Regional Seismic Reflection Profiles Across the Middle America Trench and Convergent Margin of Costa Rica
J.C. Crowe, R.T. Buffler, 1983. "Regional Seismic Reflection Profiles Across the Middle America Trench and Convergent Margin of Costa Rica", 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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During 1977 and 1978, the University of Texas Institute for Geophysics acquired approximately 1,200 km (746 mi) of marine multichannel CDP seismic reflection profiles, both landward and seaward of the Middle America Trench off Costa Rica's Pacific margin. The Costa Rica survey is part of a larger comparative geological and geophysical study of the entire (Middle America) subduction-type convergent margin and trench from southern Mexico to Panama, designed to investigate margin and trench structural style, sedimentation patterns and tectonic processes. The bathymetry for offshore Costa Rica, and a detailed multichannel track chart are shown in Figures 1 and 2. The grid of reflection profiles, designated Costa Rica (CR) 1-9, and Nicoya Peninsula (NCY) 1-8, provide regional coverage of Costa Rica's shelf, slope, trench floor, and seaward deep ocean basin.
From a global data set comprised of spreading rates, transform fault azimuths, and earthquake slip vectors, Minster and Jordan (1978) used systematic inversion methods to model present-day plate motions, and concluded that the Middle America Trench is a convergent plate boundary. Subduction of the Cocos Plate beneath Costa Rica (Caribbean Plate) presently occurs along an azimuth of N30°E±2 at a velocity of 9.4±0.3 cm/yr (3.7±0.1 in/yr) (McNally and Minster, 1981; Minster and Jordan, 1978; see also Molnar and Sykes, 1969).
A late Oligocene-early Miocene age for the Cocos Plate oceanic crust in the study area is supported by analysis of sea floor magnetic anomalies (e.g., Hey 1977; Hey et al, 1977), and DSDP drilling off Guatemala (von Huene et al, 1980) and the Cocos Ridge (Heath and van Andel, 1973). Up to 0.7 km (0.4 mi) of sedimentary strata overlying oceanic crust are observed on the study area reflection profiles. DSDP drilling on the Cocos Plate 22 km (13.7 mi) seaward of the trench off Guatemala recovered similar oceanic sediments. The pelagic and hemipelagic depositional sequences (DSDP Site 495) document the northeastward migration of the Cocos Plate away from the Pacific-Cocos-Nazca triple junction, through the equatorial carbonate belt and zone of high organic productivity (at or near CCD), and finally into proximity of hemipelagic/terrigenous sedimentation (e.g., the Central American margin), since the late Oligocene-early Miocene (von Huene et al, 1980).
The onland geology of Costa Rica has been the subject of numerous investigations (Dengo, 1962; Weyl, 1980). Some of the more recent papers that focus specifically on the Nicoya Peninsula of Costa Rica include de Boer (1979), Galli-Oliver (1979), Kuijpers (1980), Schmidt-Effing et al (1981), and Lundberg (1982). In addition, profile CR-7 is discussed in detail in a recent paper concerning variations in tectonic and structural style along the entire Middle America Trench (Shipley et al, 1982).
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.