Gregory A. Davis

The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.

Metamorphic core complexes form as the result of major continental extension, when the middle and lower continental crust is dragged out from beneath the fracturing, extending upper crust. Movement zones capable of producing such effects evolve in space as well as with time. Deforming rocks in the footwall are uplifted through a progression of different metamorphic and deformational environments, producing a characteristic sequence of (overprinted) meso- and microstructures. The movement zone is folded as the result of the bowing upwards of the lower crust to form a broad basement culmination, as the result of isostatic rebound due to tectonic denudation, but most likely also as the result of local isostatic adjustments due to granite intrusion in the middle crust. A succession of splays branch off from the master detachment fault at depth, excising substantial portions of the lower portions of the upper plate as successive detachment faults eat upwards through it. At the same time, detachment faults incise into progressively deeper levels of the lower plate, although the amount of incision is limited, because the locus of movement remains at approximately the same level in the lower plate. The detachment faults presently observed in the metamorphic core complexes are relatively young features, formed late in the geological evolution of these bodies, and are only the last in a succession of low-angle normal faults that sliced through the upper crust at the upward terminations of major, shallow-dipping, ductile shear zones in the extending Cordilleran orogen. Excisement of listric fault bottoms can explain some of the enigmatic domino-like fault blocks, and other structural relations observed in these terranes. Evidence in support of this model is illustrated from detachment terranes in the northern Colorado River region of southern Nevada, southeastern California and western Arizona, where multiple generations of detachment faults have produced remarkable excisement and incisement geometries.

Geometric and temporal evolution of an extensional detachment fault, Hohhot metamorphic core complex, Inner Mongolia, China

The Early Cretaceous Hohhot metamorphic core complex and its master Hohhot detachment fault are ;400 km west of Beijing in the Daqing Shan (Mountains) of Inner Mongolia. The complex developed across the east-trending Yinshan fold-and-thrust belt within ,4 m.y. following cessation of thrusting ca. 125 Ma (see note added in proof in main text). Postcontractional extension was initiated within a mid-crustal zone of mylonitic and ductile shear that was in part controlled by Carboniferous(?) strata sandwiched between its Proterozoic and Archean crystalline basement and an overlying thrust sheet of similar crystalline rocks. The Hohhot detachment fault appears to have rooted into deep, kinematically active levels of the mid-crustal shear zone. Higher, inactive levels of the mylonitic section were transected by the fault and carried upward in its footwall. Geometries of the footwall mylonitic rocks indicate localized ramp-flat geometries of the fault within and across them. The crosscut top of the mylonitic sequence defines a mylonitic front that departs from the gently south dipping detachment fault and dips northward into its footwall. Early Cretaceous extension was widespread elsewhere in northern China, and was particularly pronounced in the Yunmeng Shan core complex north of Beijing. The gravitational collapse of orogenically thickened crust acting in concert with localized centers of deep-seated plutonism appear to have led to the development of isolated metamorphic core complexes within a broad region of more distributed extensional deformation.

Structural evolution of the Whipple and South mountains shear zones, southwestern United States.

The Whipple and South mountains of the southwestern United States have undergone a strikingly similar sequence of deformations. In both ranges, gently dipping mylonitic fabrics have been overprinted by successively more brittle structures associated with a low-angle detachment fault. Kinematic indicators reveal that the mylonitic rocks, brittle structures, and detachment faults are kinematically coordinated and were all formed by top-to-the-northeast shear. The structural evolution of both areas can be explained in terms of major, shallow-dipping shear zones that accommodated Tertiary crustal extension. We suggest that detachment faults and associated zones of brecciation, cataclasis, and seismic slip were originally continuous downdip along the low-angle shear zones into mylonitic gneisses formed below or near the ductile-brittle transition. As the mylonites were drawn out from beneath the brittlely extending upper plate, they were progressively uplifted above the ductile-brittle transition and were overprinted by successively more brittle structures.

The enigmatic Yinshan fold-and-thrust belt of northern China: New views on its intraplate contractional styles

The east- to east-northeast–trending Yinshan belt lies within North China, extending westward at least 1100 km from Chinas eastern coast to Inner Mongolia. This intraplate Jurassic-Cretaceous belt underwent contractional and normal faulting, folding, and contemporaneous terrestrial sedimentation and magmatism. Current views on its contractional deformational style favor relatively limited “thick-skinned” faulting of Archean basement and cover units. These views are challenged, however, by recent discoveries in the eastern part of the belt of south-directed ductile nappe formation and large-displacement (>40–45 km) “thin-skinned” northward thrust faulting, both involving Archean and younger rock units. Collision of the Siberian and North China plates upon closure of a Jurassic and Early Cretaceous Mongolo-Okhotsk ocean more than 800–1100 km to the north may have been responsible for Yinshan north-south contraction. Some patterns of contraction, e.g., Jurassic-Cretaceous ductile nappe formation, appear to have been influenced by a superposed magmatic regime related to westward subduction of a Pacific basin plate beneath the North China plate.

Garlock Fault: An Intracontinental Transform Structure, Southern California

The northeast- to east-striking Garlock fault of southern California is a major strike-slip fault with a left-lateral displacement of at least 48 to 64 km. It is also an important physiographic boundary since it separates along its length the Tehachapi–Sierra Nevada and Basin and Range provinces of pronounced topography to the north from the Mojave Desert block of more subdued topography to the south. Previous authors have considered the 260-km-long fault to be terminated at its western and eastern ends by the northwest-striking San Andreas and Death Valley fault zones, respectively. We interpret the Garlock fault as an intracontinental transform structure which separates a northern crustal block distended by late Cenozoic basin and range faulting from a southern, Mojave block much less affected by dilational tectonics. Earlier ideas that the Garlock fault terminates eastward at the Death Valley fault zone appear to us to be in error, although right-lateral offsetting of the Garlock along that zone by about 8 km is necessary. Displacement along the Garlock fault must increase westward from its eastern terminus, a point of zero offset now buried beneath alluvial deposits in Kingston Wash to the east of the Death Valley fault zone. Much of the displacement on the Garlock fault due to east-west components of basin and range faulting appears to have been derived from block faulting in the area between Death Valley and the Nopah Range. Westward displacement of the crustal block north of the Garlock by extensional tectonics within it totals 48 to 60 km in the Spangler Hills–Slate Range area and probably continues to increase westward at least as far as the eastern frontal fault of the Sierra Nevada. Westward shifting of the northern block of the Garlock has probably contributed to the westward bending or deflection of the San Andreas fault where the two faults meet. Many earlier workers have considered that the left-lateral Garlock fault is conjugate to the right-lateral San Andreas fault in a regional strain pattern of north-south shortening and east-west extension, the latter expressed in part as an eastward displacement of the Mojave block away from the junction of the San Andreas and Garlock faults. In contrast, we regard the origin of the Garlock fault as being directly related to the extensional origin of the Basin and Range province in areas north of the Garlock. Recent models for development of that province related to intracontinental spreading east of an east-dipping subduction zone along the Cenozoic margin of western North America may best account for the differential east-west extension which has occurred in the crustal blocks to the north and south of the Garlock fault. Other possible examples of intracontinental transform faults in the southwestern Cordillera with geometries similar to that of the Garlock fault include the left-lateral Santa Cruz–Sierra Madre fault zone along the southern margin of the western Transverse Ranges, and the right-lateral Las Vegas shear zone and Agua Blanca fault of Baja California.

Geology of the Spring Mountains, Nevada

The northwest-trending Spring Mountains, Nevada, contain a 45-mi-wide (75-km) cross section of the eastern part of the North American Cordilleran orogenic belt and geosyncline. This cross section is probably the most southerly exposed section which exhibits structure and stratigraphy “typical” of the eastern part of the Cordillera. Stradgraphically, the transition from Paleozoic craton to miogeosyncline is present from east to west across the Spring Mountains. The sedimentary succession through the middle Permian thickens from 8,800 ft (2,660 m) east of the Spring Mountains to approximately 30,000 ft (9,000 m) in the west. Thickening of individual formations accounts for 6,800 ft (2,070 m) of added section, addition of formations at unconformities accounts for 4,600 ft (1,400 m) of added section, and addition of a thick terrigenous late Precambrian sequence accounts for 9,800 ft (3,000 m) of added section. Three major thrust plates are exposed in the Spring Mountains, each structurally higher plate containing a thicker sequence of Paleozoic rocks. The easternmost thrust is the Keystone thrust, except where the earlier Red Spring thrust plate is present below the Keystone as isolated remnants. The Keystone thrust appears to be a decollement thrust, but complications at depth suggest that additional thrust slices may be present below the thrust or several thousand feet of late Precambrian terrigenous rocks may be present above the thrust. The structurally higher Lee Canyon thrust plate probably contains at least 4,000 ft (1,200 m) of these terrigenous rocks at its base, and the Wheeler Pass thrust plate contains at least 11,000 ft (3,300 m) of these rocks. Pregeosynclinal basement could be involved in some of the higher thrust plates, particularly the Wheeler Pass plate, but depths of exposure are inadequate to determine its role. Thrust faulting has produced a shortening of from 22 to 45 mi (36.6 to 75 km) in the geosynclinal rocks based on geometric constructions of cross sections at depth. This range probably represents a minimum figure. Some folding and thrusting occurred during the early Late Cretaceous, but data within the Spring Mountains only establish a much wider time bracket, post–Early Jurassic to pre–late Cenozoic for the easternmost thrust faults and post–Early Permian to pre–late Cenozoic for the westernmost thrusts.

Maximum effective moment criterion and the origin of low-angle normal faults

Abstract The origin of the low-angle normal faults of metamorphic core complexes has been debated for over two decades. Proponents of Andersonian fault mechanics have long argued that it is mechanically unfeasible for slip to occur along shallowly dipping normal faults. A new theory, named the maximum effective moment criterion, is proposed here for their origin. Using an effective moment approach formulated as M eff = FH , where F is the force tangentially acting on the unit shear boundaries and H represents its arm. The maximum value appears at angles of ±54.7° with the σ 1 . Since extensional crenulation cleavages (eccs) and the contractional crenulation cleavages (cccs) occur in conjugate pairs with ∼110° angle between them, it is suggested that they tend to be oriented in the directions of maximum effective moment. The differential stress for formation of the ecc or ccc are less than that for fracturing. The orientations of conjugate eccs depending on relative magnitudes of simple shear versus coaxial strain components. The synthetic ecc set is much better developed than the antithetic set due to anisotropy. Upward propagation of the synthetic ecc set from mid-crustal domains of mylonitization through strain localization and strain softening is considered an effective mechanism for the formation of the low angle-normal faults of metamorphic core complexes.

Structure, Metamorphism, and Plutonism in the South-Central Klamath Mountains, California

In the south-central Klamath Mountains 50 miles of the the north-trending central metamorphic belt and adjacent parts of the eastern Paleozoic and western Paleozoic and Triassic belts have been mapped and studied in detail. Within the central metamorphic belt a sequence of three lithologically distinctive metamorphic units has been recognized (from bottom to top): (1) siliceous metasedimentary rocks and greenstones of the Stuart Fork Formation; (2) the Salmon Hornblende Schist; and (3) siliceous, calcareous, and amphibolitic rocks, predominantly metasedimentary, of the Grouse Ridge Formation. The age of these metamorphic rocks is uncertain; they are known only to predate intrusion of Late Jurassic (Nevadan) granitic rocks. Ultramafic rocks, mainly alpine-type peridotites, were emplaced before the granitic rocks and occur primarily in a single large sheetlike body which separates the central metamorphic belt from the eastern Paleozoic belt. Granitic plutons, including quartz diorites, trondhjemites, granodiorites, diorites, and gabbros, in decreasing order of abundance, range in size from less than 1 to about 80 square miles in area. Two orogenic phases in the central metamorphic belt have been distinguished by structural and textural features. A late deformation uniformly affected the metamorphic terrane and the ultramafic rocks but predated granitic rocks. It was accompanied by some metamorphism in the lower to middle greenchist facies and produced upright folds that trend south and plunge gently. An earlier phase affected the various rock units differentially; it produced widespread recumbent folding and upper greenschist- to amphibolite-facies metamorphism in Salmon and Grouse Ridge rocks, but involved the underlying Stuart Fork Formation less severely, producing at least local recumbent folding and lower greenschist-facies metamorphism. The preferred interpretation of this upward increase in structural complexity and metamorphic grade is that the Salmon-Grouse Ridge sequence is a thrust sheet which overrode the Stuart Fork rocks concurrently with emplacement of the ultramafic rocks during the culmination of early recumbent folding and metamorphism. Thrusting was then followed by upright folding during the waning stages of regional metamorphism. The first deformational phase, and possibly the second, occurred during late Paleozoic time as indicated by recent isotopic ages of Salmon Hornblende Schist.

Stress magnitude, strain rate, and rheology of extended Middle Continental Crust inferred from quartz grain sizes in the Whipple Mountains, California

Knowledge of the magnitude of differential stress and strain rate during the formation of mylonitic shear zones in metamorphic core complexes provides constraints on the mechanical behavior of the middle continental crust during extension. We analyzed the differential flow stress during the mylonitization of quartzofeldspathic rocks in the Whipple Mountains, California, using grain-size piezometers and kinetic laws for grain growth. Mylonitic gneisses collected from two widely separated transects have grain sizes that cluster in a range from 32 to 61 µm. Analysis of grain growth kinetics indicates that mylonitization of the gneisses continued during cooling to temperatures ≤500°C, compatible with estimates from two-feldspar thermometry. Quartz grain-size piezometers suggest that the mylonitization occurred under differential stresses (σ1–σ3) of ∼40–150 MPa, or maximum shear stresses of 20–75 MPa. Extrapolation of quartzite flow laws to 500°C indicates that the mylonitization occurred at strain rates faster than 10−14 s−1. These estimates suggest that the mylonitic zone within the Whipple Mountains had an effective viscosity of the order of 1018±4−1020±4 Pa s. These low viscosities and rapid strain rates, combined with seismic reflection data showing that continental crust is layered, suggest that more realistic physical models of extension of the continental lithosphere should treat the lithosphere as a heterogeneous distribution of high-viscosity regions separated by low-viscosity zones.

Major thrust sheet in the Daqing Shan Mountains, Inner Mongolia, China

Small thrust faults in the Daqing Shan that were previously mapped as separate structures belong to a single Mesozoic thrust, herein named the Daqing Shan thrust. It extends more than 155 km along the northern margin of the Daqing Shan, obliquely cutting the Daqing Shan along the western flank of the Jinluandian peak to the southeast and taking its way to Chayouzhongqi to the east. Kinematic markers indicate tectonic transport of the thrust sheet to the NNW. Minimum displacement, based on the observable outcrops, is 22 km, and the inferred thrusting distance may be larger than 35 km. The thrust sheet covers the whole eastern area of the Daqing Shan. The thrust truncates the E-W trending, somewhat older South-directed Panyanshan thrust and, therefore, the two faults are not as a result of counter thrusting. Such major intraplate tectonic deformation that occurred in the Inner Mongolia Axis or the Yin Shan latitudinal tectonic belt during late Jurassic time calls for deep thought on its tectonic significance in dynamics. It is most likely that the Daqing Shan thrust represents major intraplate shortening during Jurassic-Cretaceous closure of the Mongolo-Okhotsk ocean about 1000 krn away to the north.