Control of inherited structural fabric on the development and exhumation of passive margins-Insights from the AraçuaíOrogen(Brazil)

The AraçuaíOrogen,in eastern Brazil,was formed during the Neoproterozoic–Cambrian amalgamation of West Gondwana.During the Mesozoic–Cenozoic opening of the South Atlantic Ocean,and the associated divergent tectonics,the orogen developed as basement to the passive margin of South America and was progressively covered by thick offshore sedimentary basins,particularly the Espírito Santo,Mucuri,and Cumuruxatiba basins,in which hydrocarbon systems have been exploited.Our understanding of the AraçuaíOrogen’s passive margin evolution,erosion,and sediment transfer to these basins ultimately depends on constraining the onshore exhumation in response to Mesozoic–Cenozoic events.Here,new and previously published data from apatite fission-track(AFT)analyses and inverse thermal history modelling of(Pre)Cambrian basement rocks from the AraçuaíOrogen resolve three discrete basement cooling and associated erosional exhumation episodes.In the Pre-Rift phase,Jurassic–Hauterivian erosion of the AraçuaíOrogen is most likely related to the adjoining intra-continental West Gondwana flexural subsidence,which increased hillslope and river erosion power.In the Rift and Transitional phases,Barremian–Albian accelerated phase of erosion is associated with the uplift of the Atlantic rift shoulders and the establishment of an oceanic base-level.In the Drift phase,reactivations in response to far-field stresses likely triggered a Late Cretaceous–Paleocene rapid erosion event.The rates at which these events unfolded vary spatially and are controlled by inherited structures.The AraçuaíOrogen experienced slower denudation rates in areas closer to the São Francisco Craton,which suggests that the tectonic reactivation and related surface uplift during the Mesozoic–Cenozoic is in first-order controlled by lithospheric rigidity.Furthermore,the structural framework of the Paramirim and Pirapora aulacogens and NE-oriented shear zones in the orogen’s southeast facilitated later reactivations.From the spatial pattern of denudation/exhumation of the AraçuaíOrogen during the Mesozoic–Cenozoic,we draw inferences on the tectonic development of the offshore basins regarding their hydrocarbon potentials.

Unravelling the protracted U-Pb zircon geochronological record of high to ultrahigh temperature metamorphic rocks:Implications for provenance investigations

The assessment of detrital zircon age records is a key method in basin analysis,but it is prone to several biases that may compromise accurate sedimentary provenance investigations.High to ultrahigh temperature(HT-UHT)metamorphism(especially if T>850℃)is herein presented as a natural cause of bias in provenance studies based on U-Pb detrital zircon ages,since zircon from rocks submitted to these extreme and often prolonged conditions frequently yield protracted,apparently concordant,geochronological records.Such age spreading can result from disturbance of the primary U-Pb zircon system,likewise from(re)crystallization processes during multiple and/or prolonged metamorphic events.In this contribution,available geochronological data on Archean,Neoproterozoic and Palaeozoic HT-UHT metamorphic rocks,acquired by different techniques(SIMS and LA-ICP-MS)and showing distinct compositions,are reassessed to demonstrate HT-UHT metamorphism may result in modes and age distributions of unclear geological meaning.As a consequence,it may induce misinterpretations on UPb detrital zircon provenance analyses,particularly in sedimentary rocks metamorphosed under such extreme temperature conditions.To evaluate the presence of HT-UHT metamorphism-related bias in the detrital zircon record,we suggest a workflow for data acquisition and interpretation,combining a multi-proxy approach with:(i)in situ U-Pb dating coupled with Hf analyses to retrieve the isotopic composition of the sources,and(ii)the integration of a petrochronological investigation to typify fingerprints of the HT-UHT metamorphic event.The proposed workflow is validated in the investigation of one theoretical and one natural example allowing a better characterization of the sedimentary sources,maximum depositional ages,and the tectonic setting of the basin.Our workflow allows to the appraisal of biases imposed by HT-UHT metamorphism and resulting disturbances in the U-Pb detrital zircon record,particularly for sedimentary rocks that underwent HT-UHT metamorphism and,finally,suggests ways to overcome these issues.