The timing of polyphase Miocene tectonics in Northern Romania

This study addresses the polyphase Miocene tectonic evolution in the Maramures area (northern Romania) by combining field observations, stratigraphic arguments and fission-track analysis (Tischler et al. in press). Fission-track analysis has been carried out on basement samples from the Rodna horst, situated in the East Carpathians (Bucovinvian nappes). This area was affected by Cretaceous medium- to low-grade metamorphism, followed by post-collisional exhumation and renewed moderate thermal overprint due to the deposition of Eocene to Early Miocene sediments. Based on paleostress analyses of mesoscale structures, three main tectonic phases can be disdinguished in the study area, all of which are postdate the earliest Miocene (Aquitanian, 20.5 Ma). In late Early Miocene (Burdigalian) the Pienide nappes, nonmetamorphic flysch series, were emplaced onto the Paleogene to Early Miocene sedimentary cover of the Bucovinian nappes...conferenc

Thermal and structural evolution of the East Carpathians in northern Romania : from Cretaceous orogeny to final exhumation during Miocene collision

Combining thermochronological methods with structural field data, this study aims to reconstruct the Tertiary burial and exhumation history of the northeastern part of the Tisza‐ Dacia block during its invasion in the Carpathian embayment and final soft collision with the European margin. Zircon fission track data additionally provide information about the last metamorphic overprint during the Cretaceous. Within the basement units of the northern Central East Carpathians (Bucovinian nappe stack) and the Preluca massif (Biharia unit) zircon fission tracks are largely reset during the last (Cretaceous) orogeny. Temperatures during this Alpine metamorphic overprint increase from external to internal within the northern East Carpathians from sub‐greenschist facies conditions to at least greenschist facies conditions. Greenschist facies conditions are also observed in the Preluca massif. The close neighbourhood of Coniacian to Campanian zircon FT cooling ages with Cenomanian sediments suggests Late Cretaceous tectonic exhumation in the northern East Carpathians. The most likely explanation for this tectonic exhumation is a ‐ so far undocumented ‐ Late Cretaceous extension related to orogenic collapse (Gosau type basins). The Tertiary evolution is characterised by the invasion of the previously amalgamated Tisza‐Dacia and ALCAPA blocks into the Carpathian embayment and Miocene soft collision of both these blocks with the European margin. The juxtaposition of the ALCAPA and Tisza‐Dacia blocks, with Tisza‐Dacia in a lower plate position, is announced by the onset of Oligocene turbiditic sedimentation. The Burdigalian SE‐directed over‐thrusting of the Pienides (non‐ metamorphic flysch units) is interpreted to express the final stages of this juxtaposition. Burial heating caused full annealing of fission tracks in apatite (i.e. >120°C) in the Central East Carpathian basement in the NE of the study area, while in the basement of the Preluca massif in the SW of the study area temperatures related to Paleogene to Early Miocene burial did not exceed 80°C. The post‐Burdigalian (post‐16 Ma) soft collision of Tisza‐Dacia with the European margin occurred in two stages expressed by predominantly sinistral strike‐slip deformation during constant NE‐SW shortening. During and following soft collision, combined uplift and erosion caused exhumation of the buried rocks along the European margin. Uplift reaches up to 9 km in the Rodna horst The first post‐Burdigalian transpressional stage (16‐12 Ma) is related to perpendicular convergence of Tisza‐Dacia with the NW‐SE striking European margin. Minor exhumation during this stage resulted in Middle Miocene apatite fission track cooling ages (15‐13 Ma) in the west of the study area. The transtensional stage (12–10 Ma) led to the formation of the Bogdan‐Dragos‐Voda fault system. Sinistral transtension allowed for the docking and fitting of Tisza‐Dacia with the NW‐ SE striking European margin. Oblique E‐W convergence led to strain partitioning with thrusting in the external thrust belt and internal strike‐slip deformation. Differential lateral movements are distributed and deformation is accommodated by E‐W striking sinistral strike slip faults and SW‐NE striking normal faults, which led to fragmentation into SW‐tilted blocks. Fragmentation and differential offset along the bounding faults resulted in the formation of triangular‐shaped graben and corresponding horst structures. Enhanced exhumation during transtensional activity led to advective heat transport and resulted in Middle to Late Miocene apatite fission track cooling ages (13‐7) in the east of the study area

Tectono-Stratigraphic Evolution of the Onshore Namibe-Benguela-Kwanza Basins, Angola - Implications for Margin Evolution Models

Rifts III: Catching the Wave 22-24 March 2016 The Geological Society, Burlington House, LondonOnshore Angolan outcrops have been studied as part of a Brazil-West African conjugate margin project. Focus has been on rift margin to break-up axis tectono-stratigraphic evolution and associated strike variability of oblique and orthogonal rifted segments (Higgins et al., this conference). In this contribution we highlight key observations from Pre- and Post-Salt outcrops that occur as an elongate coastal strip extending from Namibe in the south to Kwanza in the north. Rift initiation deposits include interbedded basalts, rhyolites and aeolian-alluvial clastics that sit with low angle discordance on Pre-Cambrian basement. Volcanics (dated 132Ma) have geochemical affinity to the Parana-Etendeka LIP. Pre-rift fluvial-lacustrine deposits are locally evident beneath the volcanics, preserved as incised antecedent valleys. Rift climax and early sag sediments are absent onshore, with deposition focused outboard of the hinge line (present day offshore), whilst areas inboard of the hinge underwent uplift, erosion and sediment by-pass. By Early Aptian times a progressively onlapping-overstepping fluvial to marine Pre-Salt succession was deposited along the entire West African margin, cumulating in deposition of Loeme Fm evaporites. This succession includes spectacular fluvial-tidal incised palaeovalleys overlain by algal laminate-sabkha facies with halite pseudomorphs that pass transitionally into the overlying evaporite. Importantly for geodynamic models, this latest sag megasequence transgresses the subaerial rift margin, and effectively ¿seals¿ rift-related faults. The presence of evaporites within all studied palaeovalleys indicates extreme aridity and rift margin drainage shut-off. Sag outcrops also include lacustrine and ¿dam and cascade¿ carbonates (travertines, Sharp et al., 2013). Secondary diagenesis is complex (karstification, dolomitisation, silicification). The onshore observation of rift-initiation clastics/volcanics overlain by latest sag transgressive sediments, separated by a significant unconformity, can be linked to offshore seismic stratigraphic megasequences and distal margin evolution. Original onshore depositional thickness of evaporite, and percentage halite, is hard to estimate - 200-500m is postulated based on halokinetic structures within the overlying Pinda Gp, the basal part of which is pre-halokinetic (Binga Mbr), whilst the overlying mixed carbonate-clastic succession (Dondo-Catumbela) is syn-halokinetic, with spectacular growth geometries. Significant hinge line rotation is also recorded by the Albian succession. In all 3 basins Pinda Gp clastics show a close spatial relationship to Pre-Salt Aptian and modern valleys, indicating W. African drainage is antecedent in nature. Late Albian platform collapse follows, associated with extensional faulting, establishment of U. Albian slope successions and widespread emplacement of shallow sub-volcanic intrusives, subaerial to subaqeous lava flows and major volcanic centres (Sumbe Volcanics). Volcanism is diachronous along strike (Kwanza - 100-97Ma, Benguela - 91-86Ma, Namibe - 88-79Ma). In the Namibe Basin volcanism is proceeded by thick (>1500m) Albian-Cenomanian alluvial clastics with divergent hinge line growth geometries. Clastics are point sourced, associated with palaeovalley and palaeofan morphologies. Disconformably overstepping the Albian and older units is a Cenomanian-Coniacian transgressive sequence (Salinas Fm), itself intruded and overlain by Sumbe Volcanics. Volcanism in Namibe was locally associated with a return to subareal to lacustrine conditions (including travertines). Finally, flat lying successive transgressive-regressive passive margin wedges of Santonian-Campanian, Maastrichtian, Eocene, Miocene, Plio-Pleistocene and Holocene age were deposited, locally associated with spectacular seismic scale clinoforms.Rifts III