Meng Wan Yeh

Structural constraints on the timing of left-lateral shear along the Red River shear zone in the Ailao Shan and Diancang Shan Ranges, Yunnan, SW China

The >1000-km-long Oligocene—Miocene left-lateral Red River shear zone (RRSZ) and metamorphic belt and the Pliocene—active right-lateral Red River fault (RRF), stretching from SE Tibet to the South China Sea, has been cited as one of the primary examples of a lithospheric scale strike-slip fault that has resulted in syn-kinematic metamorphism and partial melting and accommodated several hundred to a thousand kilometers of horizontal motion as a result of the indentation of India into Asia. Alternatively we interpret the metamorphic complexes along the RRSZ as exhumed metamorphic core complexes of older rocks, subsequently affected by Oligocene–Early Miocene left-lateral shear and localized partial melting (leucogranite dykes), Miocene low-angle normal faulting along margins (Range Front faults), and Pliocene active dextral strike-slip faulting (RRF). Along the Ailao Shan (ALS) and Diancang Shan (DCS) ranges in Yunnan, SW China, early amphibolite facies metamorphic rocks were intruded by K-feldspar orthogneisses of Triassic age (Indosinian). LA-ICP-MS U-Pb zircon dating reveals a complex history with zircon cores showing evidence of Indosinian (∼239–243 Ma) to Neoproterozoic magmatism. Zircon rims show an Oligocene (∼26 Ma) magmatic or metamorphic overprint. Biotite granodiorites and syenites of mantle origin intruded the gneisses during the Oligocene (∼35 Ma). Later biotite leucogranites intruded the orthogneisses and migmatite host rocks before a significant phase of tight to isoclinal folding. Ductile, left-lateral strike-slip shear fabrics were superimposed on all lithologies at high temperature (∼500–550 °C) for the ALS and lower temperatures (∼250–150 °C) after peak metamorphism and after granite intrusion. A few very small biotite (±Grt ± Tur) leucogranite veins and dykes crosscut the ductile strike-slip shear fabrics at Yuanjiang, in the Ailao Shan. Low-angle normal faulting along the margins of the metamorphic massif accommodated final exhumation of the Red River gneisses. Using published U-Th-Pb and 40 Ar/ 39 Ar ages of granites along the shear zone, the age of left-lateral ductile shearing along the RRSZ can be constrained as between the earlier deformed leucogranites (31.9–24.2 Ma) and the later crosscutting dykes (21.7 Ma) with exhumation-related cooling continuing until ∼17 Ma.

Multiple mantle sources of the Early Permian Panjal Traps, Kashmir, India

The Early Permian Panjal Traps of northern India are the volcanic remnants of continental rifting that led to the formation of the Neotethys Ocean and the ribbon-like continent Cimmeria. The Traps are one of at least five major mafic eruptions of flood basalts during the Late Palaeozoic however their origin and petrogenesis are poorly constrained. Basalts from the Kashmir Valley were collected and analyzed for chemical and isotopic (Sr, Nd) compositions in order to characterize their mantle source and evaluate the petrogenetic processes related to opening of the Neotethys Ocean. Samples collected from the eastern side (Guryal Ravine, Pahalgam, PJ3) of the Kashmir Valley are chemically similar to mildly alkaline to tholeiitic, within-plate flood basalts. The TiO2 contents (TiO2 = 0.8 to 3.1 wt.%), La/YbN values (La/YbN = 1.8 to 6.1) and εNd(t) values (εNd(t) = −5.3 to +1.3) along with partial melt modeling indicates that the basalts were likely derived from a spinel peridotite source. In contrast, samples collected from the western side (PJ4) of the Kashmir Valley (Buta Pathri) are more primitive in composition and show evidence for clinopyroxene fractionation. The basalts from the western side of the Kashmir Valley have higher Mg# (Mg# = 60 to 78) values and εNd(t) values (εNd(t) = +0.3 to +4.3) suggesting they were derived by slightly higher amounts of partial melting and from a more depleted spinel peridotite source. The changing bulk composition of the basalts from ‘enriched OIB-like’ on the eastern side to ‘depleted MORB-like’ compositions on the western side is likely due to the changing nature of the Panjal rift from a nascent continental setting to one transitioning to a mature ocean basin. In comparison to Pangaean and post-Pangaean flood basalt provinces, the Panjal Traps are more chemically similar to the flood basalts from the post-Pangaean provinces that are associated with plate separation.

Jurassic Dextral Movement along the Dien Bien Phu Fault, NW Vietnam: Constraints from 40Ar/39Ar Geochronology

The NNE-SSW-trending Dien Bien Phu fault, which dextrally displaces the NW-SE-trending Song Ma suture, in northwestern Vietnam, is widely considered one of the most seismically active faults in Indochina. In order to better understand the fault’s activity, this study reports new 40Ar/39Ar geochronological data for mica schists and mylonites from several areas along the Dien Bien Phu fault, showing 40Ar/39Ar ages of 194–212 Ma for mica schists and 158–198 Ma for mylonites. These ages suggest that after the collision of the South China and Indochina blocks, dextral shearing deformation initiated along the Dien Bien Phu fault in the Jurassic, significantly earlier than previously thought. In light of the relevant tectonic events in Indochina, the successive suturing/collision of the Indochina, Simao, and Sibumasu blocks may have been responsible for the initiation of dextral shearing along the Dien Bien Phu fault, which in turn resulted in offset of the Song Ma suture.

Temporal and structural evolution of the Early Palæogene rocks of the Seychelles microcontinent

The Early Palæogene Silhouette/North Island volcano-plutonic complex was emplaced during the rifting of the Seychelles microcontinent from western India. The complex is thought to have been emplaced during magnetochron C28n. However, the magnetic polarities of the rocks are almost entirely reversed and inconsistent with a normal polarity. In this study we present new in situ zircon U/Pb geochronology of the different intrusive facies of the Silhouette/North Island complex in order to address the timing of emplacement and the apparent magnetic polarity dichotomy. The rocks from Silhouette yielded weighted mean 206Pb/238U ages from 62.4 ± 0.9 Ma to 63.1 ± 0.9 Ma whereas the rocks from North Island yielded slightly younger mean ages between 60.6 ± 0.7 Ma to 61.0 ± 0.8 Ma. The secular latitudinal variation from Silhouette to North Island is consistent with the anticlockwise rotation of the Seychelles microcontinent and the measured polarities. The rocks from Silhouette were emplaced across a polarity cycle (C26r-C27n-C27r) and the rocks from North Island were emplaced entirely within a magnetic reversal (C26r). Moreover, the rocks from North Island and those from the conjugate margin of India are contemporaneous and together mark the culmination of rift-related magmatism.

Age and tectonic setting of the East Taiwan Ophiolite: implications for the growth and development of the South China Sea

The South China Sea is one of the youngest marginal seas and understanding its development is important for reconstructing the tectonic evolution of Southeast Asia. The South China Sea is thought to have been actively spreading between 32 Ma and 15.5 Ma. The East Taiwan Ophiolite (ETO) is one of the few preserved remnants of the South China Sea on land and provides an opportunity to investigate the age and the tectonic setting of the accreted easternmost portion. The age of the ETO was obtained by LA-ICP-MS in situ zircon U–Pb methods and yielded a mean 206 Pb– 238 U age of 14.1±0.4 Ma, suggesting that magmatic activity in the South China Sea continued ~1.5 million years beyond current estimates. Cr-spinel data (Cr no. = 42–54) and depleted e Nd (t) values (i.e. +9.1 to +11.4) from the serpentinized peridotites and gabbros and the light rare earth element depleted patterns (La/Yb ≤ 1) of the ETO mafic rocks are consistent with a ridge setting (i.e. N-MORB composition). Therefore, the ETO likely represents the terminal portion of the South China Sea spreading ridge that was sheared off during the northward translation of the Luzon arc.

Evidence for Cool Extrusion of the North Indochina Block along the Ailao Shan Red River Shear Zone, a Diancang Shan Perspective

Much debate surrounds the temporal-thermal-structural evolution of the >1000-km NW-SE-trending Ailao Shan–Red River (ASRR) shear zone. This mainly reflects the contradictory interpretations of the timing and P-T conditions for the occurrence of left-lateral shearing event due to a diverse data set of mineral ages. Our new microstructural, petrological, and geochronological data from the southern section of the Diancang Shan (DCS) block along the west wall of the ASRR support slow cooling from ∼300°C to ∼150°C from the Middle Eocene to the Late Miocene, accompanied by left-lateral shearing activity. This conclusion is based on the following: (1) The lower greenschist facies assemblages of biotite replaced by muscovite or chlorite, the albitized K-feldspar, and the stable association of K-feldspar + chlorite rather than muscovite + biotite are universally observed for the synkinematic Sn+1 and mylonitic S/C fabrics from both quartzofeldspathic and amphibolite domains. (2) New 40Ar/39Ar data for K-feldspar and muscovite of various structural domains such as the K-feldspar phenocrysts (∼20 to ∼40 Ma) from the early Triassic magmatic protolith (zircon U-Pb ages of 230–250 Ma), pre- to synkinematic muscovite (∼17 to ∼23 Ma) in mylonites, and K-feldspar (∼5 to ∼10 Ma) in postmylonitic cross-cutting veins produce an age spectrum that increases from ∼5 to ∼40 Ma under lower greenschist facies. Together these data indicate that the DCS block was slowly extruded along the ASRR shear zone through much of the Tertiary. These results contradict earlier interpretations of rapid cooling from synkinematic amphibolite metamorphic conditions caused by shear heating based in part on the assumption that all Miocene ages were cooling ages rather than crystallization ages.

Structural inversion in the northern South China Sea continental margin and its tectonic implications

The northern South China Sea (SCS) continental margin was proposed to be an active margin during the Mesozoic. However, only a few papers discussed the Mesozoic structural evolution in this region. Here, we provide information based on the seismic profile interpretations with age control from biostratigraphic studies and detrital zircon U-Pb dates of well MZ-1-1 in the western Dongsha-Penghu Uplift of the northern SCS continental margin. The industrial seismic profiles reveal evidence for structural inversion as represented by folds and high-angle reverse faults, formed by reactivation of pre-existing normal faults. The inversion event likely started after the Early Cretaceous, and developed in Late Cretaceous, but ceased before the Cenozoic. The areal extent of the structural inversion was restricted in the western Dongsha-Penghu Uplift and was approximately 100 km in width. Based on the paleogeographic reconstruction of SCS, the structural inversion was likely formed by a collision between the seamount (volcanic islands) swarm of the current North Palawan block (mainly the Calamian Islands) and the northern SCS continental margin around Late Cretaceous. Article history: Received 30 May 2016 Revised 14 March 2017 Accepted 27 March 2017