Abstract: Accretion Tectonics in Thailand Along the Nan-Uttaradit Suture Zone

S. Hada, S. Bupopas, V. Thitisawan, S. Salyyapongse, Y. Pasajawatwong, W. Yanoiyothin, K. Ishii, S. Yoshikura

A reconnaissance geological study was conducted on the Nan-Uttaradit suture zone in Thailand to clarify its tectonic evolution. One strong view accepted today is that Thailand was created by continent/continent collision; for example, during the Indosinian (Triassic) orogeny. A belt of mafic and ultramafic rocks extending along Nan-Uttaradit and Sa Kaeo-Chanta Buri in Thailand has been interpreted to form part of this collisional suture zone between the San-Thai and Indochina cratonic terranes located to the west and east, respectively.

The Shan-Thai and Indochina terranes bounding both sides of the suture zone are composed of coherent units such as volcanic-arc sequences and calcareous sediments on a continental shelf. On the other hand, the exposed rocks of the Nan-Uttaradit suture zone in the southeastern part of Thailand comprise two belts: a western belt of the chert-clastic sequence and an eastern belt of serpentinite melange that may occur as inferred fault-bounded packages.

The chert-clastic sequence consists of a coarse clastic unit and a chert unit occasionally accompanied by a greenstone in descending order. The sequence is tectonically piled up to form an imbricate structure. Because of this geologic structure, chert and clastic units occur alternately. This was formed by successive offscrape-accretion mainly during the Middle Triassic based on the radiolarian biostratigraphy. In contrast, characteristic "rock mixture" of chaotic and "block-in-matrix" fabric are differentiated in the eastern belt of the suture zone. The eastern part of the belt consists largely of sheet-like bodies of metamorphic rocks. The western half of the belt is a typical serpentinite melange zone and is characterized by chaotic complexes, which are complicated admixtures incor orating a great variety of rocks of oceanic, island-arc, and continental affinities, and are of various ages. The geochemical data of basaltic rocks indicate they are assigned both to the ocean-floor basalts and a remnant of an ocean island. Ages of the limestone blocks in the melange range from late Early Permian to middle Middle Permian, although the age is restricted to late Middle Permian in the Indochina terrane. Most of radiolarian fossils extracted from chert blocks indicate Early to Late Permian ages. Zircon U-Pb dating was conducted on one granitic lens found in the melange. The four data points define a discordia yielding an upper intercept date of 486.5 + 5 Ma, which is early Oligocene.

Rocks in the suture zone generally dip toward the west, and stretching lineations generally plunging toward the west are observed. The kinematic indicators, such as partially rotated pebble and an asymmetric microstructure, have been observed in outcrops and thin sections made parallel to the stretching lineations. The sense of rotation determined from these structures is regionally from west to east.

The Nan-Uttaradit suture zone has been inferred to extend south to the Bentong-Raub line in peninsular Malaysia. Although this is certainly a boundary between the Than-Thai and Indochina terranes, the nature of this line is still a subject of animated controversy. Although the Bentong-Raub line is characterized by a few isolated lenses of serpentinites and cherts, and the occurrence of an oligostromal sequence, other lithotypes typical in subduction or accretionary complex are absent. The present Benton-Raub line is possibly to be interpreted as a suture boundary modified by the strike-slip movement after the time of suturing. Such a nature is also observed on the boundary between Indochina terrane and the melange belt of Nan-Uttaradit suture zone. Very recently, Late Devonian to Late Permian radiolarians have been recovered from the chert lenses along the line.

From these facts, it is concluded that the time of rifting of Indochina and Shan-Thai from Gondwana seems to be, respectively, Late Devonian and Middle Permian. Their amalgamation may have occurred near the edge of the Eurasian continent during the Late Triassic.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994