R.T. Buffler, 1983. "Structure of the Mexican Ridges Foldbelt, Southwest Gulf of Mexico", 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 Mexican Ridges foldbelt is an enormous belt of folded Tertiary strata that extends along the entire western Gulf of Mexico slope from about 25 N to about 19N lat. (Figure 1). The folds form long, linear, subparallel topographic features on the sea floor. The nature and origin of these folds has been the subject of debate since their discovery in the mid-1960s.
In 1975, the University of Texas collected a grid of multifold seismic data across the foldbelt in the southwestern Gulf (Figures 1 and 2). These lines were discussed in detail by Buffler et al (1979). The lines presented here are duplications of most of the figures included in this 1979 paper. The reader is referred to this paper for a more detailed discussion of (1) previous descriptions of the foldbelt, (2) descriptions of the University of Texas seismic data, and (3) conclusions regarding the origin of the foldbelt in light of the new deep-penetration seismic data. These figures are repeated again in this atlas because of the unique nature and origin of this large foldbelt. As far as we know, nothing quite like it has been described anywhere else in the world. in addition, this belt may have enormous petroleum potential, as deep-water turbidite sands probably occur within the deformed Mexican Ridges seismic unit (drilled in DSDP holes 90 and 91; Figure 1), and "bright spots," possible gas hydrates, and fluid contacts are observed on several lines in the crests of anticlines.
The main conclusion inferred from the seismic data is that the foldbelt is probably a huge late-Tertiary gravity-slide feature detached from flat-lying older Tertiary strata below along a decollement or deformed zone within weak, possibly geopressured, slope shales. It is characterized by a large growth-fault system at its upper limit at the present shelf-break. The folds and associated imbricate thrust faults indicate regional compressional stresses acting in an east to west direction. Deformation apparently began in Miocene time and probably is continuing today.
A much more detailed analysis of the entire southern Mexican Ridges foldbelt was completed recently (Pew, 1982) Pew analyzed a more extensive set of seismic data, including older singlechannel data in the area.
Details about the collection and processing of the University of Texas seismic data shown here are included as an Appendix.
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.