New insights from low-temperature thermochronology into the tectonic and geomorphologic evolution of the south-eastern Brazilian highlands and passive margin

The South Atlantic passive margin along the south-eastern Brazilian highlands exhibits a complex landscape,including a northern inselberg area and a southern elevated plateau,separated by the Doce River valley.This landscape is set on the Proterozoic to early Paleozoic rocks of the region that once was the hot core of the Aracuai orogen,in Ediacaran to Ordovician times.Due to the break-up of Gondwana and consequently the opening of the South Atlantic during the Early Cretaceous,those rocks of the Araquai orogen became the basement of a portion of the South Atlantic passive margin and related southeastern Brazilian highlands.Our goal is to provide a new set of constraints on the thermo-tectonic history of this portion of the south-eastern Brazilian margin and related surface processes,and to provide a hypothesis on the geodynamic context since break-up.To this end,we combine the apatite fission track(AFT)and apatite(U-Th)/He(AHe)methods as input for inverse thermal history modelling.All our AFT and AHe central ages are Late Cretaceous to early Paleogene.The AFT ages vary between 62 Ma and90 Ma,with mean track lengths between 12.2μm and 13.6μm.AHe ages are found to be equivalent to AFT ages within uncertainty,albeit with the former exhibiting a lesser degree of confidence.We relate this Late Cretaceous-Paleocene basement cooling to uplift with accelerated denudation at this time.Spatial variation of the denudation time can be linked to differential reactivation of the Precambrian structural network and differential erosion due to a complex interplay with the drainage system.We argue that posterior large-scale sedimentation in the offshore basins may be a result of flexural isostasy combined with an expansion of the drainage network.We put forward the combined compression of the Mid-Atlantic ridge and the Peruvian phase of the Andean orogeny,potentially augmented through the thermal weakening of the lower crust by the Trindade thermal anomaly,as a probable cause for the uplift.

Chemical-Abrasion U-Pb zircon geochronology reveals 150 Myr of partial melting events in the Archean crust of the São Francisco Craton

Field observations and CA-LA-ICP-MS U–Pb zircon ages and Hf isotope compositions obtained from migmatitic orthogneisses and granitoids from the Belo Horizonte Complex,southern São Francisco Craton,indicate a major period of partial melting and production of felsic rocks in the Neoarchean.Our observations show that the complex is an important site for studying partial melting processes of Archean crystalline crust.Much of the complex exposes fine-grained stromatic migmatites that are intruded by multiple leucogranitic veins and sheeted dikes.Both migmatites and leucogranite sheets are crosscut by several phases of granitoid batholiths and small granitic bodies;both of which are closely associated with the host banded gneisses.Chemical abrasion followed by detailed cathodoluminescence imaging revealed a wide variety of zircon textures that are consistent with a long-lived period of partial melting and crustal remobilization.Results of U-Pb and Hf isotopes disclose the complex as part of a much wider crustal segment,encompassing the entire southern part of the São Francisco Craton.Compilation of available U-Pb ages suggests that this crustal segment was consolidated sometime between 3000 Ma and 2900 Ma and that it experienced three main episodes of partial melting before stabilization at 2600 Ma.The partial melting episodes took place between 2750 Ma and 2600 Ma as a result of tectonic accretion and peeling off the lithospheric mantle and lower crust.This process is likely responsible for the emplacement of voluminous potassic granitoids across the entire São Francisco Craton.We believe that the partial melting of Meso-Archean crystalline crust and production of potassic granitoids are linked to a fundamental shift in the tectonics of the craton,which was also responsible for the widespread intrusion of large syenitic bodies in the northern part of the craton,and the construction of layered mafic–ultramafic intrusions to the south of the BHC.

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.