Showing papers in "Tectonics in 1989"

Continental growth of northwest China

Abstract: The northwest corner of China is the most interior part of Central Asia and includes parts of the Siberian, Tarim, and Kazakhstan plates where the Junggar and Tarim basins are separated by the east-west trending Tianshan Range. A preliminary terrane map divides the area into thirteen separate areas which are classified into four types: (l) continental, (2) oceanic crust, (3) island arc, (4) composite. The boundaries for these terranes are faults that have been previously defined either by geologic mapping or Holocene activity. Passive consolidation of the southern margin of Paleo-Asia is marked by accretion and subduction of the Paleo-Tethys oceanic basins and by development of volcanic arcs. Paleomagnetic data indicate that the major plates and associated smaller terranes did not reach final consolidation until the Permian or later. The stratigraphic record reveals development of intracontinental basins in the late Carboniferous followed by the Pan-Asian thermal event that gave rise to widespread intrusion of A-type granites of late Paleozoic age. The frontal collision of India along the southern border of Asia in the Paleocene reactivated nearly all of the older major strike-slip and thrust faults formed during the late Paleozoic consolidation of Paleo-Asia. The complex nature of the continental growth of Central Asia has produced and destroyed many varieties of sutures, and reconstruction of the original plate and terrane configurations therefore is enigmatic.

Tilting of continental interiors by the dynamical effects of subduction

Abstract: A mechanism is proposed to explain epeirogenic motions of craton interiors in terms of the response of the lithosphere to subduction. The effects of changes in sea level are distinguished from subsidence of the basement by analyzing the tilt of chronostratigraphic sequences in which the bounding horizons were deposited at approximately the same elevation with respect to sea level. As an example, the Late Cretaceous subsidence and Tertiary uplift of the western interior of North America is examined, and a maximum tilt amplitude of 3 km, with a horizontal deflection scale of approximately 1400 km, is inferred. The link between platform sedimentation and subduction is tested by using numerical models of mantle convection which mimic the subduction and by examining the horizontal scale of the deflections to the overlying lithosphere. It is found that this scale is relatively insensitive to the temperature contrast between the slab and the surrounding mantle, the flexural rigidity of the lithosphere, and even the physical process assumed to govern the subduction. The most important factor affecting the scale is the dip of the subduction zone, and shallower subduction angles (less than 45°) can produce horizontal deflections of the order of 1000 km or more. In contrast, the vertical scale of the deflection is sensitive to all the above parameters. Using these results, two subduction models are introduced which predict both the time and length scales of the North American tilt, and it is conjectured that the process may be responsible for other regions of platform subsidence where subducted lithosphere may have passed beneath the continent at a shallow angle.

Pre-Pliocene Extension around the Gulf of California and the transfer of Baja California to the Pacific Plate

Abstract: Late Miocene (12–5 Ma) extension around the edges of the Gulf of California has been alternatively attributed to “Basin and Range” extension, back arc extension, or development of the Pacific-North America plate boundary. This extension was ENE directed and similar in structural style to extension in the Basin and Range province. Timing constraints permit nearly synchronous onset of this deformation in a belt extending SSE from northernmost Baja California to the mouth of the gulf. Where this extensional faulting continued through Pliocene time to the present, synchronous with motion on the modern transform plate boundary in the Gulf of California, no change in direction of extension can be resolved. Revised constraints on Pacific-North America plate motion support the development of this late Miocene extension as a component of Pacific-North America displacement that could not be accommodated by strike-slip displacement along the existing plate boundary west of the Baja California peninsula. This scenario implies that transfer of Baja California from the North America plate to the Pacific plate was a gradual process, beginning about 12–10 Ma, when motion of the Pacific plate relative to North America was partitioned into separate regimes of strike-slip and dip-slip displacement on opposite sides of Baja California.

The Ecors Pyrenean deep seismic profile reflection data and the overall structure of an orogenic belt

Abstract: The 250-km-long deep seismic survey across the Pyrenees from the Aquitaine basin to the Ebro basin was sponsored by French and Spanish organizations, and was carried out in 1985 and 1986. This first seismic survey of an entire orogenic belt crosses the deformed boundary between the Iberian and European plates, the geometry of which has been greatly debated during a recent past. The main results are summarized as follows: (1) The entire crust shows well-defined reflectors with a general fan shape geometry. (2) The Iberian crust seems to be thicker than the European one. Both are limited by a well-defined deep layered material above the Moho discontinuity. (3) Only the Iberian crust was significantly thickened by a southward overstacking of slabs. (4) Near the surface in the external domains, reflectors define accurately the geometry of major thrusts and structures affecting the Mesozoic and Cenozoic cover of the Pyrenees. Using ECORS data (Etude Continentale et Oceanique par Reflexion et Refraction Sismique), the structure of the studied section of the belt can be modeled considering the nature and continuity of reflectors beneath the surface trace of the North Pyrenean fault considered as the initial vertical boundary between Iberian and European plates.

Ecors deep seismic data and balanced cross sections: Geometric constraints on the evolution of the Pyrenees

Abstract: The ECORS deep seismic profile and additional geological and geophysical data are used to constrain the balancing of a structural section crossing the Pyrenees. To minimize the effects of the mid-Cretaceous strike-slip motion along the North Pyrenean fault, we have chosen to restore the geometry to the period after the Albian-Cenomanian strike-slip faulting and before the Late Cretaceous compressional tectonics. At least 100 km of shortening must be accounted for in the deep crust in order to balance the cross section. The estimated length of the top of the Iberian Paleozoic basement is 40 km shorter than the length of the layered Iberian lower crust as measured on the ECORS seismic line. A variety of restorations are thus discussed to accommodate this discrepancy. The first solution considers that the discrepancy is due to an initial absence of lower crust underneath part of the Iberian margin. This solution implies a simple shear model involving low-angle detachment faulting during the opening stage of Albian basins. The favored solution considers that the missing Iberian deep crust is currently stacked within the axial zone; it implies that Albian basins formed as pull-apart basins along the North Pyrenean fault system.

New tectonic divisions of the Grenville Province, Southeast Canadian Shield

Abstract: On the basis of geological, geophysical, and geochronological data, the Grenville Province has been divided into three first-order longitudinal belts, the Parautochthonous Belt (PB), Allochthonous Polycyclic Belt (APB), and Allochthonous Monocyclic Belt (AMB). These are set apart by three first-order tectonic boundaries, the Grenville Front (GF), Allochthon Boundary Thrust (ABT), and Monocyclic Belt Boundary Zone (MBBZ). The belts are subdivided into terranes based on internal lithological character. The GF separates the Archean to Proterozoic foreland northwest of the orogen from reworked equivalents to the southeast. Continuous at the scale of the orogen, its main characteristic is that of a crustal-scale contraction fault. The PB, although less clearly identified along the length of the orogen, in most places represents upgraded and tectonically reworked rocks of the adjacent foreland. The boundary between the PB and the APB to the southeast, the ABT, is most clearly delineated in the eastern half of the province. It is the locus of major crustal delamination along which high-grade, mostly middle Proterozoic, polycyclic terranes were tectonically transported northwest toward and onto the PB. The AMB comprises two separate areas underlain by the Wakeham Supergroup and what is currently known as the Grenville Supergroup, respectively; its basal contact, the MBBZ, is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks. This first-order zonation implies a tectonic polarity to the Grenville Province, superimposed on which are second-order features evident from contrasting tectonic styles and radiometric ages. These characteristics are consistent with a diachronous or oblique collisional model for the Grenville orogen.

Cretaceous geotectonic patterns in the New Zealand Region

Abstract: In the mid-Cretaceous, around 105±5 Ma, the tectonic regime in New Zealand changed significantly. From the Permian to the Early Cretaceous most rocks formed under the influence of convergent margin tectonics and comprise incomplete remnants of magmatic arcs, forearc basins, trench slope basins, and accretionary complexes. This tectonic pattern was ended by the approach and collision of the spreading ridge between the Phoenix and Pacific plates. Crustal extension, leading eventually to fragmentation of the continental crust, commenced immediately after ridge collision. Precollisional and syncollisional events deformed and thickened the accretionary prism, with concurrent folding and thrusting of more arcward elements. Cretaceous granulites in southwest New Zealand are thought to result from the closure of a small backarc basin or marginal sea slightly earlier than the main ridge collision. Postcollisional block faulting led to thick nonmarine sedimentation in new basins that cut across old terrane boundaries, except in the outboard area where a thick wedge of marine rocks was deposited over the subsiding accretionary prism. After oblique ridge trench collision along the New Zealand margin, the Phoenix-Pacific Ridge propagated to the southwest to link with zones of incipient spreading in the Tasman Sea and south of Australia. Subsequently, the eastern part of the ridge continued to migrate south with increasing offset along the Udinsev Fracture Zone. In the Late Cretaceous a duplicate ridge developed. The southern branch continued to move southward and collided with the trench west of the Antarctic Peninsula in the Cenozoic. The northern branch is the extant East Pacific Ridge.

K-Ar and Ar-Ar geochronology of the Himalayan collision in NW Pakistan: Constraints on the timing of suturing, deformation, metamorphism and uplift

Abstract: Mica and hornblende K-Ar and Ar-Ar data are presented from each of the three crustal components of the Himalayan collision zone in North Pakistan: the Asian plate, the Kohistan Island Arc, and the Indian plate. Together with U-Pb and Rb-Sr data published elsewhere these new data (1) date the age of suturing along the Northern Suture, which separates Kohistan from Asia, at 102–85 Ma; (2) establish that the basic magmatism in Kohistan, which postdates collision along the Northern Suture, predates 60 Ma, and that the later granite magmatism spanned a range of 60–25 Ma; (3) show that uplift amounts within Kohistan are greater toward the Nanga Parbat syntaxis than away from it and that rate of uplift near the syntaxis increased over the last 20 Ma to a current figure of about 5.5 mm a year; (4) show that much of southern Kohistan had cooled to below 500°C by 80 Ma and that the major deformation which imbricated Kohistan probably predated 80 Ma and certainly predated 60 Ma and was related to the Kohistan-Asia collision rather than the Kohistan-India one; (5) imply that uplift along the Hunza Shear in the Asian plate together with imbrication of the metamorphics in its hanging wall took place at about 10 Ma and was associated with breakback thrusting in the hanging wall of the Main Mantle Thrust; (6) suggest that the Indian plate has a lengthy pre-Himalayan history with an early metamorphism at about 1900 Ma, major magmatism at 500–550 Ma and early Jurassic lithospheric extension or inversion; and (7) show that the Indian plate rocks were metamorphosed shortly after the collision within Kohistan, which occurred at circa 50 Ma, and subsequently cooled back through 500°C at circa 38 Ma and 300°C at 30–35 Ma with ages of cooling through 200° and 100°C (as determined by fission track data) locally controlled by Nanga Parbat related uplift tectonics.

40Ar/39Ar age constraints on deformation and metamorphism in the main central thrust zone and Tibetan slab, eastern Nepal Himalaya

Abstract: In eastern Nepal, structural and petrologic observations suggest that movement on the Main Central Thrust (MCT), the development of an inverted metamorphic sequence, and leucogranite plutonism were genetically related. Samples from across the MCT zone and its hanging wall, the Tibetan Slab, were collected from four locations in the Everest region for 40Ar/39Ar studies: (1) the MCT zone, (2) the lower Tibetan Slab, (3) the upper Tibetan Slab away from intrusive rocks, and (4) the upper Tibetan Slab adjacent to and including leucogranitic material. Within the MCT zone, muscovite and hornblende yield cooling ages of 12.0±0.2 Ma and 20.9±0.2 Ma, respectively. K-feldspar gives a minimum age of 8.0±0.2 Ma (closure temperature (TC) =220±15°C). In the lower Tibetan Slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7.7±0.4 Ma and 7.5±0.6 Ma and a K-feldspar minimum age of 6.4±0.8 Ma (TC = 225±20°C). An adjacent gneiss yields a 9.1±0.2 Ma biotite isochron and a K-feldspar minimum age of 3.6±0.2 Ma (TC = 210±50°C). In the upper Tibetan Slab, samples collected >200 m from visible intrusives yield a biotite isochron age of 20.2±0.2 Ma and a complex hornblende age spectrum suggestive of excess argon. At the fourth location an augen gneiss, a pegmatite, and a tourmaline-bearing leucogranite, all in mutual contact, yield indistinguishable mineral ages. The average biotite and muscovite isochron ages for these samples are 17.0±1.4 and 16.6±0.2 Ma, respectively. The minimum age for K-feldspar from the leucogranite is 15.5±1.8 Ma. As thermobarometric data from the MCT zone in this area suggest synmetamorphic deformation at temperatures of ∼500°–550°C, the MCT hornblende (TC = ∼500°C) dates this event at ∼21 Ma. Diffusion experiments on hornblendes from the MCT zone provide data which support a maximum duration of peak metamorphic temperatures of ≤2 Ma. Biotite ages as old as ∼20 Ma from the upper Tibetan Slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the MCT zone. Very young ages in a ductile shear zone in the lower Tibetan Slab suggest that there has been deformation within the Tibetan Slab that postdates major movement on the MCT.

Tectonic evolution of the West Junggar Region, Xinjiang, China

Abstract: The West Junggar region of northwestern China consists of a Hercynian-age fold belt occupying a late Paleozoic zone of convergence between three major Eurasian plates: Siberian, Tarim, and Kazakhstan. The northern part of the West Junggar area includes part of the Irtysh-Zaysan fold belt that extends to the northwest into the USSR, where it marks a broad boundary between the Siberian and Kazakhstan plates. The central part of the West Junggar area includes the southeastern extension of the Chingiz-Tarbgatay early Paleozoic island arc sequence. The southern part of the West Junggar area is truncated by the deep fundamental Junggar-Alakol fault that has brought the Yili microplate into juxtaposiition with the West Junggar region in the vicinity of Ebin Nur Lake. To the east the Junggar area is covered by sediments of the Junggar Basin. Paleogeographic reconstructions and geologic evidence indicates that these plates began converging in the Middle Carboniferous, and by Late Carboniferous alkali granites began invading the imbricated leading edges of the coalescing plates. Paleomagnetic data reveals that the plates continued to move along strike-slip faults, bringing the Tarim plate and Yili microplate into their present positions sometime in the early Mesozoic. Six separate ophiolite occurrences in the West Junggar region are strongly tectonized and dismembered. The Tangbale ophiolite is considered to be Late Cambrian (508 ± 20 Ma.) and represents the earliest known oceanic crust in Western China. Petrologic and chemical data indicate that the Tangbale ophiolite represents a possible back arc or forearc tectonic setting situated close to a developing island arc. The Darbut ophiolite has been tectonized to a melange and is overlain by Late Devonian flysch. Ages of the radiolaria in the cherts associated with the pillow lavas indicate that it formed in the Middle Devonian. Other occurrences of ophiolite melange in the northern part of the West Junggar region may be as young as Late Devonian. These ophiolites record ocean crust formation in the Early to Middle Paleozoic within the Junggarian ocean, and their present tectonic setting indicates continental accretion and imbrication shortly after their formation. Associated blueschist blocks in the ophiolite melange further supports the concept of imbrication brought about by B-type subduction and associated obduction. Postcollisional alkali granites intrude the oceanic sequences of the West Junggar Region and are considered to represent melted lower crustal material. Lead isotopic systematics on feldspars from six separate granite plutons indicate ratios that could only be considered oceanic as far as the magmas are concerned. The implications of this finding is that the Junggar Basin basement rocks in the north and west are probably oceanic. Evidence presented in this study shows a complex history for the development of the West Junggar region. Major movements of consolidating plates in the Late Paleozoic followed by rejuvenation of these same zones of closure during the Himalayan tectonic event of the Late Tertiary demonstrates that convergence zones and fold belts may respond to vertical and horizontal stress even after many million years of inactivity.

Geometric and kinematic development of border faults and accommodation zones, Kivu‐Rusizi Rift, Africa

Abstract: Three representative basins in the Western rift system of East Africa are bordered along one side by high-angle normal faults with 2- to 5-km throws (border faults). In plan view ∼100-km-long systems of linear border faults form curvilinear border fault segments bounding the East Kivu, West Kivu, and Rusizi basins. The opposite sides of these asymmetric basins are bounded by lower relief faulted monoclines or en echelon ramps. The largely unfaulted rift flanks have been uplifted 2 km above the 1.3-km-high East African plateau, with uplift narrowing basins during Quaternary time. Maximum estimates of ∼E-W crustal extension within basins are less than 25% (< 16 km), and planar border faults may penetrate the crust. The East Kivu and West Kivu basins are linked across the rift valley by a horst that serves as a hinge for subsidence in both basins. The westward tilted East Kivu and eastward tilted Rusizi border fault segments are linked along the rift by oblique-slip transfer faults that also accommodate along-axis differences in elevation. Upper Miocene-Recent eruptive volcanic centers within the comparatively high-strain interbasinal region (accommodation zone) generally coincide with the tips of border fault segments and transfer faults. The orientations of Miocene-Recent dip-slip and oblique-slip faults show little correlation with Precambrian shear zones or foliations in metamorphic basement. Differences between the East Kivu, West Kivu, and Rusizi basins in the age of initial faulting, subsidence, and age/composition of volcanic products suggest that border fault segments developed diachronously and propagated along the length of the rift. This along-axis border fault propagation and the crosscutting geometry of transfer faults contribute to the segmentation of the Western rift valley.

The Paparoa Metamorphic Core Complex, New Zealand: Cretaceous extension associated with fragmeotation of the Pacific margin of Gondwana

Abstract: In Westland-Nelson provinces of New Zealand, high-grade metamorphic and granitic basement rocks showing mylonitic ductile deformation are juxtaposed beneath low-grade metasedimentary rocks and undeformed granites by uplift on low-angle detachment faults. Several metamorphic core complexes analogous to those described from western North America are recognized. In the Paparoa Range, basement rocks include late Precambrian(?) paragneiss and granitic rocks of both Paleozoic and Cretaceous ages. Cover rocks include Ordovician turbidites, Paleozoic and Cretaceous granites, and mid-Cretaceous breccia-conglomerates. Brittle deformation and hydrothermal alteration (silica, chlorite, hematite, carbonate ± fluorite, uranium) characteristic of the detachment zone are also superimposed on uppermost lower-plate mylonites. Kinematic indicators in the mylonitic rocks including composite S-C febrics indicate that the detachment faults on the northeast and southwest sides of the Paparoa Core Complex had opposite senses of shear, with cover rocks on both sides moving away from the metamorphic core. Ductile deformation postdates several 114±18 Ma granitic plutons but by 108 Ma had ceased to affect at least some of the rocks currently exposed. Mylonitic rocks were uplifted to the surface and eroded into evolving half-grabens by 105–100 Ma. Uplifted basement yields K-Ar dates as young as 88 Ma, and tilting of the graben sediments indicates detachment continued well into the Late Cretaceous, when both cover and basement were intruded by alkali lamprophyre dykes. The Nelson-Westland core complexes occur within an Early Cretaceous granitic province characterized by relatively radiogenic strontium. The boundary of this province, the NW trend to mid-Cretaceous half-grabens, the NNE trend of stretching lineations in mylonitic rocks, and the ESE trend of late Cretaceous lamprophyre dykes indicate that regional extension was maintained in a NNE direction for much of the Cretaceous. This regional extension may be part of an “extension corridor” which traversed the entire Gondwana continental margin from NE Queensland, Australia, to Marie Byrd Land, Antarctica. Extension preceded opening of the Tasman Sea between New Zealand and Australia at approximately 84 Ma and closely followed long-lived compression on the Pacific convergent margin of Gondwana. The presence of core complexes in western New Zealand contrasts with the Australian margin to the Tasman Sea and lends support to simple shear models of continental rifting.

The North American Midcontinent Rift beneath Lake Superior from Glimpce seismic reflection profiling

Abstract: The Midcontinent rift system is a 1.1-b.y.-old structure extending from Kansas, through the Lake Superior region, and into southern Michigan. The rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments. For most of its extent it is buried beneath Paleozoic rocks but can be traced by its strong gravity and magnetic anomalies. The rocks of the rift system are exposed only in the Lake Superior region and comprise the Keweenawan Supergroup. Much of the geology of the Keweenawan is beneath Lake Superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on-shore geology. Seismic reflection surveys by the Great Lakes International Multidisciplinary Program on Crustal Evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail. The reflection profiles across the rift reveal a deep, asymmetrical central graben whose existence and magnitude was not previously documented. They show that, in addition to crustal sagging documented by previous investigations, normal faulting played a major role in subsidence of the axial region of the rift. A sequence of volcanic and sedimentary rocks, in places greater than 30 km thick, fills the graben. Thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben. Near the axis, the prerift crust is thinned to about one fourth of its original thickness, apparently by a combination of low-angle extensional faulting and ductile stretching or distributed shear. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. The location of the accommodation zones is inferred from abrupt disruptions in the Bouguer gravity signature of the rift. Uplift of the central graben occurred when the original graben-bounding normal faults were reactivated as high-angle reverse faults with throws of 5 km or more in places. The Midcontinent rift has some striking similarities to some younger passive continental margins. We propose that it preserves a record of nearly complete continental separation which, had it not been arrested, would have created a Middle Proterozoic ocean basin.

Fission track analysis reveals character of collisional tectonics in New Zealand

Abstract: During the late Cenozoic, the Pacific plate has been obliquely converging with the Australia plate in South Island, New Zealand. This has resulted in westward obduction of continental crust of the Pacific plate, and consequently, a major mountain range, the Southern Alps, has formed east of the Alpine fault and within the leading edge of the Pacific plate. We have applied the apatite and zircon fission track dating technique to a suite of outcrop samples from three transects across the Southern Alps, starting at the Alpine fault, to better understand the uplift age and structure of the continental collision. The results show the following: (1) Uplift was underway at 7 Ma in the south at Haast Pass and by 5 Ma in the north at Arthur's Pass. (2) Profiles of the total amount of late Cenozoic uplift show an exponential increase toward the Alpine fault. Adjacent to the Alpine fault there has been 14.5 to 21.5 km of uplift in the vicinity of Haast Pass, 14.5 to 21 km near Mt Cook, and 12.5 to 15.5 km in the Arthur's Pass transect. Adjacent to the Alpine fault in the Mount Cook region the average uplift rate from ∼5.5 Ma to ∼1 Ma was 2.3 to 2.5 mm/yr; between ∼1 Ma and the present it has averaged 6.2 mm/yr. The strongly asymmetric uplift pattern is responsible for the exposure of a 13–25 km wide belt of greenschist to midamphibolite facies rocks immediately east of the Alpine fault known as the Alpine schists. (3) Late Cenozoic uplift of Torlesse greywackes and Alpine schists was preceeded during the Jurassic-early Cretaceous by 5–8 km of uplift. The Otago schist belt to the south was uplifted much more during this interval, and in addition, western parts of the Otago schists were uplifted 5–6 km during the late Cretaceous. (4) The fission track data do not support the notion of significant late Cenozoic shear heating about the Alpine fault, as previously suggested from spatial variations in K-Ar ages of the Otago and Alpine schists.

Are systematic variations in thrust belt style related to plate boundary processes? (The western Alps versus the Carpathians)

Abstract: Within the Mediterranean region, Cenozoic deformation of the Western Alps and the West to East Carpathians has resulted in two different styles of foreland fold and thrust belt. The most prominent difference between the two belts is the presence (Carpathians) or absence (Western Alps) of contemporaneous back-arc extension, but other important differences in structure, topography and metamorphism also exist. These differences in thrust belt style developed mainly during the final stages of thrust belt evolution and appear to reflect fundamental differences in the tectonic settings of the Western Alps and the Carpathians in middle and late Cenozoic time. In particular, they appear to be the result of convergence that is in the first case driven primarily by major plate motions and in the second case only by local motions of small lithospheric flakes or fragments. We suggest that the structural styles developed in these two mountain belts may be useful in identifying mountain belts that have evolved in similar tectonic settings elsewhere in the world. In this respect, the Western Alps and the Carpathians can be regarded as typical examples of two different styles of foreland fold and thrust belt (or more properly as end-member examples within a broad spectrum of foreland fold and thrust belt styles). We propose that continental subduction zones and orogenic belts can be loosely divided into segments that show no major back-arc extensional deformation adjacent to the belt (the Western Alps) and segments that exhibit back-arc extension contemporaneously with thrusting (the Carpathians). The former are found in areas where the rate of overall plate convergence exceeds the rate of subduction, and are commonly typified by extensive involvement of crystalline basement in thrusting, exposure of high grade metamorphic rocks at the surface, high topographic elevation, and large amounts of erosion (tens of kilometers). The latter are found in areas where the rate of subduction exceeds the rate of overall plate convergence and are commonly typified by thrust belts with little to no involvement of crystalline basement in thrusting, low grade to no metamorphism, low topographic elevation, little erosion and, in some instances, an anomalously deep foredeep basin system.

Origin of regional, rooted low-angle normal faults: A mechanical model and its tectonic implications

Abstract: Rooted listric low-angle normal faults (< 20°) of regional extent have been recognized widely in the past few years in the North American Cordillera and elsewhere. The low-angle geometry of these crustal-scale normal faults conflicts with Anderson's [1942] classic theory of faulting. In that theory the orientations of principal stresses are assumed to be vertical and horizontal; the predicted dip angle of normal faults is about 60° rather than 20° or less. Recent geological and geophysical studies in the mid-Tertiary extensional terrane of southeastern California and western Arizona suggest that thick mylonitic gneisses in the lower plates of low-angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust. Directed ductile flow, possibly related to the gravitational spreading of thickened lower crust, may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust. Thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface. A simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust. This model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered. In particular, appropriate stress conditions for the formation of regional low-angle normal faults (< 20°) can be produced by the superposition of two stress fields: a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant. This superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust. These results may explain both puzzling parallel belts of extension and compression and the occurrence of major low-angle normal faults in some orogenic systems.

Subduction of a Late Cretaceous Seamount of the Louisville Ridge at the Tonga Trench: A model of normal and accelerated tectonic erosion

Abstract: The hotspot-generated Louisville Ridge is a 4000-km chain of seamounts (typically 2–2.5 km high and 10–40 km in diameter) and an underlying crustal swell (1.5 km high and 100+ km wide) trending NNW across the southwestern Pacific. The northwest end of the Ridge collides with the north trending Tonga Trench (26°S) which, just north of that point, is exceptionally deep (10.8 km) and lacks both a turbidite wedge and a bordering accretionary complex. The collision zone is moving rapidly southward. Multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2–3 km beneath the lower western trench slope, which is interpreted as the flat summit of a subducted guyot, Motuku, of the Louisville chain. Projected eastward, the summit plain passes 1–2 km above the trench floor. Dredging of the nearby inner trench wall recovered uppermost Cretaceous (Maestrichtian) oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted Louisville seamount The principal effects of hotspot-ridge collision with a sediment-starved trench are (1) the impacting seamounts are subducted rather than accreted, and (2) although some seamount rocks are temporarily accreted, the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km/m.y. Accelerated tectonic erosion is related to (1) fracturing, shearing and general weakening of arc substrate rocks as they are lifted by the swell, penetrated by impacting seamounts, and left to collapse as the ridge moves away, (2) a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell, (3) a possible elevation of the subduction decollement to account for the removal of as much as 30,000 km³ of material from a 400 km sector of the trench in 1 million years, and (4) a reduction in supply of arc-derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge. "Normal" tectonic erosion in the Tonga Trench is apparently minor, and we conclude that the bulk of the ∼37,000 km³ of material which fills subducting grabens each million years is arc-derived volcanic and pelagic sediment.

Late Cenozoic tectonism and landscape development in the foreland of the Andes: Northern Sierras Pampeanas (26°–28°S), Argentina

Abstract: The Argentine Sierras Pampeanas are reverse fault-bounded mountain blocks of Precambrian to Paleozoic basement rocks in the foreland of the central Andes. Uplift in the northernmost Sierras Pampeanas fault blocks of Sierra de Quilmes, Sierra Cumbres Calchaquies, and Sierra Aconquija started about 7 Ma and became pronounced between 4 and 3.4 Ma. The movements culminated after 2.9 Ma, when the conformable Mio/Pliocene Santa Maria Group was overthrusted, faulted, and folded in the course of principal basement uplift. These movements created climate and base level conditions that resulted in the formation of five pediment levels in the deformed basin strata between 2.5 and 0.3 Ma. Tectonically induced base level changes in the piedmont resulted in three tectonism-related pediment surfaces between 2.5 and 0.6 Ma, which attest to the neotectonic activity in the Andean foreland. The chronology of young tectonic uplift contradicts models in which early uplift between 20 and 13 Ma is due to the interaction between high topography on the subducting Nazca Plate and the South American craton. In conjunction with the principal uplift of the adjacent Puna Plateau the northernmost Sierras Pampeanas are better explained by E-W compressional stresses superposed on a thinned lithosphere and inherited zones of structural weakness that facilitated uplift.

The anatomy of a pull‐apart basin: Seismic reflection observations of the Dead Sea Basin

Abstract: The Dead Sea Basin (DSB), located along the African-Arabian plate boundary, constitutes a good example of a pull-apart basin because of its large dimensions, its structural simplicity, and its active subsidence. A coherent three-dimensional picture of the DSB has been constructed on the basis of analysis of seismic stratigraphy together with the interpretation of previously published geological and geophysical data. Despite the large known vertical offsets across the basin, deformation takes place mainly along the transverse and longitudinal faults, and the intervening sediments are relatively undeformed and are hardly tilted. Comparison between E–W seismic lines indicate that the basin has widened by the collapse and tilting of arcuate blocks from the western margin but that its original shape is a full-graben. The southern and central parts of the basin are divided into equidistant segments 20–30 km long by transverse faults. Activity along these faults commenced only during the Pleistocene, long after the Dead Sea strike-slip fault system was formed, and migrated northward with time. A likely scenario for the development of the DSB is one in which the basin grows northward with time by a simultaneous propagation of the southern strand of the Dead Sea fault and a retardation of the northern strand.

Crustal‐scale thrusting and extension in the Hercynian Schwarzwald and Vosges, central Europe

Abstract: The Hercynian outcrop areas in the Schwarzwald of SW Germany and Vosges of easternmost France consist from north to south of three major lithotectonic complexes: very low- to medium grade metamorphics of the Saxothuringian Belt, the polymetamorphic Central Gneiss Complex, and the tectonic melange of the Southern Complex. Juxtaposition of these three complexes occurred by oblique convergent motions in early Carboniferous time. The Saxothuringian Belt is part of a major SE dipping accretionary wedge that consists of lower Paleozoic meta-sedimentary and metaigneous rocks and superimposed upper Devonian to lower Carboniferous synorogenic volcanics and elastics. The Central Gneiss Complex consists of polymetamorphic paragneisses, orthogneisses, metabasites, and meta-ultramafics dipping to the NW. The Southern Complex consists of an imbricated melange of north to NW dipping synorogenic elastics, volcanics, and blocks of ultramafics and gneissic basement. The Central Gneiss Complex (CGC) experienced pervasive Hercynian low-pressure/high-temperature metamorphisrn before it was emplaced against and thrust over the sedimentary and volcanic fill of the adjacent synorogenic basins. This convergence (340–330 Ma) produced mylonitic-cataclastic shear zones in the CGC. The most significant shear zones are the SE verging Todtnau fault zone between the CGC and the Southern Complex and the NW verging Lubine fault zone between the Central Gneiss Complex and the rocks of the Saxothuringian Belt. These two fault zones enclose a bivergent “pop-up” of the polymetamorphic basement. The Todtnau fault zone probably extends for more than 100 km along strike and can be traced to a depth of 10 km by deep seismic reflection studies. Detachment probably occurred along a mid crustal zone of anatexis expressed by a domain of low seismic reflectivity between about 10 and 15 km. Crustal thickening during early Carboniferous convergence led to an extensional phase of crustal reordering (between about 330 and 280 Ma) that was accompanied by the massive rise of “posttectonic” granitic plutons from middle to high crustal levels. Simultaneous creation of a structural “lamination” in the refractory lower crust neglected the main convergent tectonic boundaries; it produced a basin-and-range type lower crust and mantle-crust transition in early Permian time and involved a major advective input of heat.

Proterozoic contraction/extension tectonics of the southern SÃO Francisco Region, Minas Gerais, Brazil

Abstract: The southern Sao Francisco region comprises the southern portion of the Sao Francisco craton, which is underlain by Archean basement, and its fringing Proterozoic orogenic belts. Included in this region are three geologic provinces: the Quadrilatero Ferrifero (QF), the Sao Francisco Basin, and the Cordilheira do Espinhaco. We synthesize preliminary results of ongoing structural analysis, reconnaissance field studies, and published reports to provide a concise review of the geology of the region and to propose a working tectonic model for its Proterozoic evolution. Interpretation of superimposed folds, faults, and foliations leads to the conclusion that the southern Sao Francisco region records the effects of four principal tectonic events that occurred subsequent to deposition of the Lower Proterozoic Minas Supergroup. The first event resulted in formation of a northwest-verging fold-thrust belt. The second event led to development of high-angle reverse faults, open folds, and basement uplift. The third event is manifested in the QF by the formation of normal faults and by the intrusion of mafic dikes; these extensional structures may be associated with formation of the sedimentary basins east and south of the Sao Francisco craton. The final event resulted in formation of a west-verging fold-thrust belt that affected the QF, the Cordilheira Espinhaco, and the Sao Francisco basin. The events that we describe can be tentatively associated with regionally recognized orogenic periods (Trans-Amazonian, Uruacuano, Brasiliano).

Evolution and assembly of Archean Gneiss Terranes in the Godthåbsfjord Region, southern west Greenland: Structural, metamorphic, and isotopic evidence

Abstract: The Godthabsfjord region of southern West Greenland comprises several terranes that were assembled between 2750 and 2550 Ma and folded during amphibolite facies metamorphism. The terranes, which are dominated by gneisses of different ages and show different preassembly metamorphic and structural histories, are (1) the Faeringehavn terrane containing the 3820–3600 Ma Amitsoq gneisses, with granulite facies metamorphism at circa 3600 Ma, (2) the Akia terrane containing the 3070–2940 Ma Nuk gneisses, with granulite-amphibolite facies metamorphism at circa 2980 Ma, (3) the Tasiusarsuaq terrane containing circa 2900 Ma gneisses, with granulite facies metamorphism at circa 2800 Ma, and (4) the Tre Brodre terrane containing the 2800–2750 Ma Ikkattoq gneisses, with amphibolite facies metamorphism at 2800–2750 Ma. Metamorphic assemblages and structures formed prior to terrane assembly, were variably overprinted during amphibolite facies metamorphism and heterogeneous strain associated with assembly. Recognition of the region as consisting of several terranes indicates that the anatomy of some Archean high-grade gneiss complexes may resemble that of orogens of Proterozoic and Phanerozoic age that formed as a consequence of plate tectonic processes.

Pre‐Eocene Synmetamorphic Structure in the Mindoro‐Romblon‐Palawan Area, West Philippines, and implications for the history of southeast Asia

Abstract: The structure of the pre-Eocene rocks, considered as the "basement" of the Philippines, has been investigated in the Mindoro-Lubang, Romblon-Tablas-Sibuyan, and North Palawan areas. In the former two areas the same pre-Eocene succession of units is recognized from top to bottom: (1) a pre-Eocene olistostrome; (2) an ophiolitic nappe; (3) a schistose sequence (pelites, sandstones, basic schists and marbles); and (4) a gneissic unit. The nature of the contact between the olistostrome and the underlying units is unclear, but the ophiolite and the schistose sequence form two thrust sheets of oceanic origin thrust upon the gneissic unit that is considered a part of a continental basement called the West Philippines Block. Small-scale structures show that the early deformation stage is characterized by a submeridian (0°-N40°E) lineation formed in greenschist to amphibolite facies conditions during the thrusting. Kinematic analysis show that the thrusting was from north to south. In North Palawan, metamorphic rocks with similar microtectonic and kinematic characteristics are found. They are overlain by a Late Jurassic olistostrome which is correlated with the olistostromes found in Calamian, North Mindoro, Carabao, and Buruanga peninsula (North Panay). The microstructural features and the presence of the olistostrome suggest that the North Palawan, Mindoro, Tablas, Romblon, Sibuyan, and Carabao islands belong to the same North Palawan block of Hamilton (1979) which is a continental fragment rifted from Asia in Cenozoic times. It is assumed that the Western Panay and Zamboanga areas, which are characterized by Mesozoic ophiolites and metamorphic rocks, also belong to the North Palawan Block. All these islands experienced, to some extent, the same Mesozoic geohistory: the south verging thrusting is interpreted as the result of an oblique collision of the West Philippines Block with Asia. The contemporaneous left-lateral strike-slip faulting and the calc-alkaline magmatism widespread along the Chinese margin are also included in the geodynamic model.

Correlations between Nazca/Farallon Plate kinematics and forearc basin evolution in Ecuador

Abstract: The Tertiary evolution of the forearc basins of Ecuador shows a close correlation between the changing convergence rate of the Farallon, and later Nazca, oceanic plates and continental South America. The correlation occurs during the subduction of a relatively young slab and, in the Late Miocene, onset of the subduction of the Carnegie aseismic ridge. The Ecuador forearc basins lie on a basement of oceanic crust known as the Pinon terrane. The accretion of this terrance occurred in the Paleocene as the leading edge of the Farallon plate, the Macuchi island arc, collided with South America. In the Middle Eocene this forearc terrane was the site of major pull-apart basin formation and turbiditic sedimentation, coincident with a phase of very rapid convergence between chron 21 and chron 13 (48–37 Ma). This deformation was bounded by the trench and a major dextral strike-slip fault zone and resulted in the northward translation of the forearc with respect to continental South America. During the Oligocene a phase of extension normal to the trend of the active margin occurred, coincident with a phase of relatively slow convergence (chron 13 to chron 6, 37–20 Ma). This extension was followed in the Middle Miocene by inversion of most of the forearc basins, coincident with a return to relatively fast convergence from chron 6 (20 Ma) to the present day. Subduction of the Carnegie aseismic ridge occurred during this period (circa 8 Ma to present) and may have enhanced the compressive event. Further, northward translation of the forearc silver accompanied this later deformation. The relationships outlined for the forearc may be modeled in terms of a dynamic orogenic wedge which responds directly to changes in convergence rate at the subduction zone. The convergence rate appears to be an important control on the coupling between the downgoing slab and overriding continental plate.

Isotopic studies bearing on the tectonics of the West Junggar Region, Xinjiang, China

Abstract: Isotopic investigations from the Hercynian-age fold belt between the Kazakhstan and Siberian cratons in the West Junggar region determine the timing of tectonic evolution and the petrogenesis of the granitic rocks of the region. Sphene from the leucogabbro phase of the Tangbale ophiolite melange, the oldest member of the ophiolite sequences in the fold belt, yields an isotopic Pb-Pb age of 523.2±7.2 Ma. Zircon from a postcollision alkali granite yields slightly discordant isotopic U-Pb ages that indicate magma crystallization at 321.4±6.7 Ma, dating it in the Lower Carboniferous period. Radiometric dating thus documents a time span of circa 200 Ma for igneous activity in the area. Petrogenetic studies were made to test whether Precambrian crustal rocks might underlie the Junggar sedimentary basin. Initial lead isotope ratios determined from potassium feldspars from five alkali granites show clear affinity with ratios from mid-ocean ridge basalts in Pb isotope correlation diagrams. Sm-Nd data from sphene and apatite from one of the granites yield an initial eNd(T)=+6.1. The granite sources are depleted mantle rocks of oceanic affinity that show no involvement of recycled aged granitic crustal rocks.

Extension, detachments, and sedimentation in continental rifts (with particular reference to East Africa)

Abstract: Plots of extension versus maximum synrift sediment thickness for rifts display a wide range of values. Some rifts display almost equal amounts of vertical displacement and horizontal extension, while others display considerably more extension than vertical displacement. The former appear to be associated with deep depths to detachment and/or steeply dipping boundary faults, while the latter tend to correspond with shallower depths to detachment and/or more gently dipping boundary faults. Boundary fault shape is inferred to exert an important control on relative amounts of extension and maximum sediment thickness and in sedimentation style. The deep anoxic lakes of the Western Branch, East African Rift system are associated with relatively steeply (40°–70°) dipping faults and depths to detachment ranging between 15 and 30 km. The more coarse clastic-dominated Miocene half grabens in Northern Kenya are associated with relatively gently dipping boundary faults (50°–20°) and shallower depths to detachment (less than 15 km).

Blueschists in major suture zones of China

Abstract: Blueschist facies rocks and C-type eclogites occur as allochthonous terranes or as small tectonic blocks in almost all the accretionary fold belts and major suture zones within the interior of China and along the Circum-Pacific and Tethys-Himalayan belts. At least forty blueschist localities have been described. Blueschists around the Pacific margins and the Himalaya-Alpine belts are mostly Mesozoic in age; only those in Taiwan and New Caledonia are Cenozoic. Intracratonal blueschists of China are older than Late Permian, when the collision-amalgamation presumably occurred. Several alleged Precambrian blueschist localities [Ye, 1987] are shown in the recently published Metamorphic Map of China. A Precambrian(?) blueschist belt in Anhui-Hubei (central China) extends for about 2300 km. However, except for the pre-Sinian blueschist terrane in Aksu, the age determination for Precambrian blueschist metamorphism elsewhere has not been confirmed. Characteristic features of Chinese blueschists include the following: (1) blueschist facies metamorphism predates the continental collision-amalgamation, (2) most blueschists have undergone multistage metamorphism, commonly with increasing temperature and/or decreasing pressure for later events, (3) assemblages include sodic amphibole + epidote + albite + quartz + phengite + chlorite + sphene; lawsonite, jadeitic pyroxene, and aragonite are not common, and (4) protoliths are mainly mafic, pelagic, and clastic rocks. Blueschist facies metamorphism in China may be divided into two types on the basis of the imposed geothermal gradient, hence the progressive change in mineral assemblage. The rare lawsonite-bearing blueschists produced at lower temperatures and higher pressures are confined to the Inner Mongolia and Yarlung Zangbo suture zones where aragonite and possibly jadeitic pyroxene + quartz occur, and the blueschists are associated with subgreenschist facies rocks. Most other blueschists of China formed at higher temperatures along the transitional blueschist-greenschist facies boundary where epidote, sodic, and calcic amphiboles are ubiquitous, and the blueschists are interlayered with greenschist facies rocks. The common occurrence of intracratonal blueschists in major suture zones of China and elsewhere in Eurasia indicates pre-Mesozoic subduction-accretion of oceanic crust and flysch sediments. These zones of accretion represent growth around the periphery of major Precambrian cratons. This fact, together with the distribution of ophiolite belts and available paleomagnetic data, provides evidence for an extremely mobile history of plate movement in Eurasia. However, the Paleozoic intracontinental blueschists and ophiolites of China have formed before, rather than during, final closure between cratons. They do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction, underplating, and imbrication of oceanic materials similar to the accretion history of westernmost North America.

The Triple Junction of the North America, Cocos, and Caribbean Plates: Seismicity and tectonics

Abstract: The triple junction of the North America, Cocos, and Caribbean plates is ambiguously defined, mainly because the North America-Caribbean plate boundary does not clearly continue beyond its known surface trace (the Motagua fault zone) in western Guatemala to intersect the Middle America trench. Well-located regional shallow earthquakes (h≤70 km) show that there is no intermediate or large-magnitude seismic activity associated with a presumed extension of the North America-Caribbean plate boundary to the west, beyond its well-defined surface trace. There is, however, a clear zone of shallow seismic activity from the western section of the fault system through southern Mexico. Fault plane solutions for these events indicate a left-lateral strike-slip displacement, which is in good agreement with surface faulting. We suggest that these strike-slip faults, together with the Salina Cruz fault in the isthmus of Tehuantepec, mark the boundaries of a broad zone of deformation in southern Mexico and northern central America which takes up the interactions of the three plates. In this sense, no single point constitutes the triple junction. The geologic record suggests that the Motagua fault zone developed because the westernmost portion of the Caribbean plate became locked against North America.

Brittle and ductile deformation in a zone of rapid uplift: Central Southern Alps, New Zealand

Abstract: Schists exposed in the central Southern Alps, New Zealand, 1–5 km east of the Alpine fault zone, have been rapidly uplifted during the late Cenozoic. A regionally consistent sequence of mesoscopic structures discordant to the schistosity is recognized extending backward in time from recent brittle displacements of deglaciated surfaces through structures exhibiting brittle-ductile transitional behaviour to ductile deformational features. Kinematic analysis of these structures gives consistent principal subhorizontal shortening directions similar to the present day principal horizontal shortening direction, indicating their relation to late Cenozoic uplift of the Southern Alps. Analysis of deformed veins suggests a minimum shortening strain of about 50% perpendicular to the foliation. The veins crosscut upright mesoscopic to macroscopic folds which commonly develop high-strain zones on their limbs. Within such zones a strong stretching lineation plunges gently SW, approximately perpendicular to the stretching lineation in the mylonites along the Alpine fault. The folds and high-strain zones do not appear to be related to late Cenozoic uplift but may have originated during an earlier phase of dominantly strike-slip motion. Fluid inclusion studies give depth-temperature estimates of around 3 km and 285°C for the development of the brittle structures and 6–8 km at 310°–350°C for the brittle-ductile transition. The deformed veins show evidence of extensive fluid infiltration during the later phases; data from fluid inclusions and metamorphic assemblages give an estimate of 15–20 km and 400°–450°C for their deformation. These data, combined with uplift rates determined by other studies, allow the construction of a depth-temperature-time path which indicates nearly isothermal decompression associated with rapid uplift. A shallow brittle-ductile transition (6–8 km) is consistent with numerical modeling of thermal and mechanical behavior of the crust during rapid uplift associated with continental collision and with the presence of high heat flow in the area. Because of the small temperature change over a large amount of decompression, pressure may be an important factor in controlling the depth of the brittle-ductile transition. The marked temperature drop above the brittle-ductile transition is thought to be enhanced by the influence of convection as an effective cooling mechanism.

The Mojave Extensional Belt of southern California

Abstract: It is proposed that the regional pattern of early Miocene low- and high-angle normal faulting and dike swarm emplacement in the western and central Mojave Desert is the result of regional extension developed within a roughly east trending zone, named here the “Mojave Extensional Belt”. This zone of extension, although now disrupted by late Cenozoic right-slip faulting, can be traced from the intersection of the San Andreas and Garlock faults to the eastern Mojave Desert near Bristol Lake and the Granite Mountains fault. Restoration of post-Oligocene movement along the San Andreas fault system places the now buried western part of the Mojave Extensional Belt opposite a similar age-extended area of west central California. Surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the Mojave Extensional Belt was low-angle, normal sense, simple shear (detachment faulting). Strain within the Mojave Extensional Belt is partitioned between four domains (Edwards, Waterman, Daggett, and Bullion terranes) that each consist of one or more half-grabens. Each half-graben is composed of tilted, normal fault-bounded blocks that lie above a rooted, low-angle, brittle-ductile normal sense shear zone. Tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping (∼80°) planar to slightly curviplanar normal faults. At least one generation of high-angle faults postdates the formation of the detachments. Differential extension between the domains was accommodated by strike-slip faults (transfer zones). Similarly, the lack of early Miocene extensional elements north of the western segment of the Garlock fault suggests that the Garlock may have been active in the early Miocene and served as the northern transfer boundary of the Mojave Extensional Belt. The Mojave Extensional Belt initially began to open about 22 m.y. ago, with the major phase of extension occurring between ∼22 and 20 Ma. Local, high-angle, normal faults cut older detachments and were active between ∼20 and 17 Ma. Kinematic indicators suggest that the western part of the Mojave Extensional Belt opened in a NE–SW direction, whereas the eastern portion extended in an ENE–WSW direction (present-day reference frame). Rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the Mojave Extensional Belt was originally oriented ∼N–S and was rotated as much as 50° (clockwise) between ∼20 and 18 Ma (Early Miocene).