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In the deep crust, temperature, which is among the key pa- rameters controlling lithospheric dynamics, is inferred by extrap- olation from the surface using several assumptions that may fail in regions of active tectonics and fluid migration. In the rare case that temperatures of 700 8C or higher are exceeded in the upper and middle continental crust composed of quartz-rich felsic rocks, the a-b quartz transition (ABQT) will occur, generating a mea- surable seismic signature and offering the possibility for precisely estimating temperature from the known ABQT phase diagram. Here it is shown that all expected seismic features of the ABQT are met by the boundary between the upper and middle crust be- low the INDEPTH III profile in central Tibet. This finding implies that a temperature of 700 8C is achieved at a depth of 18 km under the southern Qiangtang block, which agrees with the depth to the top of a high electrical conductivity anomaly, likely representing partially melted crust. To the south in the northern Lhasa block, the ABQT is at 32 km depth, corresponding to a temperature of 800 8C. It thus appears that this seismic boundary representing the ABQT is the result of recent geologic processes rather than being a lithologic boundary.

Deep seismic reflection data from the 1995 Urals Seismic Experiment and Integrated Studies (URSEIS) transect of the Southern Urals document a distinct ∼5 km vertical offset of the Moho beneath the Uralian foreland, here named the Makarovo fault zone. This offset does not disrupt the overlying Riphean sedimentary section, nor does it correlate with any major surface lineaments or known Phanerozoic subduction features. It is thus interpreted to be an Archean or Early Proterozoic fault zone (possibly a terrane boundary) that disrupts an even older Moho. Most important is the implication that this structure and the Moho it offsets were not significantly modified by two subsequent cycles of continental collision that affected this region, the Baikalian (700–570 Ma) and the late Paleozoic Uralian (350–230 Ma) orogenic phases. Similarly, the pronounced reflectivity of the Moho beneath the Uralian foreland must also be a Precambrian signature, not a young feature, as would be implied by certain recent thermo-rheological and reflectivity models that associate a reflective lower crust and Moho with areas that underwent pronounced synorogenic or postorogenic collapse. The Makarovo fault zone provides yet another demonstration that relicts of ancient tectonic processes can survive for long periods of time in the lower crust, and that the Moho can be a key marker horizon in delineating the tectonic evolution of the lithosphere.

COCORP (Consortium for Continental Reflection Profiling) deep-reflection profiles crossing the northern Williston basin show (1) dipping reflections within the crust of the underlying Trans-Hudson orogen that extend downward into the upper mantle and (2) no clearly defined reflection Moho. In contrast, Lithoprobe (Canada9s national geoscience project) profiles crossing the Trans-Hudson orogen north of the basin show dipping reflections within the crust that terminate at a sharply defined subhorizontal reflection Moho—a reflection character typical of the interior of collapsed Phanerozoic orogens. We suggest that a crustal root produced during Hudsonian collision was incompletely removed by “normal” postorogenic extension in the region now underlain by the Williston basin. This remnant crustal keel much later underwent eclogite-facies metamorphism, which overprinted a new nonreflective Moho across the root and induced the subsidence of the Williston basin.

COCORP (Consortium for Continental Reflection Profiling) deep reflection profiling across the Williston basin and underlying Trans-Hudson orogen shows the following. (1)There is no evidence for a precursor rift basin beneath the axis of the Williston basin. With the exception of small-scale structures (e.g., Nesson and Cedar Creek anticlines), the basement surface beneath the Williston basin is smooth and has a dish-shaped geometry. (2) Crustal penetrating reflections within the eastern Trans-Hudson orogen dip eastward, in apparent contradiction to the "conventional view" that the Superior province underthrusts the eastern side of the orogen. (3) Basement reflections beneath the center of the orogen define a broad crustal-scale antiform, the axis of which coincides approximately with the depocenter of the overlying Williston basin. Midcrustal reflections defining the crest and western flank of this antiform also coincide generally with the location and shape of the source of the North American Central Plains conductivity anomaly. (4) Crustal penetrating reflections beneath the western side of the orogen dip westward, supporting the current view that subduction polarity was to the west beneath the Wyoming province. (5) The eastern edge of the Wyoming province is marked by east-dipping upper-crustal reflections and subhorizontal lower-crustal reflections that are truncated by the west-dipping reflectivity of the Trans-Hudson orogen.

Unusually clear indications of complex structure in the mid-to-lower crust is revealed by seismic reflection surveys in northeastern Kansas. This complexity contrasts markedly with the layer-cake simplicity of both the overlying sedimentary cover and most previous crustal models for the central United States. Seismic sections collected by COCORP (Consortium for Continental Reflection Profiling) as part of a major east-west traverse across the Neniaha Ridge and Midcontinent Geophysical Anomaly indicate that below a thin, relatively flat layered Paleozoic sedimentary section, the deep crust is characterized by numerous dipping and arcuate reflections and diffractions. In many places layered and crosscutting, these reflections suggest convoluted three-dimensional folded, faulted, and intruded structures. Specific identification of these deep features may be possible if future surveys can trace them to accessible depths. The basement above these reflection complexes contains significantly fewer reflections—consistent with, but not necessarily diagnostic of, the granitic terrane that dominates basement drill-hole samples in the region. Among the events at these shallower basement depths are several east-dipping reflections, some of which may be major faults. Travel times corresponding to expected Moho depths (about 36 km) are characterized less by specific reflections than by an apparent decrease in the density and number of reflections. While evidence of crustal heterogeneity is common among deep reflection studies, the Kansas seismic results outlined in this brief report stand out as being unusually clear representations of such.

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.

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.

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.

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 (Consortium for Continental Reflection Profiling) surveys across the mid-Michigan gravity anomaly, near the McClure-Sparks No. 1 deep well, delineate a thick layered sequence of relatively strong reflectors that define a structural trough within the Precambrian beneath the Michigan Basin. These deep reflections, which correlate spatially with the mid-Michigan gravity and magnetic high, are interpreted to be a pile of basaltic extrusive rocks intercalated with clastic rocks and correlative with Keweenawan volcanics exposed in the Lake Superior region to the northwest. A reflection-poor zone between the base of the Paleozoic, as found in the deep well, and the top of the deep layered sequence probably corresponds to the upper Keweenawan clastic assemblage. Direct evidence of faulting within the inferred volcanic sequence is lacking, although sharp flexures, structural benches, and dips of as much as 10° suggest subsidence controlled by fault-block motions. Reflections from beneath the inferred volcanic pile are rare, the deepest clear event being at about 18 km. There is no unequivocal evidence on these sections to indicate whether Precambrian structures were reactivated to affect the overlying Paleozoic strata of the Michigan Basin proper, although data quality may be insufficient to detect such effects.

New COCORP deep seismic reflection profiles from the Montana plains between the Rocky Mountains and the Williston basin image the crystalline continental basement of the Archean Wyoming cratonic province on a regional scale. The crust is, in general, reflective throughout its entire thickness. West of the Williston basin, the crust-mantle boundary is at the base of the reflective zone and is not marked by the presence of any distinctive reflections. The lowermost crust beneath the Williston basin is, in contrast, characterized by a prominent, laterally extensive zone of relatively high-amplitude reflections. If, as the spatial correlation suggest, the anomalously reflective lower crustal zone is causally related to the subsidence of the basin, then the data place constraints in addition to those of the sedimentary record on physical models for the evolution of the Williston 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.

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 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 (

COCORP seismic reflection profiles collected in 1977 on the west side of the San Joaquin Valley near Coalinga in southern California provide information on the Cenozoic and Mesozoic structures in the subsurface of the western Great Valley. The data show evidence of significant normal faulting during the Cretaceous, when this part of California was the site of a forearc basin. Neogene compression, probably associated with transform motion on the nearby San Andreas fault system, may be reactivating these pre-existing faults in a reverse sense and causing the active folding of the southern Coast Ranges. The recent Coalinga earthquake sequence that began on May 2, 1983, appears to correspond to movement on the high-angle reverse fault within the basement that is inferred from the COCORP lines.

Recent releveling measurements conducted in the Socorro area of the Rio Grande rift specifically to study possible crustal deformation indicate uplift of the central part of the rift (Socorro) relative to bench marks to the west (Magdalena). Total relative uplift measured along the newly releveled line between 1934 and 1978.7 reached 10 ± 1 cm. Combining this new information with previously reported releveling data suggests a roughly elliptical uplift affecting at least 7,000 km 2 . Maximum observed uplift near the center of this area reaches about 20 cm relative to the periphery. The uplift occurs directly above an unusual crustal discontinuity at a depth of about 20 km below this part of the rift. This discontinuity, discovered on the basis of seismic evidence, has been inferred to be the upper boundary of an extensive magma body. The spatial coincidence of the zone of uplift and the presumed magma body, the persistent microearth-quake activity, and the modeling results strongly suggest that the observed movements result from expansion of the Socorro magma body. Although available data are not sufficient to determine uniquely the temporal behavior of the doming, the observations are consistent with more or less continuous uplift averaging about 5 mm/yr during the time the movements were monitored (1909 to 1979). The average inflation rate within the magma body required to produce the observed uplift is on the order of 1 to 2 × 10 −2 km 3 /yr and is consistent with measurements in volcanic regions. Although less clearly defined, the leveling data also suggest relative uplift within the Albuquerque-Belen Basin well beyond the presumed northern boundary of the Socorro magma body. If this uplift is associated with magmatic activity, the Socorro magma body extends considerably farther north than previously expected, or another magma body lies beneath the Albuquerque-Belen Basin.

New COCORP profiles on the coastal plain of Georgia and northern Florida support the hypothesis that the Brunswick anomaly marks a late Paleozoic suture. They do not support the alternate view that this anomaly is caused by a Mesozoic rift basin. The trend of the Brunswick anomaly relative to the Appalachian gravity gradient indicates that in westernmost Georgia and adjacent Alabama, African basement (Suwannee terrane) is in proximity to autochthonous North American basement (Grenville). Farther east one or more Paleozoic accreted terranes intervene between the North American and African sides of the orogen. Offshore, the Brunswick anomaly closely parallels the east coast magnetic anomaly. This relationship implies that the east coast magnetic anomaly marks not only the present continental/oceanic crustal transition but also the northward continuation of the late Paleozoic suture between North America and Africa. Transitional crust beneath the Carolina Trough, Baltimore Canyon trough, and correlative parts of the African Atlantic margin is thus likely to have formed within a preexisting suture zone. The dramatic change in character of the U.S. Atlantic margin southward from the Carolina Trough to the Blake Plateau probably reflects the fact that two different types of prerift crust are juxtaposed in this region.

COCORP deep crustal seismic profiles in southwestern Oklahoma show strong, persistent, continuous, and undeformed layering in the basement over an area probably very much greater than 2,500 km 2 . Such layering is very unusual, judging by COCORP experience with basement rocks elsewhere in the United States. The data can be interpreted as representing a Proterozoic basin filled with clastic sedimentary and felsic volcanic rocks 7 to 10 km thick, whose base lies 10 to 13 km deep. These rocks are believed, on the basis of sparse evidence from regional geology, to have been deposited or extruded about 1,200 to 1,400 m.y. ago, and some of them may now be metamorphosed. This basin lies on the south side of the Wichita Mountains, under the Paleozoic Hardeman Basin, and is similar in depth to the Paleozoic Anadarko Basin north of the mountains. The deep basement layering is truncated on the south side of the Wichita Mountains, probably by Precambrian faults in conjunction with granitic intrusions. Pennsylvanian compression probably reactivated these Precambrian trends. Extensive Precambrian basin deposits in this area were unexpected, on the basis of evidence from sparse well control, and reports of other layered basement reflections elsewhere in the southern Midcontinent suggest that Precambrian basins may be an important feature of this region. Simple models for the evolution of southwestern Oklahoma as an aulacogen must be reformulated in the light of these new data.