T.P. Harding, 1983. "Divergent Wrench Fault and Negative Flower Structure, Andaman Sea", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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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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Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.