Tectonic evolution of the Dabashan orocline, central China: Insights from the superposed folds in the eastern Dabashan foreland

The Dabashan orocline is situated in the northwestern margin of the central Yangtze block,central China.Previous studies have defined the orthogonal superposed folds growing in its central-western segment thereby confirming its two-stage tectonic evolution history.Geological mapping has revealed that more types of superposed folds have developed in the eastern segment of the orocline,which probably provides more clues for probing the structure and tectonic history of the Dabashan orocline.In this paper,based on geological mapping,structural measurements and analyses of deformation,we have identified three groups of folds with different trends （e.g.NW-,NE-and nearly E-trending folds） and three types of structural patterns of superposed folds in the eastern Dabashan foreland （e.g.syn-axial,oblique,and conjunctional superposed folds）.In combination with geochronological data,we propose that the synaxial superposed folds are due to two stages of ～N-S shortening in the west and north of the Shennongjia massif,and that oblique superposed folds have been resulted from the superposition of the NW-and NE-trending folds onto the early ～ E-W folds in the east of the Shennongjia massif in the late Jurassic to early Cretaceous.The conjunctional folds are composed of the NW-and NE-trending folds,corresponding to the regional-scale dual-orocline in the eastern Sichuan as a result of the southwestward expansion of the Dabashan foreland during late Jurassic to early Cretaceous,coeval with the northwestward propagation of the Xuefengshan foreland.Integration of the structure and geochronology of the belt shows that the Dabashan orocline is a combined deformation belt primarily experiencing a twostage tectonic evolution history in Mesozoic,initiation of the Dabashan orocline as a foreland basin along the front of the Qinling orogen in late Triassic to early Jurassic due to collisional orogeny,and the final formation of the Dabashan orocline owing to the southwestward propagation of the Qinling orogen during late Jurassic to early Cretaceous intra-continental orogeny.Our studies provide some evidences for understanding the structure and deformation of the Dabashan orocline.

Formation of the Moping Dome in the Xuefengshan Orocline, Central China and its Tectonic Significance

Many equiaxial dome-like structures developed in the north segment of the Xuefengshan orocline, Central China are obviously inconcordant with the NE-trending linear structures in this area, which contain important records for understanding the structural framework and evolution of this belt. In this paper, taking one of the typical dome-like structures in the Xuefengshan orcline （e.g. Moping dome-like structure） as an example, based on its structural framework interpratatoin, superposed deformation analysis and paleo-stress fields reconstruction, we propose the Moping dome- like structure is composed of two populations of different-striking thrust-fold structures, -E-trending and NE-striking structures, indicative of two-stages shortening, -N- and NW-striking, respectively. Together with the geochronological analysis, we suggest the first stage of shortening occurred in Late Triassic to Early Jurassic, due to the Indosinian intercollisional orogeny of the Yangtze Block and the North China Block. The second occurred during Late Jurassic-Early Cretaceous owing to Yanshanian intracontinental orogeny, leading to the intensive superposition of the NE-trending structures onto the -E-trending structures, and the final ocurrence of the Moping dome. Thus, our study indicates the Xuefengshan arc-shape belt also experienced two-phase deformation, and resulted from the superposition of NE-SW structures onto -E-W structures in Late Jurassic-Early Cretaceous, which could provide new structural evidence for probing the Mesozoic tectonic framework and evolution of the Xuefengshan orocline.

Documentation of the Sirjan Orocline in the southeast Sanandaj-Sirjan Zone, Iran

At the southeastern part of the SanandajSirjan Zone of Iran, a group of structural elements outline a large-scale arc curvature around a vertical axis. This curvature comprises several elongated structural elements and their dividing faults, axialfold traces, layering, and foliation. The most frequent lithological units include Paleozoic metamorphic rocks, Mesozoic-Paleogene sedimentary rocks, and Mesozoic magmatic-ophiolitic complex disposed in several anticlines and synclines, forming a horseshoeshaped structure with a 240-km arc length and a 90-km wavelength. We name this structure the Sirjan Orocline, and characterize this structure here through field observations and satellite image analyses. The Sirjan Orocline formed during the late EoceneOligocene related to the most significant deformation event after regional metamorphism. The final form of this structural arc is affected by a younger tectonic event that compressed and transected this structure.

Avalonia,get bent!–Paleomagnetism from SW Iberia confirms the Greater Cantabrian Orocline

The amalgamation of Pangea formed the contorted Variscan-Alleghanian orogen,suturing Gondwana and Laurussia during the Carboniferous.From all swirls of this orogen,a double curve in Iberia stands out,the coupled Cantabrian Orocline and Central Iberian curve.The Cantabrian Orocline formed at ca.315–290 Ma subsequent to the Variscan orogeny.The formation mechanism of the Cantabrian Orocline is disputed,the most commonly proposed mechanisms include either(1)that south-westernmost Iberia would be an Avalonian(Laurussian)indenter or(2)that the stress field changed,buckling the orogen.In contrast,the geometry and kinematics of the Central Iberian curve are largely unknown.Whereas some authors defend both curvatures are genetically linked,others support they are distinct and formed at different times.Such uncertainty adds an extra layer of complexity to our understanding of the final stages of Pangea’s amalgamation.To solve these issues,we study the late Carboniferous–early Permian vertical-axis rotations of SW Iberia with paleomagnetism.Our results show up to 70counterclockwise vertical-axis rotations during late Carboniferous times,concurring with the anticipated kinematics if SW Iberia was part of the southern limb of the Cantabrian Orocline.Our results do not allow the necessary penecontemporaneous clockwise rotations in Central Iberia to support a concomitant formation of both Cantabrian and Central Iberian curvature.The coherent rotation of both Gondwanan and Avalonian pieces of SW Iberia discards the Laurussian indenter hypothesis as a formation mechanism of the Cantabrian Orocline and confirms the Greater Cantabrian Orocline hypothesis.The Greater Cantabrian Orocline likely formed as a consequence of a change in the stress field during the late Carboniferous and extended beyond the Rheic Ocean suture affecting the margins of both Laurussia and Gondwana.

Exhumation and Deformation of the Daba Shan Orocline as Determined from Modern River Sands Apatite Fission-Track

The Daba Shan orocline is located at the northeastern margin of the Sichuan Basin and has been inferred as a foreland thrust-fold belt of the Qinling Orogen since the Late Triassic.A complete understanding of rock exhumation history is critical to elucidate how and when this typical orocline structure is developed.Detrital apatite fission-track dating of modern river sands is employed to reveal the regional exhumation history of the Daba Shan orocline.Four age peaks are identified and interpreted as the results of tectonic exhumation.Two older age peaks at ~150–140 and ~116–86 Ma are agreement with two main shortening deformation episodes of the Yanshanian Movement in the eastern China.The other two younger age peaks at ~69 and ~37 Ma support that the Daba Shan was reactivated by the Late Cretaceous to Cenozoic deformation which were likely related to the subduction of the Pacific Ocean and eastward growth of the Tibetan Plateau,respectively.It is worth noting that in contrast to the ~150–140 Ma rapid rock uplift and exhumation,the Middle Cretaceous exhumation(~116–86 Ma) shifted southward and continued to spread to southern tips of the Daba Shan.These exhumation variations in temporal and spatial allow a southward thrust deformation with piggyback style during the Yanshanian.

Oil/Gas Migration and Aggregation in Intra-Continental Orogen Based on Numerical Simulation: A Case Study from the Dabashan Orocline, Central China

Geofluid, driven by tectonic stress, can migrate and aggregate in geological body. Thus, numerical simulation has been widely used to rebuild paleo-tectonic stress field and probe oil/gas （one type of geofluid） migration and aggregation. Based on geological mapping, structural data, and mechanical parameters of rocks, we reconstruct the traces for gas/oil migration and aggregation in Dabashan intra-continental orogen using numerical simulation. The study shows that gas/oil, obviously dominated by late Middle Jurassic-Early Cretaceous paleo-tectonic stress field that is characterized by NE-SW shortening in the Dabashan thrust belt and SW-emanating shortening in its foreland belt, massively migrate from the Dabashan thrust belt to its foreland belt, that is, NE to SW, resulting in the formation of some probable favorable areas for oil/gas mainly along the Tiexi -Wuxi fault, in some superposed structure （e.g., Zhenba , Wanyuan , Huangjinkou , and Tongnanba areas）, and in the Zigui Basin. Thus, our study shows that numerical simulation can be effectively applied to study oil/gas migration and aggregation in intra-continental orogen and provided some significant evidences for oil/gas exploration.

Belt-Parallel Shortening in the Northern Apennines and Seismotectonic Implications

Major seismic activity in the Northern Apennines concentrates in few zones, distributed in a peculiar way. It is argued that such context may be plausibly explained as an effect of belt-parallel?shortening, which has caused oroclinal bending of the longitudinal ridges formed during the Late Miocene to Lower Pliocene evolutionary phase. The main effects of this process, developed since the upper Pliocene, have mainly affected the outer sectors of the belt. The major seismic sources have generated in the zones where different oroclinal bendings of adjacent ridges have produced extensional/transtensional deformation. In the inner side of the Northern Apennines, belt parallel shortening has occurred at a lower rate. The main effects have resulted from the shortening of the?Albano-Chianti-Rapolano-Cetona ridge. In particular, the proposed tectonic setting may account?for the moderate seismic activity that occurs in the Firenze, Elsa, Pesa, Siena and Radicofani basins.

BLOCK ROTATIONS AT THE NORTHERN EDGE OF INDIA (SPITI,N-INDIA) AND THEIR CONTINUATION TO THE EAST (MALARI, N-INDIA)-REGIONAL SIGNIFICANCE FOR THE TECTONIC DEVELOPMENT OF THE TETHYAN HIMALAYAS

In carboniferous and triassic metacarbonates (anchizone to lower greenschist facies) of the Tethyan Himalayas the characteristic remanent magnetisations are carried by magnetite (ChRM\-1) and pyrrhotite (ChRM\-2;Klootwijk & Bingham,1980;Appel et al.,1991 & 1995;Schill et al.,1999).Magnetite may carry a primary remanent magnetisation whereas the pyrrhotite component is secondary and related to the last cooling event below 300℃. Pyrrhotite is formed in marly carbonates during low\|grade metamorphism.In Spiti the last cooling is represented by an 40 Ar/ 39 Ar age of 42～45Ma (Wiesmayr & Grasemann,1999).Five locations were sampled in the Spiti valley (Fig.1).Besides a present earth field direction,both remanent components (ChRM 1+2 ) are present in single specimens.The contribution to the total NRM is around 30% for the ChRM\-2 (coercivity of 20～100mT) and only around 8% for the ChRM\-2 (unblocking temperature of 250～330℃).Despite of the contribution stable remanence directions could be obtained for the ChRM\-2.For all sampling locations well grouping overall mean directions were obtained (Table 1,Fig.1).Results from Losar and the lower Pin valley are preliminary.They were estimated by great circle analyses or by taking the residual component after AF\|demagnetisation. Clockwise block rotations of around 10～40° in respect to stable India since 42 Ma are calculated by using the apparent polar wander path of Besse & Courtillot (1991).The α 95 \|angles show no overlapping (Fig.1 small figure).Therefore local rotations are not negligible.