The Lepontine Alps as an evolving metamorphic core complex during a‐type subduction: Evidence from heat flow, mineral cooling ages, and tectonic modeling

Abstract

New surface heat‐flow data are presented for the Valle Maggia region within the high‐grade metamorphic core of the Lepontine Alps. The steady state values range from 44.4 to 55.2 mW/m2 and are anomalously low compared with those measured around the periphery of the Lepontine. These differences are considered to reflect the combined effects of differential uplift and erosion superimposed on a transient, low heat flow regime which has existed since the Miocene. The uplift‐denudation history of the Lepontine is modeled by using various mineral geochronometers. Two major pulses of accelerated uplift are recognized at circa 22 m.y. B.P. and circa 10 m.y. B.P., respectively. These events directly coincide with times of dominant crustal convergence in the Helvetic and Jura stages of A‐subduction. From known PMax–TMax conditions of Oligocene metamorphism, estimates of low mantle heat flow and experimentally determined values of crustal heat production, best fit thermal models are presented to explain surface heat flow and mineral cooling age data for the metamorphic core. A two‐stage underplating model is shown to be consistent with the excavation history of the Lepontine and with geological and geophysical data from more external parts of the Central Alps. This information is integrated to explain the establishment of a crustal‐scale duplex in the Metamorphic Core Complex contemporaneous with thin‐skinned tectonics in the foreland.