Quaternary outer fore-arc deformation and uplift inboard of the Panama Triple Junction, Burica Peninsula

摘要

The Panama Triple Junction (Cocos-Nazca-Caribbean) represents the point that abruptly separates the thick and rapidly subducting Cocos plate to the northwest from the thin and obliquely subducting Nazca plate to the southeast along the Central American convergent margin. New structural and geomorphic analyses on the Burica Peninsula, an outer fore-arc peninsula located only –100 km inboard the Panama Triple Junction, reveal that the peninsula is dominated primarily by contractional deformation along three listric thrust faults that root in the underlying plate boundary. The geometry and spatial distribution of these thrusts indicate that this deformation occurs primarily in response to the change in crustal thickness occurring as a result of eastern migration of the flank of the Cocos Ridge coeval with migration of the Panama Triple Junction at a rate of –55 mm/yr to the southeast. Mapping and detailed elevation surveys reveal eight marine terraces on the peninsula with a distribution of inner edge elevations indicating that uplift is spatially uniform from north to south along strike in this area. However, terraces along the northwest part of the peninsula are offset across major thrust faults. Age control provided by ~(I4)C, OSL and soil chronosequences indicate that the terraces within the easternmost portion of the peninsula range in age from Marine Isotope Stage (MIS) 3 to Holocene, a result that indicates that this portion of the peninsula is younger than –60 ka. Time-averaged uplift rates calculated from marine terraces and other Quaternary marine deposits yield consistent uplift rates that range between 2.1 ± 0.1 and 7.7 ± 0.5 mm/yr for samples older than 1 ka and between 6.9 ± 1.0 and 19.3 ± 8.0 mm/yr for samples younger than 1 ka. We interpret this temporal distribution in uplift rates to suggest that the eight terraces preserved on the peninsula are produced coseismically wherein the anomalously high uplift rates calculated from the youngest samples (<1 ka) are not yet averaged over a complete seismic cycle. These observations, combined with (1) shortening estimates from balanced cross sections indicating that minimum shortening decreases from northwest to southeast and (2) the observation of growth strata within the youngest marine units, are consistent with a space-for-time model for triple junction migration. These results indicate that both triple junction migration and the change in bathymetry occurring at the triple junction boundary are far more dominant factors in outer fore-arc deformation than the change in rate and obliquity of subduction and basal tractions that also occur on either side of the triple junction.