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The prevailing view of the structural style of wrench-fault zones emphasizes the occurrence of en echelon folds and other structures that are caused by shortening. These features are often conspicuous along wrench faults in which blocks either move parallel to each other (Harding, 1973) or move with a component of convergence (e.g., convergent or transpressional wrench faults; Sylvestor and Smith, 1976).

A different group of structures characterize wrench zones where blocks move obliquely apart (e.g., divergent or transtensional wrench faults). These zones are sometimes distinguished by negative flower structures, which are defined as linear, shallow synforms that are displaced by upward-diverging strands of a wrench fault having mostly normal separations. This type of flower structure differs from the positive flower structures described elsewhere in the atlas (Harding et al, 1983). A positive flower structure is defined as a linear, shallow antiform that is displaced by upward-diverging strands of a wrench fault with predominantly reverse separations.

Two seismic profiles and a structure map from the Andaman Sea illustrate several characteristics of divergent wrench faults and negative flower structures, and demonstrate important criteria for identifying these zones. In the discussion following, I first outline the tectonic framework of the Andaman Sea region and present evidence for right-slip displacement in the area of the seismic profiles. Then the structural style of the Andaman Sea wrench fault is described.

The Andaman Sea is a marginal sea bounded on the west by the Andaman-Nicobar Ridge and on the east by the Malay Peninsula (Figure 1). The Andaman-Nicobar Ridge is the topographic expression of an outer high lying above an east-dipping subduction zone that has been active since the Oligocene or late Eocene (Curray et al, 1979). Extensive magmatism has occurred in several episodes on what is now the Malay Peninsula. At times during these episodes, which range in age from possibly the Carboniferous to Eocene (C.C.O.P, 1981, p. 45-49), this region had an arcmassif setting.

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