic development of this convergent margin. The first objective of this research is to refine the Cenozoic stratigraphic framework for the Bristol Bay and Cook Inlet basins, and then compare this stratigraphy to other basins in southern Alaska to filter out local versus regional controls on subsidence. A second objective is to develop a geodynamic framework through numerical modeling constrained by geophysical and field-based observations. A better understanding of the geodynamic factors is important for interpreting the role of Cenozoic flat-slab subduction along the southern Alaska margin.
New sedimentological and biostratigraphic data collected from the Bear Lake Formation has helped us understand the development of this part of the southern Alaska margin. The Bear Lake Formation is considered to be one of the most promising potential reservoirs in the frontier Bristol Bay basin in southwestern Alaska. We have integrated stratigraphic, sedimentologic, and macrofauna/palynological data to establish the first chronostratigraphic framework for the Bear Lake Formation. This framework allows us to correlate between major outcrop belts and to develop an understanding of the depositional systems represented by the Bear Lake Formation. Our goal in this study is to provide the explorationist with some basic stratigraphic and sedimentologic tools for evaluating the Bear Lake Formation, which have applicability to core, well log, and/or seismic data.
The Cook Inlet basin is the other major sediment repository along the southern margin of Alaska. New detrital zircon U-Pb geochronology, rare earth element geochemistry, and clast compositional data from middle Eocene-Pliocene strata in this forearc basin demonstrate the importance of sediment sources located in the far retroarc region (>300 km inboard) and along strike within the basin. Provenance studies of most forearc basins have interpret the primary sediment sources for the basin to be the nearby magmatic arc and accretionary prism.
The Yakutat microplate has recently been reinterpreted to represent buoyant crust that is presently subducting at a shallow angle beneath southern Alaska. Our ongoing work is developing a new synthesis of stratigraphic, geochronologic, and thermochronologic data from the region directly above and around the perimeter of the flat-slab subduction in southern Alaska. The upper-plate geologic record of flat-slab subduction processes is a topic of considerable discussion for both modern and ancient convergent margins. We document a change from Paleocene–Oligocene subduction-related volcanism and basin development to northward-propagating exhumation during Oligocene–Pliocene time. Our new compilation indicates that flat-slab subduction of the Yakutat microplate was shaping southern Alaska by Oligocene time and continues to the present. This is the first study to link basin development along a convergent margin with flat-slab subduction processes in the upper plate.
Another major objective of our research is to understand the discrete contributions of unique driving forces for lithospheric deformation in western Canada and Alaska. These forces have not been quantified in detail, so their relative role in influencing deformation has remained unresolved. We are developinng a synoptic analysis of the kinematics and neotectonics of Alaska and western Canada, and in particular addresses the location and nature of the present day plate boundaries in the region. We use numerical finite element models that are constrained by observations of long-term strain rates, including plate and microplate motion models, ridge spreading rates, seismicity, and Quaternary fault slip rates, as well as recently released GPS data that have not previously been used to model the region, to produce a continuous velocity and strain rate field. Our results have several important implications in Alaska and western Canada: 1) non-rigid accommodation of deformation is an important mechanism and is required in any kinematic model of the region; 2) tectonic extrusion likely does not play an important role in the current kinematics; 3) the relative motion between the Pacific and Bering plates may be a governing driver in the neotectonics of the region; and 4) the North America-Pacific-Bering plate boundary is a far-reaching zone of diffuse deformation that extends for more than 1000 km across Alaska. Using finite element models, we calculate a continuous strain rate and velocity field that provides evidence that a wide zone of diffuse deformation defines the present-day boundaries between the North America, Pacific, and Bering plates in Alaska and western Canada. In southern Alaska, boundary forces related to flat-slab subduction of the Yakutat microplate are the dominant driver for lithospheric deformation, whereas in central and northern Alaska and inboard parts of western Canada, buoyancy forces and basal tractions may be the dominant contributors.