Frederick A. Cook

Thin-skinned tectonics in the crystalline southern Appalachians; COCORP seismic-reflection profiling of the Blue Ridge and Piedmont

COCORP seismic-reflection profiling in Georgia, North Carolina, and Tennessee and related geological data indicate that the crystalline Precambrian and Paleozoic rocks of the Blue Ridge, Inner Piedmont, Charlotte belt, and Carolina slate belt constitute an allochthonous sheet, generally 6 to 15 km thick, which overlies relatively flat-lying autochthonous lower Paleozoic sedimentary rocks, 1 to 5 km thick, of the proto-Atlantic continental margin. Thus, the crystalline rocks of the southern Appalachians appear to have been thrust at least 260 km to the west, and they overlie sedimentary rocks that cover an extensive area of the central and southern Appalachians. The hydrocarbon potential of these sedimentary rocks is unknown and untested. The data show that the Brevard fault is the surface expression of an eastward-dipping splay off the main sole thrust, and they show, or imply, that other major faults of this region have similar origins. The data support the view that large-scale, thin crystalline thrust sheets may be significant features of orogenic zones.

COCORP seismic profiling of the Appalachian orogen beneath the Coastal Plain of Georgia

A southeastward extension onto the Coastal Plain of an earlier COCORP traverse, which confirmed large-scale, thin-skinned thrusting of crystalline rocks of the southern Appalachians, has provided some of the most spectacular reflections yet seen in crustal seismic data. Most of the reflectors can be interpreted as either fault surfaces or as metamorphosed strata of late Precambrian—early Paleozoic age. They are consistent with the hypothesis that a major detachment extends eastward beneath this part of the orogen, although other interpretations with a more complex pattern of detachments or sutures are also possible. Large-scale overthrusting provides a mechanism for incorporating sedimentary rocks into the lower crust and may help to explain many of the layered features on crustal seismic data. Reflections from deep beneath the Coastal Plain indicate that the structural configuration of the rocks is complex and that the remains of a collision zone are being observed. Several east-dipping horizons, which bear strong similarities to thrust faults in Valley and Ridge sedimentary rocks, are seen in the basement at shallow and mid-crustal levels beneath the Coastal Plain. The Augusta fault, for example, displays a reflection which extends at a low angle some 80 km or more southeast of its surface position. In conjunction with surface geologic information, these new data demonstrate that late Paleozoic compressive deformation was pervasive and resulted in lateral movements in the upper crust extending from the Valley and Ridge to the crystalline rocks beneath the Coastal Plain — a distance of 400 km or more. A large antiform, cresting at about 2.3 sec, or about 6 km below the surface, and other structures beneath the Coastal Plain of Georgia deserve further consideration for petroleum exploration, although metamorphism may have eliminated petroleum from these rocks. Refracted arrivals and fault geometries indicate two Triassic rift basins beneath Coastal Plain sedimentary rocks, one of which has apparently not been recognized previously.

Structure of the Riddleville Basin from COCORP Seismic Data and Implications for Reactivation Tectonics

Seismic reflection data collected during COCORPs traverse of the southern Appalachians reveal that the Triassic Riddleville basin is a half-graben defined by strong, north-dipping basal reflections truncated on the north by a major south-dipping normal fault. Lithologies observed in similar basins suggest that these high amplitude, continuous reflections may be from lacustrine sediments or basaltic layers. The high-angle border fault soles into the Augusta fault, a major south-dipping, low-angle Paleozoic thrust. Geometric relations suggest that formation of the Riddleville basin reactivated this older Appalachian fault.

Empirical mode skeletonization of deep crustal seismic data: Theory and applications

Seismic skeletonization is a pattern recognition technique used to decompose reflection seismic data into events or reflectors and their seismic attributes, and to store the results in an “event file” for an analysis toward seismic interpretation. The extraction of instantaneous frequency from the event file for any statistical analysis of the seismic attribute analysis is currently not possible, thus precluding the complete parameterization of the reflected seismic wave field in terms of its decomposition products. Empirical mode decomposition of reflection seismic data has uniquely addressed the question of computing the instantaneous frequency from its decomposition products using the Hubert transform. Although the decomposition products of the seismic skeletonization and the empirical mode decomposition are conceptually different, they share a common link to the primitive features of the seismic waveforms. In this paper, we introduce a new decomposition technique, empirical mode skeletonization, that combines the features of both seismic skeletonization and empirical mode decomposition. By this process, it is now possible to subject the reflection seismic data to an analysis that could include a parameterization of the reflected wave field. We apply the new technique to a segment of seismic line 1 of the Lithoprobe Slave Northern Cordillera Lithosphere Evolution (SNORCLE) transect to extract event-oriented instantaneous frequency attributes and also to analyze the decomposition products for any scaling behavior of the reflected wave field.

Seismic skeletonization: A new approach to interpretation of seismic reflection data

Automatic identification and isolation of events on seismic reflection data provides the basis for a new approach to seismic interpretation. The approach uses a syntactic pattern recognition method in which an objective function accommodates near-neighbor and non-neighbor trace cycle correlation and in which correlations between three traces (triplets), two traces (doublets), and single traces (singlets) are all included in configurations. The method yields a file that includes a listing of all identified events and their characteristics such as length (number of traces), dip, frequency, and amplitude. Thus, for the first time, these data can be statistically analyzed according to the relational attributes of seismic events.

COCORP seismic reflection profiling across thrust faults

Summary It is very important to have good subsurface data in order to understand the nature and behaviour of thrust faults. Deep crustal seismic reflection profiling is the best technique currently available to make detailed subsurface studies of such important problems as the attitude and extent at depth of major faults, and hence deduce the mode of deformation and tectonic forces producing them. The Consortium for Continental Reflection Profiling (COCORP) is collecting large quantities of seismic reflection data from the deep crust and upper mantle in many parts of the U.S.A. Areas of major thrusting which have been profiled so far by COCORP include the Wind River Mountains in Wyoming and the Southern Appalachians of Georgia and Tennessee. Seismic profiles have been very successful in delineating a major thrust fault of moderate dip underlying the Wind River Mountains, thus demonstrating that compressional tectonics were dominant in their formation. In Georgia and Tennessee the seismic profiles demonstrate that the major tectonic feature of the Southern Appalachians is a relatively thin overthrust sheet, which may have moved at least 260 km. Deep crustal seismic reflection profiling thus appears to be an indispensable tool for the study of areas in which thrusting and nappe formation have occurred.

Some consequences of palinspastic reconstruction in the southern Appalachians

Palinspastic reconstructions of Alleghanian compressive structures in the Valley and Ridge place severe constraints on acceptable models of crustal structure in the southern Appalachians. These reconstructions, in conjunction with available deep seismic reflection data, imply that the Carolina slate belt arc was transported westward relative to North American cratonal basement by at least 120 km during the late Paleozoic. At present, it is not possible to determine unequivocally whether this juxtaposition took place along subhorizontal intracrustal detachments or whether it was the result of the steepening of a pre-existing detachment surface to produce a high-angle lithosphere-penetrating boundary.

Identification and interpretation of azimuthally varying crustal reflectivity with an example from the southern Canadian Cordillera

A new approach to seismic interpretation, seismic skeletonization, is used to analyze crustal reflection data by identifying numbers of reflection events, their lengths, their energies, and their dips, with respect to azimuths of recorded profiles. Application of the method to crustal reflection data in the southern Canadian Cordillera demonstrates strong directional dependence of crustal reflectivity, with persistent trends at 025–040° (205–220°), 085–095° (265–275°), 115–125° (295–305°), and 140–160° (320–340°). These trends are visible in both the upper and lower crust and may be related to regional structural trends. Further applications of the technique, particularly in different tectonic regions, may allow categorization of seismic patterns and cataloguing of signatures for complex reflection data in a manner similar to seismic stratigraphy and structural patterns in stratified rocks.

Geophysical transect of the Eagle Plains foldbelt and Richardson Mountains anticlinorium, northwestern Canada

A 250-km-long east-west geological and geophysical transect has been constructed at about lat 66°40′N, from near the Yukon-Alaska border, across the Eagle Plains foldbelt and Richardson Mountains anticlinorium, to the Interior platform in northwestern Canada. It includes reprocessed industry seismic reflection profiles, regional gravity data, and drill hole information. The north-trending Richardson Mountains anticlinorium is interpreted to be a contractional (pop-up) structure, having a core of Proterozoic and lower Paleozoic rocks; the structure is bounded on the east and west by post-Early Mississippian, pre- or syn-Cretaceous thrust faults. Contractional deformation in the Eagle Plains foldbelt is probably the same age. The location of the pop-up may have been controlled by a preexisting west-facing crustal scale ramp at the top of the crystalline basement. A horizontal displacement of about 33 km is required to accommodate the pop-up; the displacement probably occurs above regional detachment(s) that project westward beneath the Eagle Plains.

3-D coherency filtering

Coherency filtering is a tool used commonly in 2-D seismic processing to isolate desired events from noisy data. It assumes that phase‐coherent signal can be separated from background incoherent noise on the basis of coherency estimates, and coherent noise from coherent signal on the basis of different dips. It is achieved by searching for the maximum coherence direction for each data point of a seismic event and enhancing the event along this direction through stacking; it suppresses the incoherent events along other directions. Foundations for a 2-D coherency filtering algorithm were laid out by several researchers (Neidell and Taner, 1971; McMechan, 1983; Leven and Roy‐Chowdhury, 1984; Kong et al., 1985; Milkereit and Spencer, 1989). Milkereit and Spencer (1989) have applied 2-D coherency filtering successfully to 2-D deep crustal seismic data for the improvement of visualization and interpretation. Work on random noise attenuation using frequency‐space or time‐space prediction filters both in two or t...