ALBERT

Description: The South Makassar Strait mass transport complex (MTC) covers an area of at least 9000 km 2 and has a total volume of 2438 km 3 . It is composed of a shale-dominated sedimentary unit with high water content. Seismic reflection data across the South Makassar Strait MTC show that it displays relatively coherent internal sedimentary stratigraphy that in the toe region is deformed into well-defined thrust-related structures (imbricates, ramps and flats, fault bend folds). It is one of the largest known coherent MTCs. The bowl-shaped central core region is as much as 1.7 km thick, and is confined to the west and east by 355° and 325° trending (respectively) lateral ramps in the upper slope area that pass via oblique ramps into a northeast-southwest–trending frontal ramp area. The core area passes via the lateral, oblique, and frontal ramps into an extensive, thin (tens of meters thick) region of the MTC (the lateral and frontal apron areas) that are internally deformed by thrusts, normal faults, and thrust faults reactivated as normal faults. The MTC anatomy can be divided into extension headwall, translational, toe, flank, lateral apron, and frontal apron domains. The headwall region is located in the upper slope area of the Paternoster platform; the main body of the slide is in the deep-water region of the Makassar Strait. The complex is interpreted to be triggered by uplift of the platform area (accompanied by inversion), and/or basin subsidence, which caused seaward rotation of ~2° of the Paternoster platform in the Pliocene. Variable uplift promoted sliding dominantly from the eastern and western margins of the headwall. The internal fault patterns of the MTC show that extension in the upper slope to lower slope in the core area changes downslope to compressional structures in the toe domain and apron. Later extensional collapse of parts of the compressional toe area occurred with negative inversion on some faults. The coherent internal stratigraphy, and evidence for multiphase extension in the eastern headwall area, suggests that the ~6–7 km of shortening in the toe region of the MTC occurred at a slow strain rate. Therefore, this type of MTC does not have the potential to generate tsunamis.

Description: The Khao Khwang fold-and-thrust belt, central Thailand, developed within a basin that formed on the southwestern margin of the Indochina block. Because of limited geochronological and provenance constraints, the time of deposition, sediment source location, and tectonic significance of the basin have been uncertain. Here, we present 837 U-Pb detrital zircon ages and 271 Hf isotope in situ analyses from Permian–Triassic clastic units within the Khao Khwang fold-and-thrust belt in order to constrain the provenance, maximum depositional ages, and depositional environment of the southwestern margin of the Indochina terrane through the late Paleozoic to early Mesozoic. The key lithological units, the Sap Bon, Pang Asok, and Nong Pong Formations, are part of the Saraburi Group and have detrital age spectra spanning from Late Triassic to Paleoarchean. The entire data set has a common age peak at ca. 450 Ma, and all samples contain grains with ages of 0.2–0.3, 0.4–0.6, 1.0–1.3, 1.7–1.8, and 2.2–2.7 Ga. A few grains predate 3.0 Ga. Multidimensional scaling analysis of detrital zircon ages from throughout SE Asia demonstrates that the age spectra of the siliciclastic units of the Saraburi Group resemble those of Permian–Triassic detritus found elsewhere in the Khorat Plateau and throughout Vietnam and southeast China, implying that these areas shared similar sources. These sources may have been the, now largely covered, Indochina basement, and/or contiguous continental crust in terranes already amalgamated to Indochina at that time. Detrital zircons as young as 205 ± 6 Ma show that some formations of the Saraburi Group, previously considered to be of Middle–Late Permian age, are no older than Late Triassic. We propose a depositional model for the region of a Permian rift or passive-margin setting that evolved into piggyback and foredeep basins during an extended period of folding and thrusting in the Triassic.

Description: K-Ar dating of illite in fault gouges is a useful tool for constraining the timing of brittle fault movement; however, this can be problematic in fault gouges hosted in clay-rich rocks due to the influence of host-rock material. Therefore, this study employs a multianalytical geochemical approach to unravel the influence of host-rock mineralogy, as well as fault zone development, on ages from fault-gouge samples in a shale detachment zone. K-Ar dating of the ≥2 µm fraction of 6 samples from the Sap Bon Formation detachment zone and associated fault zones in the Khao Khwang fold-thrust belt of central Thailand yielded an age range of 262 ± 5.4 to 208 ± 4.6 Ma. Carbon and oxygen stable isotope analysis along with X-ray diffraction mineralogy indicate that the samples with the youngest K-Ar ages are characterized by higher grade clay mineralogy, and hotter, orogenic fluid temperatures. Using these proxies and comparison to existing geochronology of the study area, we correlated K-Ar illite ages to one of three stages of fault zone evolution: detrital, diagenetic (burial), and authigenic (fault movement). The youngest K-Ar dates in the Sap Bon Formation are contemporaneous with recently published zircon province data indicating that faulting and detachment zone formation in the Sap Bon Formation were occurring by the mid-Late Triassic, with deformation continuing as late as the Rhaetian.

Description: The northern Mergui Basin (Andaman Sea) contains east-northeast–west-southwest– to northeast-southwest–striking normal fault–bound basins, and north-northwest–south-southeast–trending strike-slip faults. The two largest strike-slip faults (Manora and Mergui) pass into extensional or transtensional basins at their tips, consistent with dextral offset. The faults provide examples of early stage pull-apart basin development at fault tips instead of the more common model for development at releasing bends. Offset of isochron markers for the Ranong Formation indicate that ~8 km of dextral offset has occurred along the Mergui fault and 4.5 km of dextral offset has occurred on the Manora fault. The strike-slip faults and associated extensional faults formed relatively late for the history of the entire Mergui Basin during the Early Miocene. The northern part of the Mergui Basin developed after a phase of west-northwest-east-southeast extension during the Oligocene in the Mergui Basin to the south, indicating a rotation in the extension direction toward the north-northwest–south-southeast with time. The basin is part of a major transtensional system involving the Sumatra, West Andaman, and Sagaing faults that accommodated the northern motion of western Myanmar as India moved north relative to Southeast Asia. Fault activity in the northern Mergui Basin decreased significantly when the broad zone of Early Miocene transtension became focused on the Alcock and Sewell Rises during the Middle Miocene, and the West Andaman and Sagaing faults began to develop and interacted in a large pull-apart geometry with the Shan Scarp Fault, and later (Late Miocene or Pliocene) with the Sagaing Fault.