Yehua Shan

Paleozoic multiple accretionary and collisional processes of the Beishan orogenic collage

The Beishan orogenic collage is located in the southernmost part of the Altaids, and connects the Southern Tien Shan suture to the west with the Solonker suture to the east. The orogen was previously regarded as early Paleozoic in age in contrast to the surrounding southern Altaid collages, which are Late Paleozoic or even Early Mesozoic. This paper reviews the tectonic units of the Beishan orogen, which along a north-south traverse consists of several arcs and ophiolitic mélanges. These tectonic units were thrust imbricated and overprinted by strike-slip faulting during Permian-Triassic times, and the youngest strata involved in the deformation are Permian. Stitching plutons are Late Permian in age. Peaks of magmatic-metamorphic-tectonic activity, and paleomagnetic and paleogeographic data indicate that the Beishan orogenic collage evolved by development of several, Early to Mid-Paleozoic arcs in different parts of the Paleoasian Ocean. The Late Paleozoic collage is characterized by three active continental margins or island arcs that are separated by two ophiolitic mélanges. The northernmost active margin is represented by the Queershan arc, which may have lasted until the Permian. The Shibanshan unit is the southernmost, subduction-related continental arc along the northern margin of the Dunhuang block. In the Late Carboniferous to Permian the eastern end (promontory) of the Tarim Craton probably collided with the Chinese eastern Tien Shan arc, forming a new active continental margin, which interacted with the Beishan Late Paleozoic archipelago, generating a complicated subduction-accretionary orogen; this is suggested to be one of the last phases in the development of the long-lived Altaid accretionary orogenesis. The new model for this orogen bridges the gap between the western and eastern ends of the southern Altaids. The modern Timor-Australia collision zone with its many surrounding arcs is an appropriate analog for the Altaids in the Late Paleozoic.

Separation of polyphase fault/slip data: an objective-function algorithm based on hard division

The objective-function algorithm (OFA) on the basis of hard division is presented in the paper to separate polyphase fault/slip data. The separation is made by detection of linear structures existing in the data in Frys (1999) sigma space. Different from other exhaustive-search methods, the OFA is direct and robust in theory without any arbitrary assumptions. Polyphase fault/slip data are simulated under prescribed tensors in order to validate the method. The results show its efficiency in stress estimation. The accuracy of stress estimation is controlled by random errors in the orientation of fault striations and by similarity between prescribed stress vectors related to different tectonic phases. The similarity between controlling stress vectors has an obvious effect on the estimation either when random errors are sufficiently large or when some similarity coefficients between the vectors are large enough. The accuracy of stress inversion tends to decrease as the range in errors increases. The OFA makes a very good approach to recognition of similar controlling stress vectors from polyphase fault/slip data, which are often associated with spatial and temporal variation of the tectonic stress field in a region, thus critical to understanding of the formation of geological structures.

Transitions among Mariana-, Japan-, Cordillera- and Alaska-type arc systems and their final juxtapositions leading to accretionary and collisional orogenesis

Abstract ‘Arc system’ is used here as a collective term for a variety of arcs that occur along continental margins or in oceanic plates; it includes associated units from adjacent plates. Four major arc systems (Mariana-, Japan-, Cordillera- and Alaska-type) can be distinguished along the Circum-Pacific region. Some Japan-type arc systems in ancient orogens (e.g. the Altaids) may have been largely regarded as microcontinents because they have so-called Precambrian basement. Often the Cordillera-type arc systems can be very complicated, and if they are rifted away from the host continent they become more difficult to recognize. Commonly these arc systems interact mutually and with continental marginal sequences, leading to complicated accretionary and collisional orogens. The alternation between Western Pacific archipelagos and the Eastern Pacific active margin is the stereotype of accretionary and collisional orogenesis. More importantly, these four main types of arc systems can be juxtaposed into a final orogenic collage, which is another main expression of accretionary orogenesis. Only some parts of accretionary and collisional orogens can be terminated by attachment of a continent-size craton such as Tarim or even India, and even so the accretionary and collisional processes may continue elsewhere along strike. The significance of the interactions among these arc systems and their final juxtaposition has not been fully appreciated in ancient orogens. The Altaids together with the Circum-Pacific orogens offers a good opportunity to study such accretionary–collisional orogenesis.

The diversity of flow structures in felsic dykes

Abstract: Numerous structures and textures, which can be related to magma flow, were observed in felsic dykes intruding late Mesozoic granitoid plutons in the Jiaodong peninsula, in eastern Shandong province, eastern China. These flow structures may be classified into two categories, interior and peripheral. The former group includes magmatic bands, various types of folds (e.g. injection, sheath, similar and disharmonic folds), rotation of phenocrysts, magmatic foliation or lineation, and crenulation, whereas the latter includes hot tool marks and quarrying structures. Magmatic banding resulted from shearing of mingled magma during magma flow in the dykes. The magma seemed to flow rapidly, probably triggering turbulence in some thick dykes. Interaction at the contact between the hot, moving magma and the cold, stationary wallrock sometimes produced the peripheral structures. A few measurements of hot tool marks and of magmatic lineation reveal a roughly horizontal flow of magma within these dykes. For the dominant NE–SW-striking dyke set in the Laoshan granitoid pluton, the felsic magma probably ascended on or to the SW of the pluton to feed the dyke swarm, and then flowed laterally to drive the horizontal propagation of dykes.

Discussion on Mesozoic extensional structures of the Fangshan tectonic dome and their subsequent reworking during collisional accretion of the North China BlockJournal, Vol. 163, 2006, 127–142

Yu Wang writes: On the basis of descriptions of the stratigraphic sequences and structural features in the field, and interpretations of published data, Yan et al . (2006 a ) reach the surprising conclusion that early Mesozoic (early–middle Triassic) ESE-oriented extension resulted in formation of the Fangshan metamorphic core complex. If correct, the authors have made an important contribution to understanding of the tectonic evolution of North China. However, the way in which they cite previously published work to support their model, and discrepancies between their findings in the field and those of other workers raise a number of problems that will be discussed below. ### Citing of published work. Yan et al . (2006 a ) cited many references (e.g. Lei et al . 1994; Niu et al . 1994; Yu & Zhang 1996; Chen 1999; Fu 1999; Meng et al . 2003) in support of their conclusions about early Triassic extension and formation of metamorphic core complexes, as well as Late Cretaceous normal faulting. In many cases, however, the cited references do not lend support to their interpretations. For example, the Du Shan granite and surrounding Archaean and Proterozoic metamorphic rocks (Yu & Zhang 1996) contain no evidence for extension or development of metamorphic core complex features, but instead record NE-trending, strike-slip ductile shearing, not top-to-the-SE extension. The Malanyu dome is an old metamorphic sequence (Chen 1999), and no one has previously interpreted it as an extensional dome associated with SE-directed extensional shear. Zhang et al . (1991) did not examine any aspect of the geology in the area of the so-called Yuerya metamorphic core complex. Furthermore, no dome has been identified here by any previous geologists; instead, it is a c . 175–174 Ma (U–Pb sensitive high-resolution ion microprobe ages, Luo et al . 2001) granitic sheet-like …

Continuous restoration of deformed shapes through the construction of a reverse displacement gradient and its application to granite emplacement in central North China

This paper presents a new technique to restore 2D/3D deformed shapes by minimization of the misfit between measured and theoretical reverse displacement gradients. Owing to its relatively direct and objective nature, the procedure ensures continuity of displacement across the boundaries between abutting elements. The procedure also minimizes the additional distortion of the elements that results from the compatibility of retro-deformation between abutting elements. We apply the technique to the Mesozoic Fangshan granodioritic pluton (FGP) in western Beijing, central North China. Based upon the measured strain of deformed enclaves, about 78% area of the present central zone can be restored by retro-deformation of the marginal and transitional zones. As a result of the emplacement of the central zone, deformation also occurred in both the outer zones and the wall rocks, predominantly by ductile flow. The associated deformation in the outer zones, with or without the involvement of the wall rocks, created space for the emplacement of the central zone. In addition, vertical expansion was effective in creating space for subsequent emplacement, producing approximately one third of the volume created by later expansion. Thus, ballooning is a viable mechanism for emplacement of the FGP.