Seismic mapping of the central and southern segments of the Tanlu fault zone using P-wave receiver functions

The central and southern segments of the Tanlu fault zone, located in the collision boundary between the Yangtze Plate and the North China Craton, underwent complex tectonic deformation associated with the Pacific Plate subduction. The crustal thickness and Vp/Vs ratio are important parameters for comprehending tectonic evolution and geodynamic processes. By integrating a newly dense seismic array, our results based on P-wave receiver function analyses reveal the crust thickness varies significantly across the Tanlu fault zone. The Yangtze Plate is characterized by a thinner crust, contrasting a thicker crust imaged in the North China Craton, indicating the Tanlu fault zone is a prominent structural block boundary. The Vp/Vs ratio within the Tanlu fault zone is visibly higher than its surroundings, probably correlating with abundant fluids activity along the fault zone, as a result of the subduction of the Pacific Plate. Additionally, we observe a gradually deepening Moho discontinuity beneath the Sulu orogenic belt with a low-angle dip, presenting direct seismic evidence for supporting the ancient Yangtze Plate underthrusting beneath the Sulu orogenic belt.

Crystallographic orientation of ilmenite inclusions in amphibole – an electron backscatter diffraction study

Orientated ilmenite inclusions have been discovered in amphibole of hornblendite from the Zhujiapu area, Dabie ultra-high-pressure (UHP) metamorphic terrane, China. In order to characterize the crystallographic orientation relationships between ilmenite inclusions and amphibole host and reconstruct the mechanism of their formation, we present an electron backscatter diffraction (EBSD) analysis combined with energy dispersive spectroscopy (EDS) analysis and electron microprobe analysis (EPMA) for ilmenite inclusions and amphibole host. The inclusions can be subdivided into four groups: (1) 60.2% of ilmenites have the crystallographic orientation {0001}Ilm // {100}Amp, (101̅0)Ilm // {010}Amp, [112̅0]Ilm // <001> Amp and [112̅0]Ilm // <012 > Amp. (2) 16.5% of ilmenites have <0001> Ilm // <001> Amp, (101̅0)Ilm // {010}Amp, (112̅0)Ilm // {100}Amp and [3̅031]Ilm // <012> Amp. (3) 13.8% of ilmenites have <0001> Ilm // <012> Amp, (112̅0)Ilm // {100}Amp and [3̅031]Ilm // <001> Amp. (4) 9.5% of ilmenites have <0001> Ilm // [1̅12]Amp, (101̅0)Ilm // {201}Amp, [112̅0]Ilm // [1̅12]Amp and [ 11 21 ¯ ] I l m ${[11\overline {21} ]_{Ilm}}$ // <010> Amp. By comparing the lattice relationship between ilmenite inclusions and amphibole hosts, it is shown that the frequency of the ilmenite inclusions in different groups is related to the lattice coherency and oxygen packing. Group-1 of the ilmenite inclusions was most likely be formed via a solid-state exsolution process by cooling of the hornblendite after the intrusion was emplaced. The other three groups of ilmenite inclusions were probably formed via reduction reaction in an open system. The formation temperature of the ilmenite inclusions is estimated by using the TiO2 solubility geothermeter in amphibole. The minimum formation temperature of the ilmenite inclusions is about 1025 °C, and the maximum formation temperature of the ilmenite inclusions is about 1126 °C.

Subduction, collision and exhumation in the ultrahigh-pressure Qinling-Dabie orogen

High-pressure metamorphism and ophiolite emplacement (Songshugou ophiolite) attended suturing of the Yangtze craton to Rodinia during the c. 1.0 Ga Grenvillian orogeny. The Qinling microcontinent then rifted from the Yangtze craton at c. 750 Ma. The Erlangping intraoceanic arc formed in the Early Ordovician, was emplaced onto the Qinling microcontinent in the Ordovician-Silurian, and then both units were accreted to the Sino-Korea craton before being stitched together by the c.400 Ma Andean-style Qinling arc. Subsequent subduction beneath the Qinling-Sino-Korean plate created a Devonian-Triassic accretionary wedge that includes eclogites, and formed a coeval volcano-plutonic arc that stretches from the Longmen Shan to Korea. In the Late Permian-Early Triassic, the northern edge of the South China Block was subducted to >150 km depth, creating the diamond- and coesite-bearing eclogites of the Dabie and Sulu areas. Exhumation from the mantle by lithosphere-scale extension occurred between 245 and 195 Ma during clockwise rotation of the craton. The Yangtze-Sino-Korea suture locally lies tens of km north of the exhumed UHP-HP part of the South China Block, implying perhaps that the very tip of the South China Block was not subducted, or that the UHP-HP rocks rose as a wedge that peeled the upper crust of the unsubducted South China Block from the lower crust. The Tan-Lu fault is an Early Cretaceous to Cenozoic feature. The apparent offset of the Dabie and Sulu UHP terranes by the Tan-Lu fault is a result of this Cretaceous to Cenozoic faulting combined with post-collisional extension north of Dabie.

Fossilized high pressure from the Earth's deep interior: the coesite-in-diamond barometer

Mineral inclusions in diamonds provide an important source of information about the composition of the continental lithosphere at depths exceeding 120-150 km, i.e., within the diamond stability field. Fossilized high pressures in coesite inclusions from a Venezuela diamond have been identified and measured by using laser Raman and synchrotron x-ray microanalytical techniques. Micro-Raman measurements on an intact inclusion of remnant vibrational band shifts give a high confining pressure of 3.62 (+/-0.18) GPa. Synchrotron single-crystal diffraction measurements of the volume compression are in accord with the Raman results and also revealed direct structural information on the state of the inclusion. In contrast to olivine and garnet inclusions, the thermoelasticity of coesite favors accurate identification of pressure preservation. Owing to the unique combination of physical properties of coesite and diamond, this "coesite-in-diamond" geobarometer is virtually independent of temperature, allowing an estimation of the initial pressure of Venezuela diamond formation of 5.5 (+/-0.5) GPa.

Clinoenstatite in alpe arami peridotite: additional evidence of very high pressure

Observations by transmission electron microscopy show that lamellae of clinoenstatite are present in diopside grains of the Alpe Arami garnet lherzolite of the Swiss Alps. The simplest interpretation of the orientation, crystallography, and microstructures of the lamellae and the phase relationships in this system is that the lamellae originally exsolved as the high-pressure C-centered form of clinoenstatite. These results imply that the rocks were exhumed from a minimum depth of 250 kilometers before or during continental collision.