Journal of Southeast Asian earth sciences最新文献

The Rajmahal basalts form a north-south trending belt extending over 4100 km² along the eastern margin of the Indian Shield. The basalts form part of a widespread magmatic episode which coincided with continental break-up between India and Australia-Antarctica during the Late Jurassic-Early Cretaceous. In the sections examined, between 2 and 15 flows have been encountered, ranging in composition from tholeiitic basalt to dacite (pitchstone). The flows vary in thickness from < 1 m to > 70 m. Significant volcanic features observed include vent sites, marked by volcanic breccia, welded agglomerate, tuff and lapilli-tuff (including volcanic bombs). The tuffs occur in widely scattered areas, and are more abundant than hitherto recognised. These rocks were probably erupted during the early stages of volcanic activity, following extensive differentiation in sub-volcanic magma chambers.

Several characteristics of sediments interbedded with the lavas suggest a humid, sub-tropical lacustrine depositional environment during eruption of the lower part of the volcanic sequence, and a temperate climate during emplacement of the upper flows. Quiescent eruption of the Rajmahal basalts took place largely through fissures sited at the faulted margin of the Indian Shield. This magmatic activity is interpreted as resulting from extension of the lithosphere above a mantle plume.

Alkaline granites can be broadly divided into anorogenic (AA-type) and post-orogenic (PA-type). The former heralds the beginning of rifting and the latter is an indicator of the end of orogeny. Although it is difficult to distinguish anorogenic and post-orogenic alkaline granites, they can be identified on the basis of differences in geology, petrology and geochemistry: (1) PA-type granites are formed shortly after plate subduction and plate collision; AA-type granites are independent of subduction and collision. (2) AA-type granitic magmatism is of long time duration; PA-type granitic magmatism is generally short-lived. (3) AA-type granites tend to be associated with lithospheric rifting; PA-type granites are commonly associated with ophiolitic plate suture zones. (4) AA-type granites are often closely associated with mafic rocks and silica-undersaturated syenites; PA-type granites are the final products of a long tectonic magmatic cycle dominated by a normal calc-alkaline series. (5) AA-type granites have an extended range in R₁ (500–3000; the range for PA-type granites is very narrow RC 2300–2600). The $\text{Ga}\text{Al}$ ratio of the former is generally 4–9, while that of the latter is 2–4. The former is more enriched in Zr, Nb, Ce and Y and has smaller $\text{Y}\text{Nb}$ and $\text{Ce}\text{Nb}$ ratios than the latter.

Ten tektites from the Penglei area in Hainan province were analyzed for the abundances of major elements, 31 trace elements, and the RbSr isotopic composition. These tektites can be divided into two compositional categories, which are distinguished by high (> 80 wt %) and low (<76 wt %) SiO₂ contents. Tektite TK-1 (high SiO₂) shows a vesicular massive body and has lower refractive index, density, major and trace elements. Its chemical composition closely resembles the average composition of Muong Nong-type indochinites. The others with low SiO₂ either pitted or grooved surfaces, with schlieren structures on some surfaces, and splash-form have similar chemical compositions to those of indochinites. From the chemical composition, it is suggested that the tektites within the Indochina and the Hainan subfields are derived from similar parental material and are similar to the post-Archean upper crustal rocks. In addition, the tektites from Hainan (Hainanites) have large positive ϵ^Sr(0) ratios, indicating that the parent material for these tektites resembles old terrestrial sedimentary rocks.

From the Sr isotopic data, it is interpreted that the hainanites do not originate from continental material recently derived from the mantle or recent young sediments such as soil or loess. Based on RbSr isotopic data, it has been suggested by Blum et al. Geochim. Cosmochim. Acta 56, 483–492, 1992 that the depositional age of sedimentary target materials is close to 170 Ma (Jurassic).

Mixing calculations for various amounts and combinations of target rocks indicate that the best fit for sample TK-1 tektite is a mixture of 2% shale, 38% sandstone, 50% greywacke and 10% quartzite, and the other splash-form tektite is a mixture of 41% shale, 2% sandstone, 20% greywacke and 37% quartzite.

Ocean Drilling Program Leg 127 drilling at site 797 in the Yamato Basin of the Japan Sea indicated that the basement is composed of early Miocene (−19 Ma) basaltic-doleritic rocks, whereas at site 795 in the northern Japan Basin the basement is composed of middle Miocene (−15 Ma) calc-alkali basalt and basaltic andesite lava flows. The basaltic rocks from Hole 795B are characterized by moderate amounts of large-ion lithophile elements (including K, Rb and Sr), high-field strength elements (including Zr and Ti) and light rare-earth element (Ce around 2.5 ppm). The lithology of Hole 797C basaltic rocks can be divided into upper suite (unit 10 and above) and lower suite (unit 11 and below) based on the petrology and chemistry of these rocks (Tamaki et al., 1990). The 797C lower suite basalts are higher in Fe, Ti, Na, K, P, Ba, Hf, REEs, Cu, Cr and Ni when compared with the upper suite basalts. A distinct variation gap exists between Hole 797C upper suite and lower suite basalts in the Cr vs Y plot. The upper suite basalts are confined to a restricted area, while the lower suite basalts show a systematic variation trend. The upper suite and lower suite basalts may represent two distinct magmatic series. The mantle sources for Hole 797C lower suite basalts may be more enriched in incompatible elements than those of the upper suite basalts.

The results of a detailed study on the geometrical distribution of earthquake foci in the Sumatran region allowed the authors to distinguish the foci belonging to the recent Wadati-Benioff zone from those occurring in the continental wedge. The morphology of the Wadati-Benioff zone, its main geometrical parameters and the variable depth of its penetration into the upper mantle were established. The existence of an intermediate-depth aseismic gap in the Wadati-Benioff zone was discovered and its spatial relationship with young calc-alkaline volcanism was confirmed. The subduction process was correlated with the stratigraphy and geology of the Sumatran region. The duration of the present cycle of subduction was estimated to be 6–8 Ma. The occurrence of Oligocene volcanism and the locations of deep earthquakes point to the existence of a Tertiary subduction zone underlying the present slab. The seismotectonic pattern of the continental wedge was described by 11 seismically active fracture zones, the orientation and tectonic function of which were checked by fault plane solutions.

The Lubok Antu melange is composed of blocks of mudstone, shale, sandstone, chert, limestone, hornfels, basalt, gabbro and serpentinite embedded in a strongly cleaved, pervasively sheared, chloritised mudstone matrix. Chert blocks are common and widespread in the melange. Fifty-three species of Radiolaria were identified from 14 samples collected from these chert blocks. The Radiolaria can be grouped into three assemblages. Assemblage I is composed of 17 species. The presence of Ristola altissima (Rüst) and Parvicingula excelsa Pessagno and Blome is indicative of late Tithonian (Late Jurassic). Assemblage II consists of 21 species. The occurrence of an index form Staurosphaera septemporata (Parona) indicates middle Valanginian to Barremian age. The presence of Squinabollum fossilis (Squinabol), Archaeodictyomitra vulgaris Pessagno, Obesacapsula somphedia (Foreman), Thanarla praeveneta Pessagno, Pseudodictyomitra pseudomacrocephala (Squinabol), Rhopalosyringium majuroensis Schaaf, Stichomitra communis Squinabol and Holocryptocanium tuberculatum Dumitrica in Assemblage III suggests that the age of this assemblage is late Albian to Cenomanian. All the results show that there are three different ages of the chert blocks present in the Lubok Antu melange.

The repeated need for ad hoc modifications in plate-tectonic models to explain the evolution of southeastern Asia reveals their inability to fully explain the complex features and dynamics of this region. As one example, the hypothesis does not provide a mechanism to explain the 180° turns and twists along the strike of several foldbelts and island arcs in the region (e.g. Banda arc). Convection-cell configuration renders such 180° contortions and Rayleigh-Bénard-type convection impossible. However, during the last 10 years, new data bearing on the convection-cell problem have become available in the form of seismotomographic images of the earth's interior. These images show that (i) mantle diapirs as proposed by traditional plate-tectonic models do not exist; (ii) there is no discernible pattern of upper or lower mantle convection, and thus no longer an adequate mechanism to move plates; and (iii) the lithosphere above a depth of about 80 km is permeated by an interconnected network of low-velocity channels.

Seismic-reflection studies of the low-velocity channels discovered on the seismotomographic images reveal that these channels have walls with a 7.1–7.8 km s⁻¹ P-wave velocity. Commonly, the interiors of the channels are acoustically transparent, with much slower P-wave velocities, in places as low as 5.4 km s⁻¹. The author and co-workers have interpreted the low velocities as evidence for the presence of partial melt in the channels, and they postulated that this melt moves preferentially eastward as a result of the earth's rotation. They named these channels “surge channels” and their new hypothesis for earth dynamics “surge tectonics”.

Surge channels underlie every type of tectonic belt, which includes mid-ocean ridges, aseismic ridges, continental rifts, strike-slip fracture zones, and foldbelts. In southeastern Asia, surge channels—mainly foldbelts—lie between all platform and cratonic massifs. These massifs, platforms, and tectonics belts—the surge channels—form an anastomosing E-W pattern southern Asiatic Russia, Mongolia, western China, the Qinghai-Tibetan region, and northern India and Pakistan. Such an anastomosing pattern indicates that flow is an active process in the surge channels.

Surface studies of phenomena that might be associated with the surge channels soon revealed that all active channels are characterized by higher-than-normal heat flow (> 55 mW m⁻², thermal springs and elevated ground-water temperatures, volvanic phenomena, bands of microearthquakes, and linear belts of faults, fractures, and fissures. The latter are especially visible on satellite images. The bands of high heat flow, thermal springs, microearthquakes, and faults-fractures-fissures almost exactly coincide. The fault-fracture-fissure systems are interpreted to be streamlines caused by flow in the surge channels—a consequence of Stokes's Law (an expression of Newton's Second Law of Mo