Development of the Sagaing-Namyin Fault was either immediately preceded or accompanied
in northern Thailand, 100 to 500 km to the east (Fig. 7), by formation of northerlytrending
sedimentary graben and half-graben occupied by basins which now form intramontane
alluvial plains (Gibling and Ratanasthien 1980). In general the basin successions are
fluviatile with thick conglomerates and lacustrine sediments, lignite, coal, and in the Mae Sot
Basin, oil shales. To the south, the Chao Phraya Basin extends offshore into the Gulf of
Thailand where a late middle Miocene non-marine succession (Batchelor 1979) occupying
graben and half-graben lies unconformably on Lower Tertiary sediments (Hamilton 1979).
The maximum sedimentary thickness in northern Thailand approaches 3 km and in the Gulf
probably exceeds 4 km. Basin development has been related to mid-Tertiary differential
subsidence with development of growth faults, followed in the Pliocene or Quaternary by
block faulting with major uplift (Anon 1980), the late Cenozoic tectonic regime in general
being tensional (Suensilpong et al. 1978).