THE NATURE AND TIMING OF MID-CRETACEOUS CORDILLERAN OROGENESIS: A TALE OF BASIN CLOSURES, STRIKE-SLIP, AND SUBDUCTION ZONES

The Cordilleran orogeny is a continuum of Mesozoic deformation but Early to mid-Cretaceous events are particularly prominent. Nonetheless, many Cordilleran geologists in the US have a limited view of this period because of lower-48 blinders: the 2D concept of the “Sevier Orogeny” where backarc contraction in the thrust belt is linked to east-dipping subduction beneath the Great Valley forearc and Sierran arc. The geology to both the north and south, however, indicate this concept has several problems. The extreme view of these problems gives rise to ribbon continent models, which have their own fundamental flaws, yet they along with the more realistic Baja BC model point out observations that cannot be reconciled with simple 2D views of the late Mesozoic orogen. The most important problems in this context are the unresolved magnitude of post-mid Cretaceous strike slip faulting that obscure mid-Cretaceous paleogeography. Dextral strike slip north of 44N is well documented and even minimum slip estimates restore parts of the Chugach forearc accretionary complex to paleolatitudes as far south as southern California, with inboard terranes successively less displaced. This implies the traditional view of the Franciscan-Great Valley-Sierra triplet directly adjacent to its present backarc may oversimplify the system. Unresolved strike-slip magnitudes and their relative timing lead to problems in resolving older tectonics due to the potential for miscorrelations across strike. Most significant is implications for the mid-Cretaceous contractional orogen that developed during closure of an ocean basin with a suture recognizable from Idaho to SW Alaska. The presence of the closed ocean basin is not in doubt but its nature and size is. Specifically, a popular theory is that the closed ocean basin was a backarc basin that opened in the Late Jurassic after initial accretion of Wrangellia, yet that theory largely rests on interpretations of geology in highly deformed metamorphic rocks and detrital zircon studies of sediments within the closed basin. The Jurassic collision followed by backarc opening hypothesis is well documented in parts of the lower 48, but differences in timing and rock associations lead to questions in applying this model to the Wrangellia composite terrane; a topic to be reviewed in this presentation.