A Crustal-Scale View at Rift Localization and Hyper-Extension Along the Fossil Adriatic Margin of the Alpine Tethys

Abstract:

Fossil rifted margins, whereby originally extended continental crust is subsequently stacked in orogenic belts, provide the opportunity to track rift-related tectonics across different crustal levels. As a result, the crustal and/or lithospheric architecture at different stages of the rift history can be elucidated. In this study, the tectono-thermal evolution of the fossil Adriatic continental margin, sampled in the Italian Southern Alps, is investigated to shed light on the processes responsible for rift localization and hyper-extension in the Mesozoic Alpine Tethys system. This area records distributed upper crustal stretching at 245-190 Ma followed by rift localization at ca. 185-180 Ma along the future western edge of the Adriatic micro-plate, culminating in exhumation of sub-continental mantle at 165-160 Ma. The minor orogeny-related overprint allowed investigation of the Temperature-time evolution of upper crustal rocks from different parts of the margin by (U-Th)/He zircon (ZHe) thermochronology, providing insights on the thermal evolution of rocks in the 160-200°C temperature window. Our study reveals a progressive westward younging of ZHe ages, from 280-240 Ma in the proximal margin to 215-200 Ma at the eastern edge of the future hyper-extended margin, where rocks located underneath ca. 3 km of overburden attained temperatures in excess of 160°C in the late Triassic. Published studies from the originally underlying lower crustal rocks of the Ivrea Zone, also cropping out in the Southern Alps, indicate that protracted fluid flow and minor magmatism were affecting the base of the Adriatic crust in the late Triassic, too. The spatial distribution of the detected heating-cooling cycle suggests that rift localization along the future western edge of the Adriatic plate was favored by a crustal-scale thermal anomaly, established at 215-210 Ma, followed by thermal decay by 200-190 Ma. Subsequent crustal necking, starting at 185-180 Ma, led to excision of continental crust and mantle exhumation. Importantly, this study demonstrates that rifted margins are likely to undergo crustal-scale heating-cooling cycles. Understanding the timing, amplitude, lateral extent and tectonic context of these thermal anomalies is currently one of the main challenges in the study of rifted margins.