OROGENIC WIDENING IN THE QUATERNARY ARGUES AGAINST CLIMATE CONTROL OF DEFORMATION IN THE CHUGACH/ST. ELIAS OROGENIC BELT, SOUTHERN ALASKA

Critical taper wedge theory is the gold standard by which climate control of convergent orogenic belts is inferred. As is widely recognized and reproduced by coupled erosion-deformation models, an increase in erosion in the face of a constant tectonic influx should be accompanied by a narrowing of orogenic belt width. Numerous papers now argue on the basis of thermochronologic data that deformation in the Chugach/ St. Elias Range (CSE) of southern Alaska responded directly to Quaternary climate change because glaciers dominated erosion of the orogenic belt. There is no question that glacial erosion has dominated the CSE for the past 5 to 6 Ma. What is unclear is whether glacial erosion occurs independently of rock uplift, which is required if climate, as communicated via changes in glacier distribution in time, dictates patterns of orogenic deformation. Exhumation on the windward side of the CSE varies between individual structures and as a function of which part of that structure is sampled. Loci of high exhumation rates varies in space as a function of structure to a greater extent than they cluster in time, which would be expected for a climatically-induced acceleration of exhumation. Shortening rate across the foreland fold-and-thrust belt on the windward side of the CSE since 5.6 Ma is ~13 – 14 mm/yr. Jumps in the thrust front location by 30 and 15 km occurred at 1.8 and 0.25 Ma, respectively. At the same time that global climate was changing toward Quaternary conditions, therefore, the CSE thrust belt incrementally accreted material to the toe of the wedge roughly 45 km into the undeformed foreland relative to the thrust front position at 1.8 Ma. Thus, the thrust belt response to an increase in glacial coverage is in fact the opposite of the expected response of a critical-taper wedge to an increase in hinterland erosion rate. The actual record of frontal accretion, thrust belt width increase, and cooling associated with individual structures implies that the CSE was at or above critical taper throughout the Quaternary. If correct the tectonic influx equaled or exceeded the erosional efflux, which implies that glacial erosion was paced by, not independent of, tectonic rock uplift rate.