S. Kaufman

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.

Cenozoic and Mesozoic structure of the eastern Basin and Range province, Utah, from COCORP seismic-reflection data

COCORP seismic-reflection data collected from the eastern Basin and Range in west-central Utah provide information on Cenozoic extensional tectonics, Mesozoic thrusting, and their interrelationships. Those data show a series of remarkably continuous, low-angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15–20 km. Over that distance, none of these events are significantly cut by any high-angle normal faults. A major detachment beneath the Sevier Desert can be traced from a surface zone of normal faulting to a depth of 12–15 km, with a regional apparent westward dip of 12°. Tentative correlation of upper- and lower-plate events suggests 30–60 km of extensional displacement on this detachment. Whether this structure is a reactivated Mesozoic thrust is uncertain. West-steepening splays off the end of the detachment reach depths of 20 km and may represent a major Mesozoic ramp or zones of distributed ductile shearing during extension. Some events are interpreted to be Mesozoic thrusts, of which at least one (beneath the House Range) has been reactivated during the Cenozoic. The Snake Range decollement dips gently east and has a sense of Cenozoic displacement opposite to that of other Cenozoic detachments farther east. Deep events are most numerous beneath the east side of the Sevier Desert where they occur to depths of 30 km, at the top of or perhaps partly within the anomalously low velocity upper mantle.

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.

Crustal structure of eastern Nevada from COCORP deep seismic reflection data

The western Nevada segments of the COCORP 40°N deep seismic-reflection survey of the North American Cordillera reveal the geometry of structures of Cenozoic and possibly earlier ages to travel-times of > 10 s, corresponding to depths of >30 km. The most striking feature of the data is a band of prominent reflections, typically at traveltimes of 9.5 to 10.5 s, that are present discontinuously across the entire data set. Few reflections are observed from beneath the base of this reflective zone, which is interpreted as the crust-mantle transition. This “reflection Mono” is inferred to be continuous across the survey area, varying gradually in depth but without resolvable offsets. It appears to have taken its present form or position during basin-range crustal extension. The middle to lower crust in much of the survey area is characterized by discontinuous reflections that are typically subhorizontal and locally dip gently west. These reflections may represent intrusions or shear zones related to basin-range or pre-basin-range extension, but some are likely to be inherited from earlier compressional deformation. Reflections from the upper crust are interpreted as images of basin-fill strata, basin-range normal faults, and Mesozoic and Paleozoic thrusts related to back-arc thrusting and accretion of oceanic and arc-related rocks.

Nature of the Wind River thrust, Wyoming, from COCORP deep-reflection data and from gravity data

Critical data for the interpretation of Laramide structure, a major tectonic problem bearing on the formation of the Rocky Mountains, have been obtained by the Consortium for Continental Reflection Profiling (COCORP) in the form of deep crustal seismic-reflection profiles. The Wind River Mountains in Wyoming have been uplifted by the Wind River thrust, which can be traced on COCORP seismic-reflection profiles to at least 24-km depth with an average dip of 30° to 35°. This Laramide uplift is thus the result of extensive horizontal compression with a minimum horizontal displacement of 21 km and a minimum vertical displacement of 13 km. The crust appears to have deformed essentially as a rigid plate. Gravity anomalies across the uplift can be modeled by a thrust, with the same geometry as indicated by the seismic-reflection profiles.

New COCORP profiling in the southeastern United States. Part I: Late Paleozoic suture and Mesozoic rift basin

New COCORP profiling in the southeastern United States has revealed a broad zone of dipping reflections that extends downward through the crust beneath the coastal plain in western Georgia. The zone is over 50 km wide, and most of the reflections dip moderately steeply toward the south. Regional considerations suggest that this feature marks the late Paleozoic suture between North America and Africa. Where crossed by the COCORP survey, the suture occurs beneath the north flank of the Triassic-Early Jurassic south Georgia basin. The main depocenter of the south Georgia basin occurs about 90 km to the south and is formed by a large half graben containing more than 5 km of rift basin fill. Farther south, the Paleozoic Suwannee basin sequence beneath northern Florida is poorly imaged on the COCORP profiles. However, weak reflections suggest that these strata (including basal felsic volcanics) may have an aggregate thickness of about 6 km in north-central Florida. At the northwest end of the COCORP traverse, a prominent horizon imaged in the upper crust beneath the inner Piedmont probably marks the southern Appalachian detachment. The detachment appears to be cut off by the Towaliga fault, implying that the Towaliga fault is in part a down-to-the-north normal fault. Intermittent Moho reflections occur at 11–12-s two-way time along the length of the COCORP survey, indicating that the crust in the region has a roughly uniform thickness of about 33–36 km (assuming an average crustal velocity of 6 km/s).

COCORP Arizona transect: Strong crustal reflections and offset Moho beneath the transition zone

The Consortium for Continental Reflection Profiling (COCORP) transect across the southwest margin of the Colorado Plateau in Arizona reveals prominent subhorizontal zones of reflections, here termed the Bagdad reflection sequence (BRS), which dominate the upper and middle crust in the transition zone (TZ) between the Colorado Plateau and the Basin and Range province. A Cenozoic origin for the BRS is considered most likely; it may represent fragmented or sheared zones related to detachment faulting or a series of intrusions, possibly emplaced along preexisting zones of weakness. Moho reflections observed beneath the TZ and the Basin and Range province contrast with the nonreflective Moho beneath the Colorado Plateau, suggesting that extension and its associated igneous processes probably played a key role in the development of reflectors at the Moho. The Moho and a prominent midcrustal reflector are offset by about 3 km in a normal sense in at least one place; this is the best example of offset Moho yet found by COCORP and contrasts sharply with COCORP observations of a smooth Moho elsewhere. This offset is probably a late tectonic expression of crustal extension and thinning across the TZ.

Continuous seismic reflection profiling of the deep basement, Hardeman County, Texas

Our understanding of the crust and upper mantle would be enhanced if geophysical studies of the deep basement rocks provided information of resolution and character more nearly like that of geological observations of basement rocks at and near the surface. A test of the continuous seismic reflection profiling technique, the geophysical method with by far the highest resolution and the best potential in this regard, at a site in the midcontinent provided abundant information on intrabasement diffractors and reflectors to depths as great as about 45 km. Conventional equipment and techniques, including nonexplosive vibratory sources, were used with minor modification. In the upper part of the section below the sediments, there are reflectors continuous over the entire length of a profile that give evidence for warping, faulting, unconformities, and other structural features. An age of 1,265 ± 40 m.y. for a sample from a nearby hole indicates that these are Precambrian rocks and not part of the Cambrian basement rocks of the Wichita Province. Detailed correlation with the Precambrian section is inhibited by scarcity of geological information. In the lower part of the section, reflections are not, in general, continuous over more than a few kilometres, but zones and discontinuities within the basement may be distinguished on the basis of spatial density, length, and dip of reflectors. Zones of low reflector density may be plutons; curvature of reflections may indicate deep folded structures. The scale of such features is a few kilometres, and it contrasts with the markedly larger scale of the smallest features of the deep basement that can be resolved by other methods. The method appears to have outstanding potential.

COCORP profiling across the Southern Oklahoma aulacogen: Overthrusting of the Wichita Mountains and compression within the Anadarko Basin

COCORP (Consortium for Continental Reflection Profiling) deep reflection profiles recorded across the Wichita Mountains and Anadarko Basin suggest that significant crustal shortening occurred in the final stages of the evolution of the Southern Oklahoma aulacogen. The crystalline rocks of the Wichita Mountains were thrust in Pennsylvanian time northeastward over sedimentary rocks of the Anadarko Basin along a series of faults with moderate (average 30° to 40°) and southwesterly dips. These faults can be traced possibly as deep as 20 to 24 km. Listric thrust faults and hanging-wall anticlines developed in the sedimentary rocks of the basin. These features contrast with conventional interpretations of Pennsylvanian structures as the result of predominantly vertical movements along high-angle faults, and they suggest that Pennsylvanian downwarping of the Anadarko Basin was at least partially due to thrust loading. Truncations of reflections from Cambrian-Ordovician rocks in the deepest part of the basin suggest normal faulting, which would support ideas of an early extensional stage in the aulacogen cycle. The distinctive Precambrian layering seen on earlier COCORP data recorded south of the Wichita Mountains cannot be recognized under the Anadarko Basin, and the Proterozoic basin containing that layering may have been bounded on its north side by a Precambrian fault. This inferred fault was probably twice reactivated during formation of the Southern Oklahoma aulacogen—once during late Precambrian(?)-Early Cambrian extension, and again during Pennsylvanian compression. The popular view that aulacogens originated from radial rifting of updomed, homogeneous continental crust is probably too simplified, and a more important constraint on their location and development may be the nature of pre-existing lines of weakness.

Major Proterozoic basement features of the eastern midcontinent of North America revealed by recent COCORP profiling

COCORP profiling in the eastern midcontinent of North America has (1) traced an extensive sequence of Precambrian layered rocks beneath southern Illinois, Indiana, and western Ohio; (2) detected a broad zone of east-dipping basement reflectors associated with the Grenville front beneath western Ohio; and (3) discovered a wide region of west -dipping reflectors penetrating most of the crust beneath eastern Ohio. The Precambrian layered assemblage may be as much as 11 km thick beneath southern Illinois, extends at least 170 km in an east-west direction, and contains several strong reflectors that have a lateral continuity of 80 km or more. Industry seismic data indicate that the layering is extensive in a north-south direction as well. Possible explanations for the layering include the silicic igneous rocks of the ca. 1.48 Ga eastern granite-rhyolite province, which are penetrated by basement drill holes throughout the region, perhaps intermixed or underlain by mafic igneous or sedimentary rocks. The 40-50-km-wide zone of strong, east-dipping (25°-30°) reflectors beneath west-central Ohio corresponds to the position of the Grenville front as determined from potential field and drill-hole data. These dipping reflectors in the upper and middle crust are interpreted to result from ductile deformation zones (mylonites) like those exposed at the Grenville front in Canada and imaged on the GLIMPCE seismic reflection lines in Lake Huron. Both the COCORP and GLIMPCE lines show a remarkably similar reflection geometry, despite the more than 500 km separating the two profiles. Easternmost Ohio appears to be underlain by pronounced west-dipping (<40°) reflectors in the middle and lower crust, which are also interpreted as marking a region of pervasive ductile deformation 80 km or more in width. Analogy with similar reflection packages elsewhere suggests that these reflections may mark a major collision zone. The west-dipping reflectors may be correlative with similar reflectors imaged on another COCORP survey in northern Alabama. The correlations suggested by these new results, though tentative, imply that the eastern midcontinent is composed of a relatively simple assemblage of crustal blocks.