Low-temperature constraints on the Alpine thermal evolution of the central parts of the Sredna Gora Zone, Bulgaria
Abstract: The central parts of the Sredna Gora Zone in Bulgaria have experienced a complex Alpine tectonic evolution. The main tectonic and thermal events since the end of the Triassic are the Late Jurassic–Early Cretaceous (Early Alpine) and Late Cretaceous–Paleogene (Late Alpine) compression separated by Late Cretaceous volcanic-arc magmatism and intra/back-arc extension and basin formation. During most of the Cenozoic, the area was mainly under post-orogenic extension. Here, we present the first apatite and zircon fission-track results and new muscovite and biotite 40Ar/39Ar analysis on upper Carboniferous–Permian granitoids together with Upper Cretaceous volcanic and subvolcanic rocks and Upper Maastrichtian–Danian conglomerates from the Panagyurishte basin, which allowed us to reveal the Alpine thermal and tectonic evolution of the central parts of the Sredna Gora Zone. Our new results disclosed the existence of several thermal and cooling episodes related to different tectonic and magmatic events in the studied area. The 40Ar/39Ar dating of samples from the metamorphic basement constrain the thermal peak of an Early Alpine thermal event at about 140–138 Ma at temperatures between ~ 300 and 400 °C. Through the apatite FT dating and thermal modelling, the time of a Late Alpine (post-Danian) event was constrained at 65–55 Ma, during which the tectonically buried sediments of the Panagyurishte basin reached temperatures of
Provenance of the Lower Cretaceous clastic rocks from the Gadvan Formation (Zagros Basin, Iran): Evidences from whole-rock geochemistry and petrography
Abstract: The petrography and geochemistry of clastic rocks from the Gadvan Formation in the Abadan Plain (southwest Iran) were analysed to infer their weathering intensity, compositional maturity, provenance, and tectonic setting. The Index of Compositional Variability (0.47–0.71) indicates high compositional and mineralogical maturity. The Chemical Index of Alteration and the Plagioclase Index of Alteration suggest high intensity of chemical weathering in the source area. In addition, a remarkable high content of REE and LREE/HREE and Th/U ratios, as well as high C-value (1.7) suggest high chemical weathering in the source area. Rounded zircon grains, mineral homogeneity, and a lack of feldspar grains could be related to high weathering and the effect of recycling. Elemental ratios (La/Sc, La/Co, Th/Sc, Th/Co, GdN/YbN, Cr/Ni, Co/Th, La/Yb, and La/Th), bivariate diagrams (La/Sc vs. Co/Th, La/Sc vs. Th/Co, Cr/Th vs. Th/Sc, Th/Yb vs. Ta/Yb, and La/ Yb vs. La/Th), and an enrichment of Nb, Zr, Th, La, Cr, Ni contents imply felsic to intermediate parent rocks, which are similar to the massive granitoids of the Arabian Shield. This could be supported by the dominance of zircon grains, as well as inclusions of rutile and tourmaline in quartz grains in the Gadvan sandstones. Moreover, further petrographical evidences, such as undulatory quartz grains, rare biotite and a small orientation of grains is also related to low-grade metamorphism in a felsic source rock. Finally, the Mudrock Maturity Index and tectonic discrimination diagrams reveal a convergence process in a collisional setting, in which the Zagros Mountains originated.
The structural pattern and tectonic evolution of the Muráň fault revealed by geological data, fault-slip analysis, and paleostress reconstruction (Western Carpathians)
Abstract: The Muráň fault is perhaps the most distinctive, steeply-dipping, brittle structure in the Western Carpathians. An analysis of brittle deformation was used to gain the succession of tectonic evolution of the Muráň fault by paleostress tensors. Movement on this fault depended on spatial orientation of the principal paleostress axes representing the paleostress fields. The kinematic analysis of fault-slip data confirmed the predominant strike-slip nature of the fault during the entire history, which had sometimes been disrupted by quiescence periods or normal faulting. The Muráň fault may be as old as 85 Ma and originated as a ductile shear zone in deeper crust. It is possible to consider the Muráň fault as sinistral transpressional strike-slip fault during the latest Cretaceous to earliest Paleocene. During this time period, with given orientation of the paleostress field, the fault originated as a semi-brittle to brittle shear zone. A significant re-organization of the paleostress field was carried out approximately on the boundary of the Paleocene and Eocene periods. During this deformation, movement on the Muráň fault changed to dextral, and the secondary positive and negative flower structures in Mesozoic rocks were most likely formed in this time as well. These structures originated after the Danian, since sediments of the Gosau Group are incorporated into these structures. In the late Eocene, activity of the Muráň fault gradually began to decrease, and the fault structure is more or less covered by the upper Eocene transgressive deposits of the Central Carpathian Paleogene Basin. The Neogene evolution is characterised by a continuous change of the orientation of the principal maximum axis σ1 (SHmax, respectively) from the NW–SE through N–S to NE–SW position. The Muráň fault started to become sinistral transpressional to transtensional up to a normal fault, however, the movement along the fault was only several tens of metres. The Quaternary period is characterised by an extensional tectonic regime with the orientation of principal least axis σ3 in the WNW–ESE direction. Late Pleistocene to Holocene normal faulting is indicated by borehole analysis in the alluvial planes of the Rimava and Muráň rivers.
The petrography and geochemistry of clastic rocks from the Upper Cretaceous Terani Formation of the Cauvery Basin, Southern India
Abstract: The petrography and geochemistry of clastic rocks from the Upper Cretaceous Terani Formation of the Cauvery Basin were studied to decipher their intensity of weathering, provenance, and tectonic history. Texturally, the Terani sandstones are moderately sorted with sub-angular and sub-rounded grains, indicating short transport and low maturity. The average Quartz–Feldspar–Rock fragment (Q–F–R) ratio of the Terani sandstone is Q89–F3–R8. Geochemically, the Terani clastic rocks are classified as sublitharenites, Fe-sand, shale, and Fe-shale types. The chemical index of alteration (CIA), plagioclase index of alteration (PIA), and chemical index of weathering (CIW) suggested moderate to high intensity of weathering in the source area. The enrichment of rare earth element (REE) contents in the Terani clastic rocks relative to UCC (Upper Continental Crust) indicates a higher concentration of heavy minerals. Likewise, the average values of Eu/Eu* (0.16), La/Sc (2.94), La/Co (2.15), Th/Sc (1.08), Th/Co (0.79), Th/Cr (0.12), and Cr/Th (8.39) revealed that the Terani clastic rocks were derived from a combination of felsic and intermediate source rocks. The chondrite normalized REE patterns of clastic rocks are characterized by a relatively flat HREE (Gdcn/Ybcn = 1.71), enriched LREE (Lacn/Smcn = 4.15), and negative Eu anomaly (Eu/Eu* = 0.16), which suggest the contribution of sediments with less HREE depleted source rocks from the Archaean group. A comparison of the REE pattern and Eu anomalies from this study with potential source rocks infers that the Terani Formation received a major contribution of sediments from the Dharwar Craton.
A first account of the semi-endophytic coralline algae Lithophyllum cuneatum from the Caribbean Sea and its evolutionary and biogeographic significance
Abstract: The semi-endophytic coralline alga Lithophyllum cuneatum, which grows partially embedded by its host on its surface and lacks haustoria penetration to this host, was formerly known only from reef environments of the Pacific and Indian Ocean. Here, we report it for the first time from coral reefs of the Caribbean Sea (Belize). The morpho-anatomical characteristics of the Caribbean specimens from Holocene sediment cores, which were collected in offshore reef environments, match those of the type material and other specimens reported from the Pacific and Indian Oceans, including the preservation of diagnostic characteristics (cuneate thallus morphology, morphology of the conceptacles and their pore canals, and dimensions of the cells). Similar to L. cuneatum from the Holocene of the Indian and Pacific oceans, Holocene specimens from Belize share two unique hosts represented by the coralline algae Porolithon onkodes and Neogoniolithon sp. The unique occurrence of this species in the Caribbean Sea can be explained either (1) by pre-Pliocene dispersal toward the west from the present-day Indian Ocean area along the Tethyan seaway and/or (2) by dispersal toward the east via the Pacific (Fiji) Ocean when the Panama Isthmus was still open. Although morphologically-equivalent coralline algae can belong to either cryptic or pseudocryptic species, both scenarios imply a broader, more continuous geographic distribution of lineage leading to semi-endophytic Lithophyllum cuneatum prior to the Pliocene, which is in contrast to the more fragmented distribution during the Holocene. Although the lack of information about the geographic range of L. cuneatum prior to the Holocene can be coupled with sampling biases and cannot discriminate among these scenarios, other cases of such disjunct distributions, which were formerly documented among marine invertebrates, indicate that the geographic distribution of this species was less fragmented in the past, and thus supports the Tethyan dispersal hypothesis, including the relict character of its present-day geographic distribution.