J. A. Brewer

Andean tectonics related to geometry of subducted Nazca plate

Seismological and geological data show that tectonic segmentation of the Andes coincides with segmentation of the subducted Nazca plate, which has nearly horizontal segments and 30° east-dipping segments. Andean tectonics above a flat-subducting segment between 28°S to 33°S are characterized by (from west to east): (1) a steady topographic rise from the coast to the crest of the Andes; (2) no significant Quaternary, and possibly Neogene, magmatism; (3) a narrow belt of eastward-migrating, apparently thin-skinned, Neogene to Quaternary shortening of the Andes; and (4) Plio-Pleistocene uplift of the crystalline basement on reverse faults in the Pampeanas Ranges. From about 15° to 24°S, over a 30°-dipping subducted plate, a west to east Andes cross section includes: (1) a longitudinal valley east of coastal mountains; (2) an active Neogene and Holocene andesitic volcanic axis; (3) the Altiplano-Puna high plateau; (4) a high Neogene but inactive thrust belt (Eastern Cordillera); and (5) an active eastward-migrating Subandean thin-skinned thrust belt. Tectonics above a steeply subducting segment south of 33°S are similar west of the volcanic axis, but quite different to the east. Early Cenozoic tectonics of western North America were quite similar to the Neogene Andes. However, duration of segmentation was longer and the width of deformation was greater in the western United States. Patterns of crustal seismicity are systematically related to Plio-Quaternary structural provinces, implying that current deformational processes have persisted since at least the Pliocene. Horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench. Above flat-subducting segments, crustal seismicity occurs over a broad region, whereas over steep segments, it is confined to the narrow thrust belt. Strain patterns in the forearc region are complex and perhaps extensional, and a broad region of the Altiplano-Puna and Eastern Cordillera appears to be aseismic.

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.

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.

COCORP profiling across the Rocky Mountain Front in southern Wyoming, Part 2: Precambrian basement structure and its influence on Laramide deformation

The influence of Precambrian basement structures on subsequent deformation is of considerable relevance to studies of continental evolution. COCORP deep seismic-reflection profiles were recently recorded in southeastern Wyoming, where several major, but temporally separated, tectonic elements of the western United States are superimposed. Of these, a fundamental boundary between Archean and Proterozoic basements and the eastern front of Laramide deformation were the principal targets of the reflection survey. The former may represent an ancient Proterozoic plate boundary; the latter is a prominent physiographic feature that signifies crustal deformation far within the North American craton, more than 1,500 km from the nearest coeval plate margin. The major crustal feature controlling a lateral, north-south variation in Laramide tectonic style appears to be the Archean-Proterozoic crustal boundary, known in the nearby Medicine. Bow Mountains as the Mullen Creek-Nash Fork shear zone. COCORP data in the Laramie Mountains and the Laramie Basin suggest that this shear zone dips ∼ 55° to the southeast. Northwest of the shear zone, the seismic basement is also characterized by southeast-dipping events, suggesting that the early Proterozoic tectonics that produced the shear zone were distributed over a wide region. Complex reflections down to 15-km depth or more under the Laramie Basin may correspond to structures or erosional truncations in metasediments overlying the Archean basement complex. Deep crustal events (between 35 and 40 km) north of the shear zone are short and discontinuous, in contrast with flat, laterally continuous reflections south of the shear zone at about 48-km depth which are interpreted as the crust-mantle transition. Thus, COCORP data and published results of regional refraction and gravity surveys suggest that the crust is significantly thinner in the Archean basement terrane northwest of the shear zone than it is in the Proterozoic province to the southeast. Differences in crustal thickness may be partly responsible for the difference between Laramide structures in Wyoming and Colorado, and a thin crust may also have facilitated Laramide deformation farther east in the Black Hills, located north of the shear zone.

The Laramide orogeny: Evidence from cocorp deep crustal seismic profiles in the wind river mountains, wyoming

Abstract Deep crustal reflection data that are critical for the interpretation of Laramide structure have been obtained by the Consortium for Continental Reflection Profiling (COCORP). The Laramide orogeny, which occurred from the late Cretaceous to early Eocene, is characterized in Wyoming by large uplifts of Precambrian basement, commonly flanked by reverse faults. The attitude of these faults at depth has been a major tectonic problem and is very important for deciding whether horizontal or vertical crustal movements were primarily responsible for the basement uplifts. COCORP has run 158 km of deep seismic reflection profiles (recording to 20-sec two-way travel time) across the southeastern end of the Wind River Mountains, the largest of these Laramide uplifts. Reflections from the thrust fault flanking the Wind River uplift can be clearly traced on the profiles to at least 24-km depth and possibly as deep as about 36 km with a fairly uniform apparent dip of 30°–35°. Other reflection events subparallel to the main Wind River thrust are present in the seismic profiles and may represent other faults. There is at least 21 km of crustal shortening along the thrust. There is no evidence in the reflection profiles for large-scale folding of the basement; the Wind River Mountains were formed predominantly by thrust movements. Gravity anomalies in the Wind River Mountains can be modeled by a thrust that displaces dense material in the lower crust. If the thrust ever cut the Moho, the effect is not observed in the gravity today. A proposed model for the presence of uplifted basement in Wyoming invokes a shallowly dipping, subducted Farallon plate beneath the North American continent; drag between the two plates localized compressional stresses in an area over 800 km into the North American plate causing large thrusts to develop. The earths crust seems to have fractured as a fairly rigid plate

Thermal effects of thrust faulting

Abstract Calculations based on simple models of overthrust sheets in crystalline basement rocks show that significant thermal effects may result from their movements. If rates are sufficiently high (e.g. plate tectonic rates), the thrust sheets sufficiently thick (5, 10 and 15 km are modelled here), the distances moved sufficiently large, and for reasonable values of the coefficient of friction along the thrust plane overthrusting can cause metamorphic mineral zonations and heat flow anomalies observable in the field. Regions where large-scale overthrusting has occurred should be characterized by a decrease with depth of grade of metamorphic mineral assemblages and anomalously low heat flow. The theoretical effects are presented as a series of maximum temperature vs. depth and heat flow vs. time plots.

COCORP profiling across the Rocky Mountain Front in southern Wyoming, Part 1: Laramide structure

COCORP deep seismic-reflection profiles have been recorded showing significant crustal deformation of the North American craton; profiles were made across the Laramie Mountains, the Wyoming section of the Front separating the southern Rocky Mountains from the Great Plains, and eastern margin, in Wyoming and Colorado. These mountains are underlain by a series of westerly dipping seismic reflectors traceable as deep as 10 to 12 km, arranged en echelon with various dips (20° to 50°), which in some cases can be traced to the surface position of faults flanking the Laramie Mountains. Although other interpretations are possible, the reflectors are thought to be thrust faults, whose distribution and orientation suggest that the mountains were uplifted by horizontal crustal shortening during the Laramide orogeny. These inferred thrusts apparently do not generate such pronounced fault-zone reflections as those seen on COCORP lines recorded across the Wind River Mountains in Wyoming, probably because fault displacements were much smaller. Differential movements along the thrusts may explain variations in the character of the Rocky Mountain Front in the region of the COCORP lines. On the basis of morphological continuity, the Front in Colorado may also have been uplifted by crustal shortening with variations of structural style caused by adjustments to Precambrian and Ancestral Rockies structures. COCORP data have now been recorded across two mountain ranges lying on the eastern and western margins of the region of Laramide basement uplifts in Wyoming. Both ranges (Laramie and Wind River) apparently are underlain by moderately dipping thrust faults. Probably, crustal shortening by lateral compression was the dominant cause of the Laramide basement uplifts. These mountain-building compressional stresses were transmitted to the interior of the continent, 1,000 to 1,500 km from an active plate margin.

Proterozoic basin in the southern Midcontinent of the United States revealed by COCORP deep seismic reflection profiling

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.

Complex Archean lower crustal structure revealed by COCORP crustal reflection profiling in the Wind River Range, Wyoming

Abstract A COCORP deep crustal reflection profile across the Wind River uplift crosses exposed Archean rocks and resolves an unusual complex deep crustal structure at a depth of 24–31 km in an area where depth relations in Precambrian rocks can be inferred. The different levels of exposure across the beveled plunge of the Wind River uplift reveal supracrustal rocks at shallower levels with migmatites and pyroxene granulites at deeper levels. For the first time, deep crustal structure from reflection profiling may be interpreted in terms of exposed basement geology. A folded, multilayered deep structure shown by relfection data resembles multiply folded pyroxene granulite interlayered with granitic gneiss exposed in the central Wind River uplift; isoclinal folding is suggested in the folded layered seismic structure. Earlier seismic reflection studies suggested a simpler lower crust. These data indicate that lower crustal structure may have a complexity similar to deeply eroded Precambrian granulite-facies rocks. If this seismic feature represents folded metamorphic rocks, it seems unlikely that this Archean crust could have been thickened by underplating after 2.7 b.y. B.P. and the crust would have to be at least 30 km thich when this structure was formed.

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.