A SPECULATIVE MODEL ON THE NORTH AMERICA CORDILLERA IN THE EARLY CRETACEOUS: TECTONIC ESCAPE RELATED TO ARC COLLISION OF THE GUERRERO TERRANE AND A CHANGE IN NORTH AMERICA PLATE MOTION

Many terranes of western North America were accreted to the continent from Middle Jurassic to Early Cretaceous time. The location of many of these accretionary events is poorly known because the amount of syn- and post-accretion translation is widely debated. Thus the paleogeography of the Cordillera before the Cenozoic is poorly known. I believe selective datasets indicate that the Early Cretaceous of the North America Cordillera had a major arc – continent collision in the south and northward tectonic escape in the north. I adopt four conclusions from other workers for the Early Cretaceous: (i) the Guerrero terrane was an oceanic arc that collided with Mexico about 120 Ma; (ii) the Baja British Columbia (Baja BC) block had a moderate amount of northward translation (~1600 km) in the Late Cretaceous – early Tertiary that was preceded by ~800 km of southward translation (sinistral faulting) in the Early Cretaceous; (iii) the sinistral faulting in Baja BC occurred at the same time, and at the same latitude, as a dextral fault system of 200 – 400 km offset farther east in California, Nevada, and Idaho; (iv) the large volume of magmatism in the Sierra Nevada in the Late Cretaceous (100 – 85 Ma) was mainly due to lithospheric-scale underthrusting of North America under the Sierra Nevada and not arc processes. These conclusions lead to a speculative model in which the earliest Cretaceous (145 - 125 Ma) was dominated by sinistral oblique convergence. There was a major change in the tectonics of the Cordillera at 125 - 120 Ma that may have been driven by an arc - continent collision of the Guerrero terrane in Mexico and a change to more westerly absolute motion of the North America plate. These events resulted in major tectonic escape of the central and northern Cordillera to the north away from the arc collision in Mexico from 125 to 105 Ma. The collision+escape was accompanied by renewed eastward thrusting in the Sevier – Rocky Mountain thrust belt. This model is similar in many ways to the modern tectonics of the eastern Mediterranean to Caucasus region. The 105 - 85 Ma interval had major convergence and the beginning of northward translation of the Baja BC block. This time interval had two belts of magmatism, one from subduction and the other the result of the underthrusting of North American crust to the west.