河南西南部典型白垩纪剖面的岩石磁组构特征及其构造意义

对河南西南部西峡盆地阳城-丹水上白垩统岩石磁学、磁组构的研究表明,该陆相沉积地层磁性矿物以及特征剩磁载体以赤铁矿为主,并含少量磁铁矿,且磁铁矿与赤铁矿相对含量比例随剖面由老到新显示出逐渐增加的趋势。岩石磁组构主要受顺磁性矿物以及反铁磁性矿物的控制。全剖面磁化率椭球结果显示在原生沉积组构基础上叠加了最早期阶段的弱变形组构;同时,全剖面以及各岩石地层单元磁化率椭球体主轴方向的变化揭示了两个不同方向及大小的构造挤压作用导致磁化率最大轴从NNW-SSE向转至NW-SE向,后又到NNW-SSE向的变化模式,一方面是由于华南、华北板块NE-SW向的构造挤压导致;另一方面是由两板块构造挤压而引起的桐柏-大别造山带上地壳物质相对下地壳发生东向构造逃逸和挤出过程中与东部块体物质间发生的挤压作用引起,这两种不同的挤压应力作用在盆地发展的不同阶段各占主要地位。     

Mesozoic extension in the Basque–Cantabrian basin (N Spain): Contributions from AMS and brittle mesostructures

In this work we analyse and check the results of anisotropy of magnetic susceptibility (AMS) by means of a comparison with palaeostress orientations obtained from the analysis of brittle mesostructures in the Cabuérniga Cretaceous basin, located in the western end of the Basque–Cantabrian basin, North Spain. The AMS data refer to 23 sites including Triassic red beds, Jurassic and Lower Cretaceous limestones, sandstones and shales. These deposits are weakly deformed, and represent the syn-rift sequence linked to basins formed during the Mesozoic and later inverted during the Pyrenean compression. The observed magnetic fabrics are typical of early stages of deformation, and show oblate, triaxial and prolate magnetic ellipsoids. The magnetic fabric seems to be related to a tectonic overprint of an original, compaction, sedimentary fabric. Most sites display a NE–SW magnetic lineation that is interpreted to represent the stretching direction of the Early Cretaceous extensional stage of the basin, without recording of the Tertiary compressional events, except for sites with compression-related cleavage.Brittle mesostructures include normal faults, calcite and quartz tension gashes and joints, related to the extensional stage. The results obtained from joints and tension gashes show a dominant N–S to NE–SW, and secondary NW–SE, extension direction. Paleostresses obtained from fault analysis (Right Dihedra and stress inversion methods) indicate NW–SE to E–W, and N–S extension direction. The results obtained from brittle mesostructures show a complex pattern resulting from the superposition of several tectonic processes during the Mesozoic, linked to the tectonic activity related to the opening of the Bay of Biscay during the Early Cretaceous. This work shows the potential in using AMS analysis in inverted basins to unravel its previous extensional history when the magnetic fabric is not expected to be modified by subsequent deformational events. Brittle mesostructure analysis seems to be more sensitive to far-field stress conditions and record longer time spans, whereas AMS records deformation on the near distance, during shorter intervals of time.     

U–Pb zircon and microfabric data of (meta) granitoids of western Cameroon: Constraints on the timing of pluton emplacement and deformation in the Pan-African belt of central Africa

Striking characteristics of the western Neoproterozoic belt of Cameroon (NFBC) are the large volume of granitoids and crustal-scale shear zones. New structural and geochronological data from this area are provided to put constraints on the tectonic evolution of this segment of the belt and to make further correlations between major shear zones exposed on both sides of the Atlantic Ocean.

Three different complexes have been identified in the study area: the migmatitic complex of Foumbot (MCF), the metagranitoid complex of Bangwa (BC), and the Batié pluton (BP). The MCF was intruded by the BC, while the BP cuts through the BC. U–Pb zircon dating of metaleucogranite and metagranodiorite of the BC yielded concordant to subconcordant ages of 638 ± 2 Ma and 637 ± 5 Ma, respectively. A concordant U–Pb zircon age of 602 ± 1.4 Ma has been obtained from porphyrogranite of the BP. These ages are interpreted as emplacement ages. Continuous deformation from magmatic to solid-state flow along the BP margins and the (sub)parallelism of the steep solid-state foliation in the BP margins with the foliation in the surrounding BC and MFC suggest synkinematic emplacement of the BP along crustal-scale NNE to ENE-trending strike–slip shear zones. Subhorizontal foliations in migmatitic-gneiss xenoliths found in the BC suggest that the major transcurrent motion was preceded by thrusting.

The new data confirm previous assumptions that the western NFBC is equivalent to parts of the Borborema province of Brazil. There are geochronological correlations between the studied (meta)granitoids and Brasiliano pre- to syn-transcurrent granitoids of the Borborema province.     

Timing of granite emplacement and cooling in the Songpan–Garzê Fold Belt (eastern Tibetan Plateau) with tectonic implications

New U–Pb and Rb–Sr geochronology on syn- and post-orogenic granites provide constraints on the timing of major tectonic events in the Songpan–Garzê fold belt, west Sichuan, China. The Ma Nai granite was probably syn-kinematic with the main deformation and yields an age of 197±6 Ma that is interpreted as an upper age limit of the Indosinian event. Zircons and apatites from the post-kinematic Rilonguan granite also yield Jurassic ages (195±6 and 181±4 Ma). The post-orogenic Markam massif gives two ages of 188±1 and 153±3 Ma. Both granites are undeformed and cut structures in the Triassic sedimentary rocks. These results demonstrate that the major deformation and décollement tectonics in the Songpan–Garzê fold belt occurred prior to the Early Jurassic. The wide range of ages obtained for post-kinematic granites (from Early Jurassic to Late Jurassic) suggests that, locally, magmatic activity persisted for a long time (at least 50 Ma) after the Indosinian compressional tectonism. No Tertiary ages have been obtained, suggesting that these granites were not affected strongly by the India–Asia collision.