Jens C. Grimmer

Tectonics of the Qinling (Central China): tectonostratigraphy, geochronology, and deformation history

Abstract The Qinling orogen preserves a record of late mid-Proterozoic to Cenozoic tectonism in central China. High-pressure metamorphism and ophiolite emplacement (Songshugou ophiolite) assembled the Yangtze craton, including the lower Qinling unit, into Rodinia during the ∼1.0 Ga Grenvillian orogeny. The lower Qinling unit then rifted from the Yangtze craton at ∼0.7 Ga. Subsequent intra-oceanic arc formation at ∼470–490 Ma was followed by accretion of the lower Qinling unit first to the intra-oceanic arc and then to the Sino-Korea craton. Subduction then imprinted a ∼400 Ma Andean-type magmatic arc onto all units north of the northern Liuling unit. Oblique subduction created Silurian–Devonian WNW-trending, sinistral transpressive wrench zones (e.g., Lo-Nan, Shang-Dan), and Late Permian–Early Triassic subduction reactivated them in dextral transpression (Lo-Nan, Shang-Xiang, Shang-Dan) and subducted the northern edge of the Yangtze craton. Exhumation of the cratonal edge formed the Wudang metamorphic core complex during dominantly pure shear crustal extension at ∼230–235 Ma. Post-collisional south-directed shortening continued through the Early Jurassic. Cretaceous reactivation of the Qinling orogen started with NW–SE sinistral transtension, coeval with large-scale Early Cretaceous crustal extension and sinistral transtension in the northern Dabie Shan; it presumably resulted from the combined effects of the Siberia–Mongolia—Sino-Korean and Lhasa–West Burma—Qiangtang–Indochina collisions and Pacific subduction. Regional dextral wrenching was active within a NE–SW extensional regime between ∼60 and 100 Ma. An Early Cretaceous Andean-type continental magmatic arc, with widespread Early Cretaceous magmatism and back-arc extension, was overprinted by shortening related to the collision of Yangtze–Indochina Block with the West Philippines Block. Strike–slip and normal faults associated with Eocene half-graben basins record Paleogene NNE–SSW contraction and WNW–ESE extension. The Neogene(?) is characterized by normal faults and NNE-trending sub-horizontal extension. Pleistocene(?)–Quaternary NW–SE extension and NE–SW contraction comprises sinistral strike–slip faults and is part of the NW–SE extension imposed across eastern Asia by the India–Asia collision.

Early- to mid-Silurian extrusion wedge tectonics in the central Scandinavian Caledonides

We present evidence for an extrusion wedge in the Scandian fold-thrust belt of the central Scandinavian Caledonides (Seve nappe complex). Rb-Sr multimineral geochronology in synkinematic assemblages indicates simultaneous movements at the normal-sense roof shear zone and at the reverse-sense floor shear zone between 434 Ma and 429 Ma. A Sm-Nd age of 462 Ma from a mylonitic garnet mica schist documents prograde garnet growth and possible incipient subduction. Pressure-temperature pseudosection calculations provide evidence for eclogite facies metamorphic conditions and nearly isothermal decompression at ∼670 ± 50 °C from 17.5 to 14.5 kbar in garnet-kyanite mica schists during reverse-sense shearing, and from 15 to 11 kbar in garnet mica schists during normal-sense shearing. These data and the presence of decompression-related pegmatites dated at 434 Ma and 429 Ma indicate that the Seve nappes form a large-scale extrusion wedge. This wedge extends along strike for at least 150 km and marks an early stage of ultrahigh-pressure metamorphism, exhumation, and orogenic wedge formation in this part of the Scandinavian Caledonides predating the major, post–415 Ma ultrahigh-pressure exhumation processes in southwestern Norway.

Köli nappes in the north-central Swedish Caledonides – new views on stratigraphy and structural evolution

Abstract The Köli Nappes of the Scandinavian Caledonides are composed of oceanic and island-arc terranes of Cambro–Silurian age and represent the upper part of the major Caledonian Upper Allochthon. Field, structural, microscopic, geophysical and geochemical data, together with a review of published information show new aspects on stratigraphy, structural evolution, nappe geometry and late Ordovician greywacke provenance in the Köli Nappes of southwestern Västerbotten and northern Jämtland counties, Sweden. It is proposed here to divide the lower part of the Köli succession of this area, i.e. the correlative of the Seima Formation, into three formations, the Kvemoen Mica Schist, the Ankarede Volcanite, and the Gilliks Formations. Trace element geochemistry of Ordovician Gilliks Formation greywackes indicates a provenance from a continental magmatic arc. New data also show that the overlying quartzite and limestone succession interpreted as the Vojtja and Slätdal Formations here is representing the top of the Stikke Nappe succession, which thus now reaches up into the highest Ordovician. The early repetition and inversion of the stratigraphy is an effect of D1-D2 deformation. The subsequent D3 episode comprises thrusting of, for example, the Middle Köli Nappe over the Lower Köli Nappe. A new aspect is that fault bends in D3 thrusts may give rise to fault-bend folds in overlying units, which are also D3 structures. The D3 episode also includes movement of the Köli units relative to the underlying Seve Nappes with top-to-the-west-directed shearing at the Seve-Köli boundary. It is succeeded by D4 regional-scale folds, related to nappe stacking in the underlying allochthons. The east-dipping limbs of these folds are thus locked for later, top-to-the-west shear deformation. Therefore, at least these parts of the Seve-Köli boundary cannot be regarded as re-activated during late (D5) and post-Caledonian gravitational deformation. Further work will have to show, whether the observed latest stage deformation represents late Caledonian tectonics or is related to subsequent extensional tectonics.

Traces of the Transscandinavian Igneous Belt in the central Scandinavian Caledonides: U-Pb zircon dating and geochemistry of crystalline basement rocks in the Middle Allochthon

Basement-slices are frequent components in the lower nappes of the central Scandinavian Caledonides. New geochronological and geochemical data provide evidence that three of these basement-slices in the Middle Allochthon are derived from the Transscandinavian Igneous Belt. Dated samples in this study comprise a quartz monzonite from the Stalon Nappe Complex and a quartz monzodiorite and a monzonite from the Ammarnäs Nappe Complex. A Ce/Yb-Ta/Yb-plot of the mafic rocks from the Ammarnäs Nappe Complex suggests a calc-alkaline to shoshonitic geochemical character. Low Ni- and Cr-contents and low Mg# indicate that the mafic rocks originated from an already evolved magma. Enrichment of Fe-Ti oxides and V>200 ppm indicate presence of cumulus phases to various degrees. These basement-derived rocks in the Middle Allochthon yielded concordant U-Pb zircon SIMS ages of 1799 ± 10 Ma, 1787 ± 6 Ma and 1797 ± 5 Ma. They are therefore interpreted to represent rocks detached from the Palaeoproterozoic Transscandinavian Igneous Belt and incorporated in the Middle Allochthon during Scandian orogeny.