Paleomagnetism of the Khorat Group in northeast Thailand

Abstract

Paleomagnetic measurements were made on three specimens from each of 179 sites distributed through all the formations of the Khorat Group exposed at the surface (Middle or Upper Triassic to Middle Cretaceous) in Northeast Thailand. One specimen from each site was tested for its stability of magnetisation by stepwise thermal demagnetisation at temperatures up 600°C, and cleaning temperatures for other specimens were selected according to the demagnetisation results of the pilot specimen for their site. After the measurements were completed 95 (54 percent) of the sites were rejected as unsatisfactory on the grounds for poor stability of the pilot specimen and / or poor in-site agreement of the directions of magnetisation and / or poor agreement of the site mean direction with the formation mean direction. Most poor agreements between site means and formation means were in strongly deformed strata and could be attributed to imperfect tectonic corrections applied to the directions of magnetisation. Fold tests of the accepted site mean directions were significant for the two lowest formations of the group (Lom Sak and Nam Phong Formation) which are the most strongly folded; directions in the younger formations, which have gentle dips (5 - 10 degrees), showed marginal or no improvement in Fisher's k after tectonic correction. Mean directions of magnetisation for each formation are in fair agreement. That for Lom Sak Formation (basic and intermediate tuff with titanomagnetite) is somewhat divergent and is comparatively close to the present field direction. The Nam Phong, Phu Kradung, Phra Wihan and Sao Khua formation mean directions are remarkably similar. Overall means are as follows:- All formations; D, 034; I, +31; k, 45; A95 9; Lat, 57(dP6) Long, 188(dM 10) All formations except Lom Sak: 037; +32; 9; 54 (dP 5) 186 (dM 10). Nam Phong, Phu Kradung, Phra Wihan and Sao Khua formations (lower to lower Upper Jurassic): 033; +37;246; 59 (dP 4); 181 (dM 7). The paleomagnetic declincations do not decrease between the base and top of the Khorat Group, and clockwise rotation therefore occurred at some time after the middle of the Cretaceous. Inclinations are essentially uniform and indicate a paleolatitude not significantly different from the present day latitude of Northeast Thailand. By contrast the Jurassic and Cretaceous paleolatitudes of nearby South China indicate a situation substantially north of the present day latitude. South China and Thailand probably have rotated clockwise partially in sympathy, possible around a single Euler pole near Northeast Thailand, but some differential movement, probably along the line of the Red River Fault, must have occurred also. Most sites have normal polarity, but infrequent thin reversed intervals provide evidence for tentative world correlations using a paleomagnetic reversal time-scale compiled from several sources. The polarity sequence determined for the Khorat Group substantially confirms and refines that proposed by Bunopas in 1981. Contrary to the correlations of Bunopas, the Lom Sak Formation in now considered to be mainly Middle Triassic (not Upper Triassic), and the Phu Phan Formation is considered to be indeterminate in age but probably Lower Cretaceous and conformable with the overlying Khok Kruat Formation (Lower to Middle Cretaceous). A lower Cretaceous age for the Phu Phan Formation would imply an unconformity and substantial stratigraphic hiatus between the Phu Phan and underlying Sao Khua Formation.