APATITE FISSION TRACK DATING OF MESOZOIC SILICICLASTIC ROCKS FROM KHORAT GROUP IN PHU PHAN MOUNTAIN RANGE, NORTHEASTERN THAILAND

Abstract

Field and remote-sensing investigations have been performed for mapping the Mesozoic siliciclastic rocks of the Phu Phan Ranges (PPR), northeastern Thailand. Stratigraphically, the PPR study area has been remapped and is occupied by 5 distinct successive formations, including the Phu Kradung Formation with the average thickness of 93 m, the Phra Wihan Formation with the average thickness of 64 m, the Sao Khua Formation with the average thickness of 63 m, the Phu Phan with the average thickness of 84 m, and the Khok Kruat with the average thickness of 49 m. Results on field and remote sensing analyses reveal that the PPR rocks are structurally deformed into series of anticlines and synclines with the NW-SE trend. Geological transects have been performed over the PPR study area. It has been recognized that the major fold has been displaced by NE-SW, short left-lateral strike slip faults. A total of 21 samples of the Khorat Group rocks have been collected for petrographic and apatite fission track (AFT) analyses. Petrographically, all the samples are very fine- to medium-grained sandstones, viz. feldspathic litharenite and litharenite. The sandstone petrography and modal analysis of quartz, feldspar, and lithic fragments points to the provenance of recycled orogen. It is considered that the source regions are the Loei-Phetchabun and the Truong Son Belts. The studied samples have essentially clastic textures with siliceous and calcareous cements and contain abundant quartz (av. 72.7%) with minor feldspar (av. 3.6%) and rock fragments (av. 23.7%). Apatite and zircon are significant accessories. Apatite invariably forms long prismatic habits and occurs as discrete detrital grains. About 120 grains from qualified 6 samples have been analyzed for fission track dating. The current AFT result from this study along with those of the previous studies reveals an interesting scenario. Three age ranges of the PPR and surrounding regions have been recognized viz., 78 to 60 Ma, 55 to 42 Ma, and about 37 Ma. The first episode (78 – 60 Ma) corresponds to the tectonic movement in response to the NE–SW compressive stress within the Khorat region, giving rise to the major deformed structure in the study area. Perhaps the Phu Phan anticlinal structures have been formed. Two successive basins within the Khorat region may have been developed at this stage. The second episode (55 – 42 Ma) indicates an extensive exhumation (major uplift and erosion) developed within the whole Khorat region, becoming a Khorat Plateau. The final episode about 37 Ma is marked by the minor movement along the strike-slip fault which slightly displaced the Phu Phan anticlinal structure in response to the clock-wise rotation of the Indochina block. The AFT results from this current and previous studies advocate an average exhumation rate of the Phu Phan Mountain range to be ca. 0.0139 mm/yr (or 13.9 m/Ma).