Crustal growth and tectonic evolution of the Tianshan orogenic belt, NW China: A receiver function analysis

Abstract We applied tomographic inversion and receiver function analysis to seismic data from ocean-bottom seismometers and land-based stations to understand the structure and its relationship with slow slip events off Boso, Japan. First, we delineated the upper boundary of the Philippine Sea Plate based on both the velocity structure and the locations of the low-angle thrust-faulting earthquakes. The upper boundary of the Philippine Sea Plate is distorted upward by a few kilometers between 140.5 and 141.0°E. We also determined the eastern edge of the Philippine Sea Plate based on the delineated upper boundary and the results of the receiver function analysis. The eastern edge has a northwest–southeast trend between the triple junction and 141.6°E, which changes to a north–south trend north of 34.7°N. The change in the subduction direction at 1–3 Ma might have resulted in the inflection of the eastern edge of the subducted Philippine Sea Plate. Second, we compared the subduction zone structure and hypocenter locations and the area of the Boso slow slip events. Most of the low-angle thrust-faulting earthquakes identified in this study occurred outside the areas of recurrent Boso slow slip events, which indicates that the slow slip area and regular low-angle thrust earthquakes are spatially separated in the offshore area. In addition, the slow slip areas are located only at the contact zone between the crustal parts of the North American Plate and the subducting Philippine Sea Plate. The localization of the slow slip events in the crust–crust contact zone off Boso is examined for the first time in this study. Finally, we detected a relatively low-velocity region in the mantle of the Philippine Sea Plate. The low-velocity mantle can be interpreted as serpentinized peridotite, which is also found in the Philippine Sea Plate prior to subduction. The serpentinized peridotite zone remains after the subduction of the Philippine Sea Plate and is likely distributed over a wide area along the subducted slab.

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Petrogenesis and Tectonic Significance of the ~276 Ma Baixintan Ni-Cu Ore-Bearing Mafic-Ultramafic Intrusion in the Eastern Tianshan Orogenic Belt, NW China

Minerals ◽

10.3390/min11040348 ◽

2021 ◽

Vol 11 (4) ◽

pp. 348

Author(s):

Minxin You ◽

Wenyuan Li ◽

Houmin Li ◽

Zhaowei Zhang ◽

Xin Li

Keyword(s):

Orogenic Belt ◽

Olivine Gabbro ◽

Early Permian ◽

Eastern Tianshan ◽

Nw China ◽

Tianshan Orogenic Belt ◽

Tectonic Significance ◽

Ultramafic Intrusion ◽

Back Arc ◽

Duke Island

The Baixintan mafic-ultramafic intrusion in the Dananhu-Tousuquan arc of the Eastern Tianshan orogenic belt is composed of lherzolite, olivine gabbro, and gabbro. Olivine gabbros contain zircon grains with a U-Pb age of 276.8 ± 1.1 Ma, similar to the ages of other Early Permian Ni-Cu ore-bearing intrusions in the region. The alkaline-silica diagrams, AFM diagram, together with the Ni/Cu-Pd/Ir diagram, indicate that the parental magmas for the Baixintan intrusion were likely high-Mg tholeiitic basaltic in composition. The Cu/Pd ratios, the relatively depleted PGEs and the correlations between them demonstrate that the parental magmas had already experienced sulfide segregation. The lower CaO content in pyroxenites compared with the Duke Island Alaskan-type intrusion and the composition of spinels imply that Baixintan is not an Alaskan-type intrusion. By comparing the Baixintan intrusion with other specific mafic-ultramafic intrusions, this paper considers that the mantle source of the Baixintan intrusion is metasomatized by subduction slab-derived fluids’ components, which gives rise to the negative anomalies of Nb, Ti, and Ta elements. Nb/Yb-Th/Yb, Nb/Yb-TiO2/Yb, and ThN-NbN plots show that the Baixintan intrusion was emplaced in a back-arc spreading environment and may be related to a mantle plume.

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