Equatorial Fracture Zone (Romanche Fracture)
Published:January 01, 1983
P. Lehner, G. Bakker, 1983. "Equatorial Fracture Zone (Romanche Fracture)", 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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A segment of seismic profile A-3 across the equatorial fracture zones is shown. In addition to Shell seismic, a large number of Lamont-Doherty sparker profiles were used to complete the structure map.
The Romanche fracture is one of a group of transform systems that separates the Jurassic Central Atlantic from the Cretaceous South Atlantic. These fractures reflect a major indentation in the Pangea breakup pattern.
The Central Atlantic north of the St. Paul Fracture began to open during Early Jurassic and formed an open ocean basin, 4,000 km (2,485 mi) wide, surrounded by extensive carbonate shelves when the South Atlantic began to open up in Aptian-Albian time.
Typical for all three fracture systems are 25 to 50 km (15.5 to 31 mi) wide trenches with steep south flanks and relatively gentle north flanks. The north flank of the Romanche fracture appears to be tectonically disturbed and there is evidence for reverse faults and some folding.
Line A-3 crosses the Romanche fracture 400 km (248.5 mi) to the east of the Mid-Atlantic ridge. Unlike other fracture systems, which become inactive away from the transform segment, the region between Romanche and St. Paul fractures shows signs of repeated tectonic activity up to Recent times.
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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.