Good-quality multichannel seismic data have revealed structural details across some of the major tectonic alignments in the western Pacific and eastern Indian Oceans along which lithospheric subduction has been proposed.

The data were gathered by R/V Gulfrex for Gulf Oil Corporation as part of the reconnaissance survey of the petroleum potential and tectonic framework of the region.

The evidence indicates that the alleged lithospheric subduction at oceanic trenches is largely restricted to the crystalline substratum, whereas the sedimentary layers undergo a "reorganization" as they are scraped off the substratum and piled on a surface much smaller than that on which they were originally deposited. This tectonic disturbance of apparently compressive nature affects the sedimentary sequence through the oceanward migration of a "tectonic front" which abruptly separates the undisturbed strata from the severely disrupted sequence. The disrupted rock mass is largely homogeneous seismically and could be construed as melange.

It can now be seen why interpreters of earlier and low-resolution seismic data concluded that trench deposits are "undisturbed." They were obtaining reflection events only from those markers not yet reached by the migrating tectonic front. They had no way of postulating a lithologic continuity across the front itself.

Besides trench-related compression, the Gulfrex data also indicate that a similar phenomenon probably occurs along pseudocontinental margins in the northeastern Sulu Sea and offshore southeast of Honshu, Japan. No bathymetric trench is present in either area, and the sedimentary rocks scraped off the substratum contribute to the buildup of the lower continental slopes.

The significance of this phenomenon to the explorationist is that, although hydrocarbons may be synthesized from dispersed organic material as the result of "tectonic milling," it is highly unlikely that reservoir or seal conditions could exist in these highly disturbed areas unless the melange were covered by, or juxtaposed against, undisturbed sedimentary units in which the fluids could be trapped.