Active Margins, Part 2—Tonga Trench, Profiles 0-1200 and G-150
P. Lehner, H. Doust, G. Bakker, P. Allenbach, J. Gueneau, 1983. "Active Margins, Part 2—Tonga Trench, Profiles 0-1200 and G-150", 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
Download citation file:
The Tonga arc is of particular interest in plate tectonics since it was in this area that the concept of a descending "cold slab" of oceanic lithosphere was first demonstrated by Oliver and Isacks (1967).
The subduction zone of the Tonga trench is not located at the margin of a continent but entirely within the oceanic domain of the Pacific. It probably originated in Late Cretaceous time along a north to south trending transform fault.Along the outer wall of the Tonga trench the oceanic basement of the Pacific plunges from a depth of around 6 km (3.7 mi) to a depth of nearly 10 km (6.2 mi). Seismic indicates that a thickness of 300 to 400 m (984 to 1,312 ft) of sediment overlies oceanic basement. JOIDES core hole 204 encountered pelagic clays and oozes of Eocene to Recent age overlying volcanoclastics.
Seismic resolution on the inner wall of the trench is poor. A discontinuous weak reflection band, interpreted as the top of the oceanic basement, can be traced below the poorly defined accretionary wedge to a depth of about 14 km (8.7 mi). The accretionary wedge probably consists of scrapings from the Pacific floor mixed with volcanoclastics, Eocene limestones, and basement material slumped down from the Tonga platform.
The Tonga platform is underlain by a strongly faulted basement, which (according to outcrop evidence) consists of gabbros, diorites and andesitic basalts. This basement is overlain unconformably by shallow marine limestones of middle Eocene age. The bulk of the sedimentary sequence on the Tonga ridge consists of Miocene volcanoclastics covered with a veneer of limestones, ranging in age from Pliocene to Quaternary.The western rim of the Tonga platform is formed by a faulted escarpment which forms the margin of the Lau basin. The volcanic cones visible on profile 150 form part of the Tonga volcanic trend. The Lau basin is a typical small ocean basin of Neogene age.
Figures & Tables
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.