Interpretation of Seismic Reflection Data in Complexly Deformed Terranes: A Geologist's Perspective
- Muawia Barazangi and
- Larry Brown
Published Online: 15 MAR 2013
Copyright 1986 by the American Geophysical Union.
Reflection Seismology: The Continental Crust
How to Cite
Hatcher, R. D. (1986) Interpretation of Seismic Reflection Data in Complexly Deformed Terranes: A Geologist's Perspective, in Reflection Seismology: The Continental Crust (eds M. Barazangi and L. Brown), American Geophysical Union, Washington, D. C.. doi: 10.1029/GD014p0009
- Published Online: 15 MAR 2013
- Published Print: 1 JAN 1986
Print ISBN: 9780875905143
Online ISBN: 9781118670118
- Seismic reflection method—Congresses
Horizontal seismic reflectors have been used recently to speculate on the existence and continuity of major tectonic features such as thrust faults in crystalline rocks, or as boundaries between crystalline basement and cover sedimentary rocks. A large body of published seismic reflection profiles, in areas where structure can be verified using drilling and/or downplunge projection, supports this interpretation. However, it has been recently shown with the Arizona A-1 hole and elsewhere that prominent, continuous horizontal reflectors in crystalline rocks do not necessarily prove a major break is present, indicating a great deal of work remains before the nature of seismic reflection events in the deeper crust is understood.
The seismic reflection method commonly detects gently dipping layers having contrasting acoustic properties, although it may be theoretically possible to detect steeply dipping layers. Correct interpretation of recurrent crustal reflection patterns could provide enormous insight into crustal structure and evolution. However, geologic interpretations from the same sets of reflectors are numerous. Single layered subhorizontal reflectors may be thrusts, unconformities, mylonite/cataclasite zones, stratigraphic contacts, or facies boundaries. Inclined reflector packages may be as imbricate thrusts or normal faults, duplexes, root zones or ramps. Curved reflectors may indicate the tops of plutons, broad open folds or refolded folds. Transparent zones are the most difficult to interpret and may indicate zones of structural complexity, structurally simple areas or rocks that contain no acoustic contrast.
The interiors of mountain chains contain a complex assemblage of rocks of low to high metamorphic grade which have markedly different mechanical and probably acoustic properties. Reflection coefficients occur at interfaces between rocks of different densities, and anisotropies or compositions without presence of tectonic discontinuities. Sub-horizontal tectonic discontinuities, such as thrust faults, may provide additional acoustic contrasts, provided there is a difference in properties of the rocks on either side of the discontinuity. Later thrusts in orogenic belts are less deformed and, if they juxtapose rocks of contrasting acoustic properties, should provide excellent reflectors. However, unless independently verifiable, continuous reflectors in orogenic terranes may not prove to be tectonic or even lithologic boundaries.
Models of crustal structure should, of necessity, include geologic, seismic reflection, and data from other geophysical techniques, such as potential fields. These should increase understanding of the nature of the deeper continental crust.