S U M M A R YLower to Middle Cretaceous red sandstones were sampled at four localities in the LanpinSimao fold belt of the Shan-Thai Block to describe its regional deformational features. Most of the samples revealed a characteristic remanent magnetization with unblocking temperatures around 680 • C. Primary natures of magnetization are ascertained through positive fold test. A tilt-corrected formation-mean direction for the Jingdong (24.5 • N, 100.8 • E) locality, which is located at a distance of 25 km from the Ailaoshan-Red River Fault, revealed northerly declination with steep inclination (Dec./Inc. = 8.3 • /48.8 • , α 95 = 7.7 • , N = 13). However, mean directions obtained from the Zhengyuan (24.0 • N, 101.1 • E), West Zhengyuan (24.0 • N, 101.1 • E) and South Mengla (21.4 • N, 101.6 • E) localities indicate an easterly deflection in declination; such as Dec./Inc. = 61.8 • /46.1 • , α 95 = 8.1 • (N = 7), Dec./Inc. = 324.2 • /−49.4 • , α 95 = 6.4 • (N = 4) and Dec./Inc. = 51.2 • /46.4 • , α 95 = 5.6 • (N = 13), respectively. The palaeomagnetic directions obtained from these four localities are incorporated into a palaeomagnetic database for the Shan-Thai Block. When combined with geological, geochronological and GPS data, the processes of deformation in the Shan-Thai Block is described as follows: Subsequent to its rigid block clockwise rotation of about 20 • in the early stage of India-Asia collision, the Shan-Thai Block experienced a coherent but southward displacement along the Red River Fault prior to 32 Ma. This block was then subjected to a north-south compressive stresses during the 32-27 Ma period, which played a key role in shaping the structure of Chongshan-Lancang-Chiang Mai Belt. Following this some local clockwise rotational motion has occurred during the Pliocene-Quaternary time in central part of the Shan-Thai Block as a result of internal block movements along the reactivated network of faults.
S U M M A R YIn order to describe features of tectonic deformation in the Indochina Peninsula, Early Jurassic to Early Cretaceous red sandstones were sampled at three localities in the Shan-Thai and Indochina blocks. Stepwise thermal treatment of most samples revealed the presence of characteristic remanent magnetization, which is generally unblocked by 680 • C. This component from Phong Saly (21.6 • N, 101.9 • E) and Borikhanxay (18.5 • N, 103.8 • E) localities yield positive fold tests with Late Jurassic-Early Cretaceous directions of Dec/Inc = 28.8 • /32.1 • (k s = 15.4, α 95 = 8.8 • , N = 22) and Dec/Inc = 42.1 • /46.9 • (k s = 20.1, α 95 = 7.9 • , N = 18), respectively. Additionally, a syn-folding mid-Cretaceous characteristic magnetization is observed in the samples of Muang Phin locality (16.5 • N, 106.1 • E), which gave a mean direction of Dec/Inc = 30.8 • /39.9 • , k = 102.6, α 95 = 3.0 • , N = 23. This reliable Late Jurassic to Mid-Cretaceous palaeomagnetic directions from three different localities are incorporated into a palaeomagnetic database for Shan-Thai and Indochina blocks. Based on these compilations, tectonic deformation of the Shan-Thai and Indochina blocks is summarized as follows: (1) the Shan-Thai and Indochina blocks experienced a clockwise rotation of about 10 • as a composite unit in the early stage of India-Asia collision and (2) following this, the Shan-Thai Block underwent an internal tectonic deformation, whereas the Indochina Block behaved as a rigid tectonic unit during the same period. Comparison of our palaeomagnetic results with seismic tomographic images suggests that the strength of continental lithosphere beneath these blocks played an important role in the process of deformation rather than any other tectonic regime. In contrast to the Shan-Thai Block, an existence of continental roots beneath the Indochina Block prevented its internal deformation.
Although seafloor massive sulfide (SMS) deposits are crucially important metal resources that contain high‐grade metals such as copper, lead, and zinc, their internal structures and generation mechanisms remain unclear. This study obtained detailed near‐seafloor images of electrical resistivity in a hydrothermal field off Okinawa, southwestern Japan, using deep‐towed marine electrical resistivity tomography. The image clarified a semi‐layered resistivity structure, interpreted as SMS deposits exposed on the seafloor, and another deep‐seated SMS layer at about 40‐m depth below the seafloor. The images reinforce our inference of a new mechanism of SMS evolution: Upwelling hydrothermal fluid is trapped under less‐permeable cap rock. The deeper embedded SMS accumulates there. Then hydrothermal fluids expelled on the seafloor form exposed SMS deposits.
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