Detrital Thermochronology Using Conglomerates and Cobbles

Abstract

Detrital thermochronology data from cobbles, either within modern sediments or basin stratigraphy, can provide excellent constraints on the exhumation history of the adjacent orogen or hinterland. This approach is especially powerful if multiple techniques are applied to each cobble. With cobbles, all grains have a common thermal history; thus, for apatite fission-track (AFT) thermochronology, ages are better defined than those for single-grains, and track-length measurements permit meaningful thermal models. Well-constrained basin stratigraphy is required to generate lag-time plots that combined with thermal modelling may constrain the rate and timing of cooling/exhumation events in the orogen, as well as later basin inversion. Limitations in this approach include the number of cobbles that need to be analysed to provide a representative sampling of the source region and to capture the pre-depositional exhumation history. Caveats also apply regarding closure temperature assumptions, integrated exhumation rates, time from erosion to basin deposition, variable provenance of the cobbles, paleo-relief in the source area, as well as burial-related partial annealing. Two examples illustrate this detrital cobble approach applied to basin stratigraphy. On the southern flank of the Pyrenean orogen, cobble AFT thermochronology from Eocene-to-Oligocene syntectonic conglomerates records three episodes of cooling/exhumation in the hinterland due to progressive movement of thrust sheets, followed by burial and Late Miocene re-excavation. The cobble thermochronologic record complements in-situ thermochronologic data from the orogen. South of the European Alps, thermochronology on conglomeratic clasts in the Gonfolite basin record exhumation in the Bergell region of the Central Alps. There, a stationary peak at {\textasciitilde}30 Ma records the emplacement of plutonic and volcanic rocks, whereas a moving peak starting at {\textasciitilde}25 Ma that decreases in age up-section records exhumation-related cooling.