New lithostratigraphic and chronostratigraphic, geochronologic, and sedimentary petrologic data illuminate the history of development of the North American Cordilleran foreland basin system and adjacent thrust belt from Middle Jurassic through Eocene time in northwestern Montana. The oldest deposits in the foreland basin system consist of relatively thin, regionally tabular deposits of the marine Ellis Group and fl uvial-estuarine Morrison Formation, which accumulated during Bajocian to Kimmeridgian time. U-Pb ages of detrital zircons and sandstone modal petrographic data indicate that by ca. 170 Ma, miogeoclinal strata were being deformed and eroded in hinterland regions. Sandstones of the Swift and Morrison Formations contain detrital zircons derived from the Intermontane belt. The Jurassic deposits probably accumulated in the distal, back-bulge depozone of an early foreland basin, as suggested by the slow rates of tectonic subsidence and tabular geometry. A regional unconformity separates the Jurassic strata from late Barremian(?) foredeep deposits. This unconformity possibly resulted as a combined effect of forebulge migration, decreased dynamic subsidence, and eustatic sea-level fall. The late Barremian(?)-early Albian Kootenai Formation is the fi rst unit that consistently thickens westward, as would be expected in a foredeep depozone. The subsidence curve at this time begins to show the convex-upward pattern characteristic of foredeeps. By Albian time, the foldand-thrust belt had propagated to the east and incorporated Proterozoic rocks of the Belt Supergroup, as indicated by sandstone compositions, detrital zircon ages in the Blackleaf Formation, and by crosscutting relationships in thrust sheets involving Belt Supergroup rocks in the thrust belt. A major episode of marine inundation and black shale deposition (Marias River Shale) occurred between the Cenomanian and mid-Santonian, and was followed by a regressive succession represented by the Upper Santonian-midCampanian Telegraph Creek, Virgelle, and Two Medicine Formations. Provenance data do not resolve the timing of individual thrust displacements during Cenomanian-early Campanian time. The Upper Campanian Bearpaw Formation represents the last major marine inundation in the foreland basin . By latest Campanian time, a major epi sode of slip on the Lewis thrust system had commenced, as recorded in the foreland by the Willow Creek and St. Mary River Formations in the proximal foredeep depozone.The fi nal stage in the evolution of the Cor dilleran fold-and-thrust belt and foreland basin system is recorded by the Paleocene-early Eocene Fort Union and Wasatch Formations, which were preserved in the distal foreland region. Regional extensional faulting along the fold-and-thrust belt began during the middle Eocene. The results presented here enable the establishment of links between previous geological work in Canada and the better known parts of the Cor dilleran foreland basin in the United States.

Andean retroarc compression associated with subduction and shallowing of the oceanic Nazca plate resulted in thin-skinned thrusting that partitioned and uplifted Cenozoic foreland basin fill in the Precordillera of west-central Argentina. Evolution of the central segment of the Precordillera fold-thrust belt is informed by new analyses of clastic nonmarine deposits now preserved in three intermontane regions between major east directed thrust faults. We focus on uppermost Oligocene-Miocene basin fill in the axial to frontal Precordillera at 31-32°S along the Río San Juan (Albarracín and Pachaco sections) and the flank of one of the leading thrust structures (Talacasto section). The three successions record hinterland construction of the Frontal Cordillera, regional arc volcanism, and initial exhumation of Precordillera thrust sheets. Provenance changes recorded by detrital zircon U-Pb age populations suggest that initial shortening in the Frontal Cordillera coincided with an early Miocene shift from eolian to fluvial accumulation in the adjacent foreland basin. Upward coarsening of fluvial deposits and increased proportions of Paleozoic clasts reflect cratonward (eastward) advance of deformation into the Precordillera and resultant structural fragmentation of the foreland basin into isolated intermontane segments. Apatite (U-Th)/He thermochronometry of basin fill constrains to 12-9 Ma the most probable age of uplift-induced exhumation and cooling of Precordillera thrust sheets. This apparent pulse of exhumation is evident in each succession, suggestive of rapid, large-scale exhumation by synchronous thrusting above a single décollement linking major structures of the Precordillera.

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Geochronologic, provenance, and sediment accumulation records from the long-lived (>100 m.y.) retroarc basin at the transition from the central to southern Andes provide improved resolution to examine the duration and controls on mixed-mode deformation and an enigmatic foreland depositional hiatus. Detrital zircon U-Pb ages for the Malargüe and Neuquén basin systems of western Argentina reveal shifts in exhumation and accumulation compatible with magmatic-arc and thrust-belt sources during unsteady Cretaceous-Neogene deformation. Fully developed foreland basin conditions were only achieved during separate periods of Late Cretaceous and Neogene shortening contemporaneous with possible episodes of enhanced coupling between a westward-advancing South American plate and the subducting Nazca slab. Separating these two contractional episodes is a 20-40 m.y. phase of reduced sedimentation and unconformity development, potentially signifying a neutral to extensional mode across the retroarc hinterland to forearc region during diminished plate coupling. We propose that the Andean orogen and its foreland and forearc basins have always been sensitive to variations in subduction dynamics, such that regional shifts in slab buoyancy and subduction geometry (particularly slab dip) superimposed on plate-scale shifts in convergence have governed mechanical coupling along the plate boundary and resulting fluctuations among contractional, extensional, and neutral tectonic regimes.

The southern Central Andes recorded retroarc shortening, basin evolution, and magmatic arc migration during Neogene changes in subduction. At 31-33°S, above the modern flat-slab segment, spatial and temporal linkages between thin-and thick-skinned foreland shortening, basement-involved exhumation of the main Cordillera, and lower-crustal hinterland thickening remain poorly resolved. We integrate new geochronological and thermochronological data for thrust sheets and Neogene foreland basin fill with structural, sedimentological, and passive seismic results to reconstruct the exhumation history and evaluate potential geometric linkages across structural domains. 40 Ar/ 39 Ar ages for volcanic horizons and zircon U-Pb ages for synorogenic clastic deposits in the Manantiales Basin constrain the minimum duration of synorogenic sedimentation to~22-14 Ma. Detrital zircon age distributions record sequential unroofing of hinterland thrust sheets until~15 Ma, followed by eastward (cratonward) advance of the deformation front, shutoff of western sediment sources, and a shift from fluvial to alluvial fan deposition at 14 Ma. Apatite (U-Th)/He cooling ages confirm rapid exhumation of basement-involved structural blocks and basin partitioning by~14-5 Ma, consistent with the timing of the Manantiales facies and provenance shifts and a coeval (~12-9 Ma) pulse of thin-skinned shortening and exhumation previously identified in the eastern foreland. Late Miocene-Pliocene (~8-2 Ma) cooling ages along the Chile-Argentina border point to hinterland uplift during the latest stage of Andean orogenesis. Finally, geophysical constraints on crustal architecture and low-temperature thermochronometry results are compatible with a hybrid thin-and thick-skinned décollement spanning retroarc domains.