Aiming Lin

Glassy pseudotachylyte veins from the Fuyun fault zone, northwest China

Glassy pseudotachylytes occur in granitic mylonites from the Fuyun fault zone, northwest China. Powder X-ray diffraction data indicate that the fine-grained matrices of the Fuyun pseudotachylytes vary from almost crystalline to complete glass (a few to 90 wt% glass). Powder X-ray diffraction patterns also indicate that clasts in the glass type pseudotachylyte veins mostly consist of quartz grains. Microlites displaying quenched or rapid-cooling textures, vesicles and amygdules, flow structures, and rounded fragments are also common in the Fuyun pseudotachylytes. The average bulk chemical compositions of these pseudotachylytes are similar to those of the host rocks. But the glass matrix has a low SiO2 component which is 5–15 wt% lower than that of the host rocks. The pseudotachylytes contain 2–3 wt% water (H2O). Using experimental data which relate the solubility of water to pressure, approximately 400 bars lithostatic pressure corresponding to approximately 1.5 km depth, of pseudotachylyte formation is estimated. A minimum temperature estimate of 1450°C can be estimated due to the presence of the pure SiO2 glass. The X-ray diffraction and chemical data indicate that the Fuyun pseudotachylytes formed during seismic faulting at shallow depths by selective melting under water-saturated conditions rather than by total melting or by crushing of the host rocks along fault surfaces.

Quaternary folding of the eastern Tian Shan, northwest China

Abstract The Tian Shan, east–west trending more than 2000 km, is one of most active intracontinental mountain building belts that resulted from India–Eurasia collision during Cenozoic. In this study, Quaternary folding related to intracontinental mountain building of the Tian Shan orogenic belt is documented based on geologic interpretation and analyses of the satellite remote sensing images [Landsat Thematic Mapper (TM)/Enhanced Thematic Mapper (ETM) and India Remote Sensing (IRS) Pan] combined with field geologic and geomorphic observations and seismic reflection profiles. Analyses of spatial–temporal features of Quaternary folded structure indicate that the early Quaternary folds are widely distributed in both piedmont and intermontane basins, whereas the late Quaternary active folds are mainly concentrated on the northern range-fronts. Field observations indicate that Quaternary folds are mainly characterized by fault-related folding. The formation and migration of Quaternary folding are likely related to decollement surfaces beneath the fold-and-fault zone as revealed by seismic reflection profiles. Moreover, analysis of growth strata indicates that the Quaternary folding began in late stage of early Pleistocene (2.1–1.2 Ma). Finally, tectonic evolution model of the Quaternary deformation in the Tian Shan is presented. This model shows that the Quaternary folding and faulting gradually migrate toward the range-fronts due to the continuous compression related to India–Eurasia collision during Quaternary time. As a result, the high topographic relief of the Tian Shan was formed.

Injection veins of crushing-originated pseudotachylyte and fault gouge formed during seismic faulting

This paper describes the injection veins of crushing-originated pseudotachylyte and fault gouge formed along the Iida-Matsukawa fault in granitic cataclasite, southern Nagano Prefecture, central Japan. The pseudotachylyte, which shows a dense and aphanitic appearances, and fault gouge occur as simple veins (fault vein) along the main fault plane and as complex network veins (injection vein) in the neighboring cataclasite. Locally, the injection veins of pseudotachylyte and fault gouge can be traced back to the fault veins. Powder X-ray diffraction patterns and petrological analysis indicate that both of the pseudotachylyte and fault gouge consist entirely of fine-grained angular clasts and that the pseudotachylytes and fault gouges have a similar X-ray diffraction patterns with that of the host granite. The similarity of chemical compositions and distribution patterns of grain size also show that the injection veins of pseudotachylyte and fault gouge have the same source material as that of fault veins. Field occurrences and petrological characteristics strongly suggest that the injection veins of pseudotachylyte and fault gouge formed during seismic faulting by a rapid intrusion and fluidization of crushed fine-grained materials generated in the shear zone in a gas-solid-fluid system.

S–C fabrics developed in cataclastic rocks from the Nojima fault zone, Japan and their implications for tectonic history

Abstract S–C fabrics similar to those found in mylonites are observed in foliated cataclastic granitic rocks from the Nojima fault zone, southwest Japan. The foliated cataclastic rocks comprise cataclasite, fault breccia, gouge, and crushing-originated pseudotachylyte. The S–C fabrics observed in these cataclastic rocks involve S-surfaces defined by shape preferred orientation of biotite fragments or aggregates of quartz and feldspar fragments, and C-and C′-surfaces defined by microshears and shear bands, respectively, where fine-grained material is concentrated. Striations on the main fault plane are oriented parallel to the cataclasite lineations. A significant microstructural difference between the foliated cataclastic rocks and S–C mylonites is the absence of dynamically recrystallized grains in the foliated cataclasites. The striations, cataclastic lineations, and the S–C fabrics in the cataclastic rocks formed from the late Tertiary to the late Holocene indicate that the Nojima fault zone has moved as a dextral strike-slip fault, with a minor reverse component since it formed. S–C fabrics in cataclastic rocks provide important information on the tectonic history and are reliable kinematic indicators of the shear sense in brittle shear zones or faults.

Tectonic characteristics of the central segment of the Tancheng–Lujiang fault zone, Shandong Peninsula, eastern China

Abstract The Tancheng–Lujiang fault zone (TLFZ), in the central segment in the Shandong Peninsula, eastern China, is composed of four major faults, called F1–F4 from east to west, forming a 50-km-wide zone. Detailed field study demonstrates that Neogene volcanic and sedimentary strata, Quaternary deposits, and topographic surfaces cover the fractured zones of faults F3 and F4 and are not deformed or displaced. The same rocks and surfaces on faults F1 and F2, however, are clearly displaced. The geological and seismological data show that the F1 and F2 are presently active as a major seismic zone in eastern China, whereas the F3 and F4 have been inactive since the Neogene. Fabrics of foliated cataclastic rocks generated in the fracture zones show left-lateral strike-slip on the F3 and F4 between Cretaceous and Neogene time, and high-angle reverse or normal faulting, with a dextral component, on faults F1 and F2 in late Quaternary time. Field evidence and analysis of the fabrics of fault-related rocks reveal the changes of shear sense and activity of faults F1–F4 of the central segment of the TLFZ from Cretaceous to Quaternary time in the Shandong Peninsula.

Microlite Morphology and Chemistry in Pseudotachylite, from the Fuyun Fault Zone, China

Microlites in pseudotachylite veins from the Fuyun fault zone, northwest China, can be divided into four morphological groups: simple, skeletal, dendritic, and spherulitic. From the margin to the center of microcrystalline and microlitic pseudotachylite veins, the size of single crystals or crystal fibers becomes progressively larger from a few µm to as much as 15 µm. The morphologies of microlites vary from simple shapes, such as acicular and globular forms, to more complex forms such as skeletal, dendritic, and coarse spherulitic forms. In glassy pseudotachylites, only simple and fine spherulitic microlites, generally smaller than 5-6 µm, occur. These microlites have a varied mineralogy but are mainly made up of plagioclase (An20-70), and alkali feldspar, including sanidine and anorthoclase, as well as clinopyroxene, grossular garnet, biotite, hornblende and some magnetic minerals. The biotite and hornblende microlites have a variable Ti content, generally higher than that of biotite and hornblende fragments from the country rocks. The wide variety of morphologies and textures of microlites within the pseudotachylite, the presence of sanidine, anorthoclase, An-high plagioclase, pyroxene, and Ti-high biotite and hornblende indicate that these microlites are primary crystals formed from rapid cooling or quenching of a melt.

How and when did the Yellow River develop its square bend

The current course of the Yellow River, China, involves an unusual 1500-km-long angular bend around the Ordos tectonic block, although sedimentary evidence suggests that the river once followed a more direct path eastward into the Bohai Sea. Geologic evidence reveals that the Yellow River formed in the Eocene as an eastward-draining river and developed its square bend around the Ordos block in late Miocene–early Pliocene time as a result of folding and uplift that are most probably related to the collision and penetration of India into Eurasia, as well as rifting around the Ordos block. The change in course of the Yellow River predates flat-lying Pliocene-Pleistocene sediment deposits that unconformably overlie folded Eocene-Miocene strata. The Yellow River provides a typical example of tectonic controls on the size, morphology, and longevity of rivers.

Coeval formation of cataclasite and pseudotachylyte in a Miocene forearc granodiorite, southern Kyushu, Japan

Abstract Cataclastic rocks and pseudotachylytes are exposed along the Uchinoura shear zone, a normal fault zone cutting the middle Miocene (14 Ma) Osumi granodiorite in southern Kyushu, Japan. Cataclastic rocks include non-foliated clast-supported to matrix-supported cataclasite and foliated clast-supported cataclastic granodiorite. In these rocks, fracturing and comminution played a major role, but dissolution and recrystallization of quartz, and plastic deformation of quartz and biotite were also active processes, especially in foliated granodiorite. Two types of pseudotachylyte are distinguished: a foliated-type characterized by a planar arrangement of clasts and microlites, and a spherulitic-type characterized by clasts surrounded by microlite overgrowths. Both types are of melt origin, as attested by the presence of microlites and rounded or embayed clasts, and by the scarcity of biotite clasts. Unlike spherulitic-type pseudotachylyte, which solidified without being deformed, the foliated-type pseudotachylyte underwent flow before complete solidification. This deformation is thought to reflect post-seismic strain accommodation immediately following the main slip episode. Kinematic indicators, which consist of Riedel-type secondary fractures branching on primary fractures, shear bands offsetting the foliation of foliated granodiorite, or asymmetrical porphyroclast systems within pseudotachylyte veins, show that all fault rocks were generated during N–S- to NW–SE-directed extensional deformation. Pseudotachylyte is closely associated both in time and space with cataclastic rocks, thus indicating that the behaviour of the Uchinoura fault zone alternated between comminution and frictional melting. Given the slow strain rates which characterize dissolution and recrystallization processes detected in cataclasites, the juxtaposition of pseudotachylytes and foliated cataclasites provides an example of aseismic and seismic displacements within the same shear zone.

Roundness of clasts in pseudotachylytes and cataclastic rocks as an indicator of frictional melting

Abstract We examine the roundness (Rd) of clasts derived from fault rocks and discuss its possible application as an indicator for fracturing and chipping or frictional melting. The roundness of quartz and feldspathic clasts derived from four frictional melting-originated pseudotachylytes and one crushing without melting-originated pseudotachylyte as well as three cataclastic rocks was measured. Between 230 and 270 clasts >10 μm in size were measured for each sample. The analyses show that Rd is lower than 0.4 in all the clasts included in the cataclastic rocks and crushing-originated pseudotachylyte, whereas it varies from 0.1 to 1.0 and 35–90% of clasts have a roundness >0.4 in the melting-originated pseudotachylyte. It is suggested that the clasts having a roundness >0.4 were rounded by frictional melting rather than by fracturing or chipping in pseudotachylytes, and that the roundness of clasts may be used as a special index of disintegration attributed to attrition and melting.

Average Slip Rate and Recurrence Interval of Large-Magnitude Earthquakes on the Western Segment of the Strike-Slip Kunlun Fault, Northern Tibet

Interpretations of satellite remote sensing images, field and trench excavation investigations, and radiocarbon dates constrain the Holocene slip rate and average recurrence interval of large earthquakes on the western segment of the strike- slip Kunlun fault related with the 2001 M w 7.8 Central Kunlun earthquake, northern Tibet. Streams and gullies developed on the alluvial fans having an average 14 C age of ∼7000 years are sinistrally offset by up to 115 m along the Kunlun fault. This constrains a slip rate of 16.4 mm/yr for the past ∼7000 years. Trenches and 14 C ages reveal that at least four seismic faulting events occurred in the past 6200 years and that the penultimate event prior to the 2001 M w 7.8 earthquake occurred during the past 400 years with an average left-lateral offset of 4–5 m. Coupling the slip rate of 16.4 mm/yr with the average offset of 4–5 m produced by individual large earthquakes, it is estimated that the average recurrence interval of large earthquakes is 300–400 years on the western segment of the Kunlun fault. Our results confirm that the Kunlun fault plays an important role as a major strike-slip fault in accommodating the horizontal eastward extrusion of Tibet.