Geochronology of the mid-German crystalline rise west of the River Rhine

The mid-German crystalline rise has its westernmost exposures at the western margin of the Rhine graben in the southern Pfalz and the northern Alsace. The outcrops are made up of granitoid rocks and minor volcano-sedimentary sequences. Radiometric ages obtained by U/Pb, Pb/Pb, Sm/Nd and Rb/Sr analyses of the igneous rocks from this area range from ∼433 to ∼325 Ma thus covering a time span from the Silurian to the end of the lower Carboniferous. Because the investigated rocks are — according to their chemical composition — largely related to subduction zone environments, the following three geodynamical scenarios are postulated, always taking subduction of oceanic crust beneath the mid-German crystalline rise into account: (a) subduction of the Rheic ocean during the Silurian from the north; (b) subduction of the Rhenohercynian ocean during the late Devonian (∼369 Ma) from the north; (c) subduction of the Saxothuringian ocean during the lower Carboniferous (∼334 Ma) from the south.     

造山与成盆作用形成于统一的动力学机制

造山带和盆地成因是当前地学研究的两个热点。本文第一部分简述了多种主要造山理论和成盆学说;其中挤压造山说与拉张成盆论在众多学说中仍占主导地位。一般都认为造山带和盆地形成于不同的地球动力学环境。第二部分结合我国地质实际,提出了造山和成盆作用形成于统一的动力学环境,即他们的形成是由于挤压后效——拉张-均衡造山、成盆。     

西秦岭造山带泥盆纪沉积地质学和动力沉积学研究:古地理、地层层序及构造演化

晚加里东到早海西期,西秦岭北带存在一较大规模的造山带,泥盆纪的古地形呈北高南低的特下。持续的海侵由南向北侵进。中泥盆世由于北秦岭造山带的向南爷冲,形成同造山的前陆拗陷盆地。南秦岭裂陷槽是早古生代小洋盆的残余海槽。     

Las Matras Block, Central Argentina (37°S-67°W): the Southernmost Cuyania Terrane and its Relationship with the Famatinian Orogeny

The Las Matras Block in Central Argentina constitutes the southernmost part of the Cuyania terrane, which was accreted to the southwestern margin of Gondwana during the Early to Mid Ordovician Famatinian orogeny. The Grenville-aged rocks of the Las Matras Block are represented by the tonalitic to trondhjemitic Las Matras pluton. A new U-Pb conventional zircon age of 1244±42 Ma confirms previous Sm-Nd and Rb-Sr isochron ages of this pluton. Mineral composition data are consistent with the tonalitic-trondhjemitic character of the pluton, and constrain its emplacement level to 1.9 to 2.6 kb. This shallow level of emplacement and the undeformed character of the pluton are distinctive features of this southernmost basement. A regional comparison indicates that the igneous-metamorphic evolution of the Grenville-aged basement rocks of the Cuyania terrane occurred over a period of more than 200 million years, with ages older than 1200 Ma up to those close to 1000 Ma. The shallowest crustal level is found in Las Matras, suggesting a southward shallowing of the exposed level of basement. The deformation and metamorphism associated with the collisional Famatinian orogeny affect both the Cuyania terrane and the adjacent western margin of Gondwana, and the Gondwana margin was also the locus of the related arc magmatism, but the compressive effects of the collision decrease in intensity toward the south. The Famatinian metamorphism and magmatism continue even further south into the Patagonia region, but the southern continuity of the Cuyania terrane into this region remains uncertain.     

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.     

Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profiling

The TRANSALP Group, comprising of partner institutions from Italy, Austria and Germany, acquired data on a 340 km long deep seismic reflection line crossing the Eastern Alps between Munich and Venice. Although the field work was split into four campaigns, between fall 1998 and summer 2001, the project gathered for the first time a continuous profile across the Alps using consistent field acquisition and data processing parameters. These sections span the orogen itself, at its broadest width, as well as the editor Fred Davey and the two adjacent basins. Vibroseis and explosion data, complementary in their depth penetration and resolution characteristics, were obtained along with wide-angle and teleseismic data. The profile shows a bi-vergent asymmetric structure of the crust beneath the Alpine axis which reaches a maximum thickness of 55 km, and 80–100 km long transcrustal ramps, the southward dipping ‘Sub-Tauern-Ramp’ and the northward-dipping ‘Sub-Dolomites-Ramp’. Strongly reflective patterns of these ramps can be traced towards the north to the Inn Valley and towards the south to the Valsugana thrust belt, both of which show enhanced seismicity in the brittle upper crust. The seismic sections do not reveal any direct evidence for the presence of the Periadriatic Fault system, the presumed equivalent to the Insubric Line in the Western Alps. According to our new evolutionary model, the Sub-Tauern-Ramp is linked at depth with remnants of the subducted Penninic Ocean. The ‘crocodile’-type model describes an upper/lower crustal decoupling and wedging of both the European and the Adriatic–African continents.     

Paleomagnetic Study of the Juiz de Fora Complex, SE Brazil: Implications for Gondwana

The Juiz de Fora Complex is mainly composed of granulites, and granodioritic-migmatite gneisses and is a cratonic basement of the Ribeira belt. Paleomagnetic analysis on samples from 64 sites widely distributed along the Além Paraíba dextral shear zone (SE Brazil, Rio de Janeiro State) yielded a northeastern, steep downward inclination direction (Dm=40.4°, Im=75.4, a₉₅=6.0°, K=20.1) for 30 sites. The corresponding paleomagnetic pole (RB) is situated at 335.2°E; 0.6°S (a₉₅=10.0°; K=7.9). Rock magnetism indicates that both (titano)magnetite and titanohematite are the main magnetic minerals responsible for this direction. Anisotropy of low-field magnetic susceptibility (AMS) measurements were used to correct the ChRM directions and consequently its corresponding paleomagnetic pole. This correction yielded a new mean ChRM (Dm = 2.9°, Im = 75.4°, a₉₅ = 6.4°, K = 17.9) whose paleomagnetic pole RBc is located at 320.1°E, 4.2° N (a₉₅=10.3°, K=7.5). Both mean ChRM and paleomagnetic pole obtained from uncorrected and corrected data are statistically different at the 95% confidence circle. Geological and geochronological data suggest that the age of the Juiz de Fora Complex pole is probably between 535–500 Ma, and paleomagnetic results permit further constraint on these ages to the interval 520–500 Ma by comparison with high quality paleomagnetic poles in the 560–500 Ma Gondwana APW path.     

Metamorphic history of high-pressure granulites in Payer Land, Greenland Caledonides

A high‐P granulite facies gneiss complex occurs in north‐west Payer Land (74°28′?74°47′N) in the central part of the East Greenland Caledonian (Ordovician–Devonian) orogen. High‐P metamorphism of the Payer Land gneiss complex resulted in formation of the assemblages Grt + Cpx + Amp + Qtz + Ru ± Pl in mafic rocks, and Grt + Ol + Cpx + Opx + Spl in rare ultramafic pods. Associated metapelites experienced anatexis in the kyanite stability field. Peak metamorphic assemblages formed around 800–850 °C at pressures of c. 1.4–1.7 GPa, corresponding to crustal depths of c. 50 km. Mafic granulites contain abundant reaction textures, including the replacement of garnet by symplectites of Opx + Spl + Pl, indicating that the high‐P event was followed by decompression while the granulites remained at elevated temperatures. Charnockitic gneisses from Payer Land show evidence of late Archean (c. 2.8–2.4 Ga) crustal growth and subsequent Palaeoproterozoic (c. 1.85 Ga) metamorphism. The gneiss complex experienced intense reworking during the Caledonian continental collision. On the basis of Caledonian monazite ages recorded from the high‐P anatectic metapelites, the clockwise P–T evolution and formation of the high‐P granulite facies assemblages is related to Caledonian crustal thickening, which resulted in formation of eclogites approximately 300 km north of Payer Land. The Payer Land granulites comprise a metamorphic core complex, which is separated from the overlying low‐grade supracrustal rocks (the Neoproterozoic Eleonore Bay Supergroup) by a late Caledonian extensional fault zone, the Payer Land Detachment. The steep, nearly isothermal, unloading P–T path recorded by the granulites can be explained by erosional and tectonic unroofing along the Payer Land Detachment.