No abstract

The Laramide province is characterized by foreland basin partitioning through the growth of basement arches. Although variable along the western U.S. margin, the general consensus is initiation of this structural style by the early Campanian (~80 Ma). This has been linked to flat-slab subduction beneath western North America, but the extent and cause for a flat slab remain debated, invoking the need for better constraints on the regional variations in timing of Laramide deformation. We present new conglomerate clast composition, sandstone petrographic, and detrital zircon U-Pb geochronologic data from the Upper Cretaceous Beaverhead Group in southwestern Montana that suggest a pre-Campanian history of basement-involved deformation. During the early stages of deposition (~88-83 Ma), two separate depositional systems derived sediment from the Lemhi subbasin and distal thrust sheets to the west as well as Paleozoic strata eroding off the exhuming Blacktail-Snowcrest arch to the east. Our data provide the first conclusive evidence for the longitudinal transport of gravel via Cordilleran paleorivers connecting sediment sources in east central Idaho to depocenters in southwestern Montana and northwestern Wyoming. Furthermore, erosion of Paleozoic strata by this time requires that the Blacktail-Snowcrest arch was exhuming prior to~88 Ma in order to remove the Mesozoic overburden. Later (~73-66 Ma) sediment flux was entirely from the foreland-propagating fold-thrust belt to the west. These results suggest that Laramide-style deformation in southwestern Montana preceded initiation elsewhere along the margin, requiring revision of existing models for Laramide tectonism.

Forearc basins are large sediment repositories that develop in the upper plate of convergent margins and are a direct response to subduction. These basins are part of the magmatic arc-forearc basin-accretionary prism "trinity" that defines the tectonic configuration of the upper plate along most subduction-related convergent margins. Many previous studies of forearc basins have explored the links between construction of magmatic arcs, exhumation of accretionary prisms, and sediment deposition in adjacent forearc basins. These studies provide an important framework for understanding firstorder tectonic processes recorded in forearc basins that are characterized by long-lived subduction of "normal" oceanic crust. Many convergent margins, however, are complicated by second-order subduction processes, such as flat-slab subduction of buoyant oceanic crust in the form of seamounts, spreading and aseismic ridges, and oceanic plateaus. These second-order processes can substantially modify the tectonic configuration of the upper plate both in time and space, and produce sedimentary basins that do not easily fit into the conventional magmatic arc-forearc basin-accretionary prism trinity.In this chapter, we discuss the modification of the southern Alaska forearc basin by Paleocene-Eocene subduction of a spreading ridge followed by OligoceneHolocene subduction of thick oceanic crust. This thick oceanic crust is currently being subducted beneath south-central Alaska and has an imaged maximum thickness of 30 km at the surface and 22 km at depth. Findings from southern Alaska suggest that forearc basins modified from flat-slab subduction processes may contain a sedimentary and volcanic stratigraphic record that differs substantially from typical forearc basins. Processes and sedimentary features that characterize modified forearc basins include the following: (1) flat-slab subduction of a buoyant, topographically elevated spreading ridge oriented subparallel to the margin prompts diachronous uplift of the forearc basin floor and exhumation of older marine forearc basin strata as the ridge is subducted. Passage of the spreading ridge leads to subsidence and renewed deposition of nonmarine sedimentary and volcanic strata that locally exceeds the thickness of the underlying marine strata. (2) Insertion of a slab window beneath the forearc basin during spreading ridge subduction produces local intrabasinal topographic highs with adjacent depocenters, as well as discrete volcanic centers within and adjacent to the forearc basin. (3) Flat-slab subduction of thick oceanic crust also results in surface uplift and exhumation of forearc basin sedimentary strata. However, the insertion of thick crust throughout the flat-slab region (i.e., lack of a slab window) inhibits subduction-related magmatism adjacent to the forearc basin. In the case of subduction of a >350-km-wide fragment of thick oceanic crust beneath south-central Alaska, exhumation of forearc basin strata located above the region of flat-slab subduction has prompted enhanced T...

The degree to which the lithosphere and mantle are coupled and contribute to surface deformation beneath continental regions remains a fundamental question in the field of geodynamics. Here we use a new approach with a surface deformation field constrained by GPS, geologic, and seismicity data, together with a lithospheric geodynamic model, to solve for tractions inferred to be generated by mantle convection that (1) drive extension within interior Alaska generating southward directed surface motions toward the southern convergent plate boundary, (2) result in accommodation of the relative motions between the Pacific and North America in a comparatively small zone near the plate boundary, and (3) generate the observed convergence within the North American plate interior in the Mackenzie mountains in northwestern Canada. The evidence for deeper mantle influence on surface deformation beneath a continental region suggests that this mechanism may be an important contributing driver to continental plate assemblage and breakup.

U-Pb ages (n = 403) of detrital zircons from the Dakota Formation in western Iowa and eastern Nebraska provide evidence for westwardflowing fluvial systems that stretched from the Appalachian highlands to the western U.S. Cordilleran foreland basin during Albian-Cenomanian time. Approximately 78% of detrital zircon grains match the ages of Grenvillian (1.3-1.0 Ga), Pan-African (750-500 Ma), and Paleozoic (500-310 Ma) bedrock sources located within the present-day Appalachian Mountains. The presence of minor detrital zircon grains of Paleoproterozoic (2.5-1.5 Ga) or Archean age (>2.5 Ga) indicates that northern source regions in Minnesota, Wisconsin, and Canada did not contribute a significant volume of sediment, as had been previously interpreted. Based on similarities between detrital zircon signatures in the midcontinent strata and time-equivalent Cordilleran foreland basin strata, Appalachian sources may have contributed a previously unrecognized volume of sediment to the Albian-Cenomanian foreland basin system. Sediment flux from the Appalachian region to the Cordilleran foreland basin during middle Cretaceous time may have been related to increased uplift and exhumation due to passage over a mantle plume track.