Jack Oliver

Fluids expelled tectonically from orogenic belts: Their role in hydrocarbon migration and other geologic phenomena

This paper presents and supports a speculative hypothesis, the essence of which follows. When continental margins in zones of convergence are buried beneath thrust sheets, fluids expelled from the margin sediments travel into the foreland basin and the continental interior. These tectonic fluids have key roles in phenomena such as faulting, magma generation, migration of hydrocarbons, transport of minerals, metamorphism, and paleomagnetism. The thrust sheet, crudely speaking, acts like a great squeegee, driving fluids ahead of it and producing widespread geologic consequences.

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.

The Moho in the northern Basin and Range province, Nevada, along the COCORP 40°N seismic-reflection transect

COCORP seismic-reflection profiles across Nevada at about 40°N image a prominent, essentially continuous band of reflectors at a two-way traveltime of 9 to 11 s. The approximate correspondence of this reflection time with estimates of the two-way traveltime to the Moho in this area provided by seismic-refraction data suggests that the prominent reflections are from the Moho. The relief on these reflectors (the “reflection Moho”) beneath Nevada, across a latitudinal transect of

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.

Focal Mechanisms of Deep and Shallow Earthquakes in the Tonga-Kermadec Region and the Tectonics of Island Arcs

Well-determined focal mechanisms based on reliable first motions of both compressional and shear waves are presented for 18 shallow, 6 intermediate, and 15 deep-focus earthquakes in the Fiji-Tonga-Kermadec region of the Southwest Pacific. The double-couple model is an adequate representation for earthquake mechanisms at all depths; most of the mechanisms are characterized by a predominance of dip-slip motions. The orientations of the mechanisms of deep and shallow earthquakes appear to be systematically and fundamentally different in respect to the orientation of the seismic zone. Whereas the shallow mechanisms all appear to accommodate movements between the adjacent sides of the seismic zone, the slip planes of the deep earthquake mechanisms are systematically nonparallel to the deep seismic zone. Hence, the deep zone of activity does not appear to be a simple thrust fault. The P, B, and T axes of the double-couple solutions tend to parallel the dip, strike, and normal directions, respectively, of the portions of the Tonga seismic zone deeper than about 80 km. The P axes tend to be more stable in orientation than the B and T axes. Large variations in the orientations of some of the deep mechanisms may reflect contortions of the deep seismic zone in a simple geometrical fashion. The shallow mechanisms indicate that thrust faulting is occurring beneath the inner (islandward) margins of the Tonga and Kermadec Trenches and that transform faulting is occurring at the northern end of the Tonga Arc. The over-all interpretation of the shallow mechanisms also includes hinge faulting south of Samoa at the juncture of the thrust and transform faults. These results, which are also in agreement with mechanism data for other regions such as Japan where both numerous and reliable data are available, are most simply interpreted by a tectonic model of an island arc in which (a) shallow earthquakes occur between a segment of lithosphere that moves downward into the mantle and the segments of lithosphere on the surface, and (b) deep earthquakes occur within the downgoing slab in response to a compressional stress within it.

Overview of the COCORP 40°N Transect, western United States: The fabric of an orogenic belt

The COCORP 40°N Transect of the Cordillera of the western United States crosses tectonic features ranging in age from Proterozoic to Recent and provides an acoustic cross-section of a complex orogen affected by extension, compression, magmatism, and terrane accretion. The key features of the transect, centered on the Basin and Range Province, include (1) asymmetric seismic fabrics in the Basin and Range, including west-dipping reflections in the eastern part of the province and predominantly subhorizontal ones in the west; (2) a pronounced reflection Moho at 30 ± 2 km and locally as deep as 34 km in the Basin and Range with no clear sub-Mono reflections; and (3) complex-dipping reflections and diffractions locally as deep as 48 km in the Colorado Plateau and Sierra Nevada. The eastern part of the transect, shot above known and inferred Precambrian crystalline basement, probably records features related to the entire history of the orogen, locally perhaps as old as 1800 Ma. In this region, major paleotectonic features probably controlled subsequent structural development. In title western half of the transect, however, most reflectors are probably no older than Mesozoic. Within the Basin and Range Province, there appears to be a strong Cenozoic overprint that is characterized by asymmetric half-grabens, low-angle normal faults, and a pervasive subhorizontal system of reflections in the lower crust; no one model of intracontinental extension is universally applicable. Processes that produce or are accompanied by thermal anomalies (magmatism, enhanced ductility, and extension) appear to be essential in developing a highly layered lower crust.

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.

Deep seismic reflection characteristics of the continental crust

Four types of seismic reflection “fabrics” appear to characterize the continental crust: (1) a cratonic fabric dominated by diffractions and dipping reflections but lacking a pronounced reflection Moho; (2) regions of thin-skinned foreland deformation with abundant shallow but sparse deep reflections; (3) a unidirectional fabric that dips toward the hinterland and may represent a crustal-scale ramp zone; and (4) zones of highly layered lower crust and high-amplitude reflection Moho. The first three types are commonly observed in transects across single mountain belts, but the fourth appears to correspond to regions of elevated lower crustal temperatures such as those encountered in rift provinces and perhaps elsewhere.