bluuebunny/crossref_metadata_2025 huggingface dataset:
Late‐Cretaceous allochthons and post‐Cretaceous strike‐slip displacement along the Cuilco‐Chixoy‐Polochic Fault, Guatemala
Authors: Anderson Thomas H. , Erdlac Richard J. , Sandstrom Melissa A.
Published: February 2008

The disposition of allochthonous masses, emplaced in late Cretaceous to Early Tertiary time, along the Cuilco‐Chixoy‐Polochic strike‐slip fault zone in western Guatemala, constrains the amount of Tertiary displacement. The absence of major disruption of the allochthons argues against lateral displacements of hundreds of kilometers during the Tertiary. In westernmost Guatemala, along the flank of the Cuilco‐Chixoy‐Polochic fault, granite, volcaniclastic and volcanic rocks, minor carbonate and phyllite comprise a sequence of south‐dipping allochthons. These tilted slabs lie against almost horizontal Jurassic and Cretaceous beds to the north. Field relations indicate that the slides were emplaced across the Cuilco‐Chixoy‐Polochic fault from south to north probably during late Cretaceous or early Tertiary time. Other fault‐bounded masses, which crop out tens of kilometers eastward, near San Sebastián Huehuetenango, probably were emplaced contemporaneously with those to the west. Locally, these allochthons lie astride the active trace of the Cuilco‐Chixoy‐Polochic fault along which offset streams record about 1 km of left‐lateral motion. Total displacement recorded by the apparent offset of allochthonous sheets is no more than several kilometers. Farther east, along much of the Cuilco‐Chixoy‐Polochic fault zone, a throughgoing strike‐slip trace has not been identified. We postulate that the pre‐Tertiary tectonic record is obscured by an extensive series of allochthons composed of regional stratigraphic suites emplaced during late Cretaceous time. In western Guatemala stratigraphic relationships between serpentinized peridotite and carbonate beds of late Cretaceous age indicate Turonian as a maximum age for initial emplacement of allochthonous material. Regional relationships indicate that deformation culminated during latest Cretaceous ‐ earliest Tertiary time. The emplacement of thrust sheets along the Cuilco‐Chixoy‐Polochic fault is a likely product of contemporaneous Cretaceous convergence along the Motagua zone to the south. Collision of the Chortis terrane against southern Yucatan resulted in crustal shortening, uplift, and consequent northward‐directed emplacement of slides. Allochthons, composed of ultramafic rocks, sedimentary cover and, less commonly, granitic rocks, were probably derived from the suture zone and the uplifted region between the Cuilco‐Chixoy‐Polochic and Motagua faults.
The Najd Fault System, Saudi Arabia and Egypt: A Late Precambrian rift‐related transform system?
Authors: Stern Robert J.
Published: February 2008

The Najd Fault System is a complex set of left‐lateral strike‐slip faults and ductile shear zones that strike NW‐SE across the Precambrian of Arabia and Egypt. This system was developed during the interval 540–620 Ma. It is up to 400 km wide with an exposed length of 1100 km; inferred buried extensions of the Najd give it a total length of 2000 km. It is the best exposed and may be the largest pre‐Mesozoic zone of transcurrent faulting on earth. Previous models for the Najd Fault System suggest it formed as a result of a major Late Precambrian continent‐continent collision. This model is not preferred here because (1) the lack of evidence for a pre‐Late Precambrian continent to the east of the Najd Fault System; (2) the difference between the orientation of the Najd Fault System and that predicted by slip‐line theory; (3) the younger age of Najd movements compared with that of collisional sutures in the Arabian Shield; and (4) lack of evidence for wide‐spread crustal uplift that would be expected to accompany collision. A new model for the origin of the Najd Fault System accounts for each of these objections: The Najd Fault System formed in response to a broad zone of NW‐SE directed crustal extension that accompanied juvenile continental crustal formation in northernmost Afro‐Arabia. This model also accounts for the following observations: (1) Strands of the Najd parallel the direction of extension in the North Eastern Desert of Egypt and Sinai; (2) the timing of the principal rifting movements (ca. 575–600 Ma) overlap with those of the Najd (ca. 560–620 Ma); (3) in spite of observation (2), the Najd Fault System is not recognized in northernmost Afro‐Arabia; instead the Najd deformation becomes increasingly ductile and these zones are more commonly intruded by sheared and foliated granites as the principal zone of extension is approached. The Najd Fault System thus represents a set of continental transforms developed in response to a major episode of Late Precambrian extensional continental crust formation in northernmost Afro‐Arabia.
Tectonic evolution of the Meatiq infrastructure, Central Eastern Desert, Egypt
Authors: Habib Mohamed E. , Ahmed A. A. , El Nady Osman M.
Published: February 2008

The Pan‐African basement exposed in the Meatiq area, Central Eastern Desert, Egypt, was found to possess tectonic styles belonging to infrastructure, transition and super‐structure levels. Tectonic style of the infrastructure was developed during an old orogeny in the Lower Pan‐African, and overprinted by those of the transition and superstructure levels of a younger orogeny in the Middle and Upper Pan‐African, respectively. In the Lower Pan‐African, old sequences of sediments and volcanics, together with ophiolitic rocks, were intensely deformed and regionally highly metamorphosed into gneisses and amphibolites. In the infrastructure level such rocks were migmatized, plastically deformed and intruded by synkinematic granitoid (diorite to alkali granite) lenses, sheets and plutons which were probably crystallized as amphibolite‐facies gneisses. Later on, these infrastructural rocks were cut by mafic to intermediate dykes and sills, isostically uplifted and cratonized. Parts were exposed to erosion. In the Middle Pan‐African the cratonized rocks sank into a transition tectonic level where they underwent large‐scale thrusting, varying degrees of cataclasis and mylonitization, and low‐ to medium‐grade regional metamorphism. An episode of plutonism guided by thrusts followed, mainly in the form of synkinematic intrusion of tonalite, granodiorite and quartz monzodiorite. In the Upper Pan‐African, isostatic uplift occurred and the mylonitic rocks attained the superstructure level. There they were subjected to two main phases of folding resulting in the major Meatiq domal structure. Subsequently, the folded rocks were intruded by a postkinematic quartz monzonite pluton.
Rotation of the Colorado Plateau: An analysis of paleomagnetic data
Authors: Bryan Phillip , Gordon Richard G.
Published: February 2008

Hamilton (1981) has proposed that the Colorado Plateau has rotated clockwise relative to cratonic North America by a total of 5°–7° since mid‐Cretaceous time. A rotation this small is unresolvable by most paleomagnetic studies. In this paper we present a new method for simultaneously comparing all of the available paleomagnetic data from on and off the plateau in order to estimate rotation of the Colorado Plateau. Our new method examines the effect of various hypothetical plateau rotations on the dispersion of data fit to a smooth model of apparent polar wander. A maximum likelihood criterion is used to estimate the amount of rotation. Confidence limits are found using an F ratio test. Assuming the plateau‐craton Euler pole to be located at 37°N, 103°W, we applied this method to North American paleomagnetic data of Carboniferous to Cretaceous age. The best paleomagnetic estimate of the rotation is 3.9° clockwise (with a 95% confidence interval of 1.4°–6.6° clockwise), in good agreement with geologic estimates of rotation. Because of the dependence of published North American apparent polar wander paths on data from the Colorado Plateau, the existence of such a rotation requires a minor revision in the apparent polar wander path, the largest change being a 3.9° eastward shift of the cusp connecting the Carboniferous‐Permian‐Triassic and Jurassic‐Cretaceous tracks.
The Late Jurassic oblique collisional orogen of SW Japan. New structural data and synthesis
Authors: Faure Michel , Caridroit Martial , Charvet Jacques
Published: February 2008

The structural configuration of SW Japan mainly reflects a late Jurassic‐early Cretaceous orogeny. The region is divided into an inner belt and an outer belt, on the Japan sea and Pacific ocean sides respectively, by a strike‐slip fault, the Median Tectonic Line (MTL). Both consist of a series of stacked nappes. The inner belt is divided into a Jurassic olistostrome known as the Tanba zone and a hinterland area comprising continental Triassic‐Jurassic sediments. The Tanba zone is sliced into two units: a lower one with late Jurassic matrix and Triassic‐early Jurassic radiolarite olistoliths, tectonically overthrust by an upper unit comprising an olistostrome with middle Jurassic matrix and blocks which include late Paleozoic limestone, basic lava, and radiolarite. The Tanba zone is overthrust by a Paleozoic nappe complex derived from the hinterland. The basal sole of the nappe corresponds to a peculiar unit called the ultra‐Tanba zone. In the Chugoku area, the hinterland is divided into an upper nappe: the Oga nappe, formed by Permo‐Carboniferous limestone and Permian clastic rocks and a lower one: the Sangun‐Maizuru nappe, formed by Paleozoic high pressure (HP) metamorphics (the Sangun schists), the Permian Maizuru olistostrome, and the dismembered Paleozoic Yakuno ophiolites. In the northern part of SW Japan, the Tanba zone is in faulted contact with the circum‐Hida and the Hida zones. The former is interpreted as the equivalent of the Oga and Sangun‐Maizuru nappes of the Chugoku domain crushed by post Cretaceous tectonics. The latter consists of Paleozoic high temperature (HT) metamorphic rocks and late Triassic‐early Jurassic granite, locally mylonitized and covered by early Jurassic sandstone. The outer belt is formed by a superficial nappe similar to the Tanba zone thrust upon a "deep domain" characterized by a synmetamorphic ductile deformation. The "deep domain" is divided into a lower unit, the Oboke unit formed by continental derived arenites and a Green Schist nappe consisting of oceanic sediments and resedimented ophiolites.The Green Schist nappe overthrusts the Oboke unit under synmetamorphic conditions with an eastward displacement. The two belts are separated by the Ryoke zone which corresponds to the southern part of the Tanba zone affected by a Cretaceous HT metamorphism and sharply cut by the MTL. A geodynamic model is proposed for the Jurassic orogeny of SW Japan assuming that the evolution of the inner and outer belts are linked. In the Late Triassic‐Early Jurassic SW Japan is an active plate margin. The upper plate or South China block consisted of the hinterland and the Tanba belt, a forearc basin; the lower plate consisted of an oceanic area and the South Japan continent. The basic mechanism of the orogeny is ascribed to the oblique subduction and collision of the South Japan continent.
Temporal variations in plate convergence and eruption rates in the Western Cascades, Oregon
Authors: Verplanck Emily P. , Duncan Robert A.
Published: February 2008

New K‐Ar age determinations on basalts, basaltic andesites, and ash flow tuffs from the central Western Cascades in Oregon range in age from 32 to 3 Ma. The ages decrease from west to east and with increasing elevation. Volcanism has been continuous throughout the evolution of the Western Cascades, with some periods of greater activity: 13–16 Ma, 22–26 Ma and 29–31 Ma. Relative volume estimates indicate that eruption rates decreased by a factor of 6 from 35 Ma to the present. The eruption rates during formation of the Western Cascades Volcanic Arc were influenced by changes in the direction and rate of convergence between the Farallon and North American plates. We have developed a model of the Tertiary convergence of these two plates, based on the mantle‐fixed hotspot reference frame, which indicates a factor of 5 decrease in convergence rate (16.0 to 3.2 cm/a). Clockwise rotation of the western margin of the North American plate led to a decrease in the convergence angle since about 35 Ma. Apparently, slower, more oblique subduction resulted in a decrease in the volume of erupted magmas.
Overthrust emplacement of New Caledonia Ophiolite: Geophysical evidence
Authors: Collot J. Y. , Malahoff A. , Recy J. , Latham G. , Missegue F.
Published: February 2008

Geophysical studies support inferences from outcrop geology that during the Late Eocene an ophiolite sheet exposed on New Caledonia was thrust southward over a rock complex consisting of sedimentary, volcanic, and metamorphic rocks that range in age from Pre‐Permian to Eocene. The outcropping ultramafic complex consists of a layered sequence, approximately 3000 m thick, including harzburgite, dunite, wehrlite, serpentinite, and gabbro. The absence of pillow basalts and sheeted dikes on land suggests that during or after overthrusting, these units were removed either by thrust faulting or by erosion. Seismic refraction profiles, collected over the adjacent Loyalty Basin, show that the lower crust and upper mantle are characterized by velocities of 4.7, 5.8, 6.8, and 8.1–8.4 km/s, suggesting the presence of a complete oceanic section that includes sediment, basalt, gabbro, and peridotite. The lower crust is probably 6.5 km thick beneath the Loyalty Basin. A seismic reflection profile that extends across the Loyalty Basin is interpreted to show the oceanic crust rising toward New Caledonia. We suggest that the ophiolite on New Caledonia is continuous with this rising oceanic crust. Studies of gravity anomalies observed both on land and offshore indicate the presence of a short‐wavelength, high‐amplitude (+180 mGal), asymmetrical, free‐air gravity anomaly along the northeastern coast of New Caledonia. A gravity anomaly profile, calculated for a geological model characterized by a 10‐km‐thick slab of oceanic crust and mantle material extending continuously from the ophiolite on New Caledonia to the oceanic crust of the Loyalty Basin, successfully matches the observed gravity data. We interpret the extension of the high‐gravity anomaly over the whole length of the eastern lagoon as evidence for the lateral extension of the root zone of the ultramafic complex. A tentative geodynamic reconstruction suggests that the proto‐Loyalty Basin crust formed a marginal basin along the eastern margin of Gondwanaland prior to Upper Cretaceous time; after the opening of the New Caledonia Basin during Upper Cretaceous to early Paleocene time a subduction zone developed along the western margin of New Caledonia. While this zone was active, the Loyalty Basin lithosphere overthrust New Caledonia in a southeasterly direction and locally rotated clockwise. This compressional tectonic event was terminated by upper Eocene time.
Relative motion of the Nazca (Farallon) and South American Plates since Late Cretaceous time
Authors: Pardo‐Casas Federico , Molnar Peter
Published: February 2008

By combining reconstructions of the South American and African plates, the African and Antarctic plates, the Antarctic and Pacific plates, and the Pacific and Nazca plates, we calculated the relative positions and history of convergence of the Nazca and South American plates. Despite variations in convergence rates along the Andes, periods of rapid convergence (averaging more than 100 mm/a) between the times of anomalies 21 (49.5 Ma) and 18 (42 Ma) and since anomaly 7 (26 Ma) coincide with two phases of relatively intense tectonic activity in the Peruvian Andes, known as the late Eocene Incaic and Mio‐Pliocene Quechua phases. The periods of relatively slow convergence (50 to 55 ± 30 mm/a at the latitude of Peru and less farther south) between the times of anomalies 30–31 (68.5 Ma) and 21 and between those of anomalies 13 (36 Ma) and 7 correlate with periods during which tectonic activity was relatively quiescent. Thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate.
Extensional shearing deformation and kilometer‐scale “a”‐ type folds in a Cordilleran Metamorphic Core Complex (Raft River Mountains, northwestern Utah)
Authors: Malavieille Jacques
Published: February 2008

Cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations. In the Raft River Mountains a mylonitic zone and a late superimposed detachment fault separate Precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous Paleozoic rocks which suffered a brittle extensive deformation. We suggest that the mylonitic ductile deformation is the result of Tertiary extensional tectonics. Strain analysis shows that (1) strain results from large‐scale progressive shearing deformation in a non‐coaxial regime, (2) E–W regional stretching lineations represent the direction of shearing and transport, (3) shear criteria indicate an east directed movement, (4) strain magnitude is increasing eastward, (5) kilometer‐scale folds parallel to the lineations are interpreted as “a”‐type folds formed by eastward shearing of a previously northward dipping layering. Measurement of striated faults in the upper plate gives an E–W direction of extension parallel to the lineations. Thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics. A model is proposed to explain the observed geometry and kinematics of the structures. The Mesozoic to Paleocene Cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes. Important uplift, abnormal heating, and granite emplacement occurred later during the collapse of the former belt. The combined effects of thermal and gravitational instabilities during Tertiary extensional tectonics enabled the development of a ductile shear zone localized at the Precambrian basement‐Paleozoic cover interface. The brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust.
Attenuation of the Coast Range Ophiolite by extensional faulting, and nature of the Coast Range “Thrust,” California
Authors: Jayko A. S. , Blake M. C. , Harms Tekla
Published: February 2008

The late Mesozoic Coast Range ophiolite and Great Valley sequence in California were juxtaposed against the Franciscan Complex during a long tectonic history that included imbricate thrust faulting, low‐angle detachment, and high‐angle reverse faulting. Many low‐angle faults previously mapped as thrusts invariably juxtapose younger over older rocks, suggesting a normal sense of offset. We infer that serpentinite melange that is present structurally beneath the Coast Range ophiolite formed above the subduction zone during convergence and was subsequently faulted and further attenuated with upper plate rocks concurrent with extension. Franciscan blueschist‐facies rock is inferred to have been transported from depth to higher structural levels concurrent with underplating and extensional unroofing in the upper plate. The present juxta‐position of the Coast Range ophiolite and Great Valley sequence with Franciscan rocks is commonly controlled by Neogene high‐angle faults. We propose that the term Coast Range thrust is no longer appropriate and that the name should be changed to Coast Range fault.
Tectonic interpretation of the morphology of Honduras
Authors: Manton W. I.
Published: February 2008

The morphotectonics of Honduras have been studied using existing geologic and topographic maps and imaging done from satellites, and the following interpretations have been made. During the late Oligocene to early Miocene a wide strike‐slip boundary existed in western Honduras and eastern Guatemala and produced east‐west trending basins containing the coarse clastic rocks of the Subinal Formation. Marine geophysical studies of the continental shelf of Honduras show that during the rest of the Miocene and the early Pliocene the boundary was inactive, but motion resumed in the late Pliocene to Pleistocene and produced horsts on the north coast of Honduras. In northwestern Honduras the east‐northeast trending alignment of the Jocotan, Copan, and Charnelecon river valleys may result from crustal flexuring rather than from faulting, as previously proposed, because the postulated fault (the Jocotan‐Chamelecon fault) cannot be reconciled with available geologic maps. The northwest and northeast flowing rivers of central and eastern Honduras define domains of strike‐slip faulting that have probably rotated 30° from an original set of conjugate fractures. Widespread deposition of conglomerates suggests that strike‐slip movements began in the mid‐Cretaceous and reoccurred at intervals, culminating in the development of pull‐apart basins in western Honduras during the late Pliocene. At the present time, no further strike‐slip movement is possible, as the faults defining each domain lie parallel and perpendicular to the Middle America trench. New conjugate fractures, however, cut Honduras along north‐south and east‐northeast trends and have also separated into domains. In the domain of north‐south fractures, easterly directed, ductile extension of the lithosphere beneath western Honduras and Guatemala is opening up rifts, but the domain of east‐northeast fractures is characterized by strike‐slip motion that has produced left‐lateral pull‐aparts in central Honduras. Early workers identified no rift valley extending from the Gulf of Fonseca to the Caribbean, and there is no fresh evidence upon which to base the existence of one today. The notion of the Honduras Depression, which has persisted over the last half century, stems from misinterpretation of the maps and writings of Karl Sapper.
Orogeny and metamorphism: A model for deformation and pressure‐temperature‐time paths with applications to the central and southern Appalachians
Authors: Jamieson R. A. , Beaumont C.
Published: February 2008

A model is presented for deformation, uplift, and exhumation in convergent orogens based on the following premises: (1) foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen; (2) the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton; on the seaward side of this transition, up to 20 km of overthrusts can be accumulated without creating topography; (3) critical shape theory can be applied to orogens as a whole, allowing predictions to be made concerning dynamic balance between mass addition and loss, uplift, exhumation, and growth of topography; and (4) pressure‐temperature‐time (P‐T‐t) data can be used to reconstruct particle paths within the deforming orogen.

Three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition, as monitored by the foreland. Rapid exhumation of metamorphic terranes is most likely in the ‘steady state’ where erosion balances accretion; a ‘constructive state,’ where mass addition is greater than mass loss, results in crustal thickening and initially little exhumation; a ‘destructive state,’ where mass loss outweighs mass addition, results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension. P‐T‐t data on peak metamorphism, pluton emplacement, uplift, exhumation, and cooling can be used in conjunction with data from the foreland to distinguish among these various states. The topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen. Application of this model to the central and southern Appalachians suggests that during the constructive phases of the Taconian and Acadian orogenies, overthrust loads accumulated mainly on the seaward side of the ocean‐continent transition as marked by the Bouguer gravity gradient. Between the Acadian and Alleghanian orogenies, the Appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton. The constructive stage of the Alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton; this resulted in widespread uplift and westward transport of older metamorphic rocks. Post‐orogenic unroofing of the southern Appalachians is compatible with the predictions of flexural models; the post‐orogenic unroofing history of the central Appalachians and Ouachitas probably reflects Mesozoic extension in these areas.
Episodic North America and Pacific Plate motions
Authors: Pollitz Fred F.
Published: February 2008

The North America plate motion for the period 20–0 m.y. is modeled by two absolute angular velocity vectors, with a change occurring at about 9 m.y. The model incorporates new geologic constraints based on (1) the recognition of a discontinuity in North America plate motion at 9 m.y., (2) a reexamination of the Iceland and Yellowstone hotspot traces, (3) improved delineation of the present North America – Pacific relative motion, and (4) a revised history of Pacific absolute motion. The two‐stage model is consistent with several features of the late Cenozoic geologic history of North America, including geological observations relating to tectonism along a large portion of both the eastern and western boundaries of North America. This plate model, combined with a previously established Pacific plate model, makes specific predictions concerning the 15–0 m.y. history of the North America‐Pacific plate boundary which compare favorably with the known history. These models together resolve most of the discrepancies previously encountered between absolute and relative motion models of the two plates. The implications of the North America plate model are the greatest for those events related to the 9 m.y. change in its motion. Among the geologic consequences of this change are rifting in the western United States, intense compressional deformation along the northern Caribbean plate boundary, a hiatus in Aleutian arc volcanic activity, and a change in the orientation of the Lesser Antilles arc. This picture reinforces the notion that the major plates behave rigidly and transmit stresses over great distances. The predictive power of the model may help in interpreting the history of the North America plate boundary in those areas where clear observations are lacking. An extra force needs to be applied to a plate to cause a change in plate motion. By treating the plate as a rigid spherical shell overlying a uniformly viscous fluid, we derive a simple class of forces sufficient to cause a given change in plate motion. Such force balance considerations indicate that an additional force applied in the northwest Pacific region, toward the northeast, would result in the observed change in North America plate motion. This force is surprisingly similar to that which apparently caused a large change in Pacific plate motion about 5 m.y. later. We propose that a common mantle process is responsible for both changes and is related to the ability of the northwest Pacific subducted lithosphere to enter into the lower mantle.
Structure and metamorphism of the Archean Murchison Belt, Kaapvaal Craton, South Africa
Authors: Vearncombe Julian R.
Published: February 2008

Critical examination of five popular prejudices applied to greenstone belts in general, and the Murchison belt in particular show that (1) the term Murchison schist belt is preferred to that of Murchison greenstone belt, which differs significantly from the Barberton greenstone belt, (2) the Murchison belt is tectonized such that no simple stratigraphy may be erected, (3) the Murchison belt lacks a synclinal keel, (4) heterogeneous oblique‐ to dip‐slip shear characterizes much of the Murchison belt, but strike‐slip deformation occurred in the Letaba shear zone along the northeastern margin of the belt, and (5) contact metamorphism was a limited phenomenon that was not responsible for the highest grade rocks of the Murchison belt. The belt comprises a number of domains, each with its own characteristic lithologies, structural style, and metamorphic history, bounded by tectonic breaks which have juxtaposed rocks from different crustal levels.
Deformation around a rising diapir modeled by creeping flow past a sphere
Authors: Cruden A. R.
Published: February 2008

A new experimental method for determining the pattern of flow and three‐dimensional progressive finite deformation of fluid elements in low Reynolds number (Re) viscous flow has been applied to creeping flow past a rigid sphere falling in a cylinder. This is intended as a model analogue of natural rock deformation associated with magmatic and solid‐state diapirs rising through ductile crust. The kinematics of finite strain in such analogue experiments may give useful information on the development of natural structures. The experiment and a new method for quantifying the progressive finite deformation of fluid volumes in the transparent Newtonian test fluid used (SGM36) are discussed in detail. The progressive finite deformation of fluid flowing along specific streamlines passing the sphere appears to be complex and largely a function of the initial distance of fluid volumes and the particular streamlines which they follow from the flow axis. The progressive deformation history in the material surrounding a rising spherical diapir is characterized by progressive flattening and rotation of material above and progressive stretching below the sphere. The results compare favorably with the external strain pattern associated with post tectonic, mesozonal diapiric intrusions.
Alleghanian Polyphase Deformation of the Hope Valley Shear Zone, southeastern New England
Authors: Getty Stephen R. , Gromet L. Peter
Published: February 2008

Structural, petrographical, and Rb‐Sr isotopic studies in the highly deformed basement rocks of southern Rhode Island have allowed the identification of the southern continuation of the Hope Valley Shear Zone (HVSZ). Previous work recognized the HVSZ as an Alleghanian tectonic boundary separating the distinct late Precambrian Esmond‐Dedham and Hope Valley terranes in southeastern New England. Two important components of the Esmond‐Dedham terrane (the late Precambrian Ponaganset gneiss and the Devonian Scituate granite gneiss) now have been traced with structural continuity into southern Rhode Island. The distribution of these gneissic units and their structures shows that the southern segment of the HVSZ has been strongly deformed, abruptly deflecting it from a NW‐SE orientation to a N‐S trend. The deflection of the HVSZ marks an abrupt structural boundary between two distinct, regionally developed mineral lineation domains: a northwestern domain with north to NNW trends (characteristic of most of the HVSZ) and a southeastern domain with NE trends. Structural relations suggest that all mineral lineations were originally parallel along the HVSZ with a north to NNW trend. The deflection formed during a subsequent phase of ductile, right‐lateral shear, producing a clockwise rotation of both lineations and the HVSZ in southern Rhode Island. Thus basement gneisses in southeastern New England have suffered polyphase Alleghanian deformation at high metamorphic grades. It has been argued that during the Late Paleozoic, southern New England was located in a structural transition zone between convergent tectonics in the central and southern Appalachians and wrench tectonics in the northern Appalachians. The structural link between these two distinct tectonic regimes is an E‐W, right‐lateral transcurrent fault. The late dextral shear deformation described in southern Rhode Island basement rocks appears to have resulted from right‐lateral movement at depth within this transcurrent fault zone.
Constraints on uplift in the Franciscan Subduction Complex from apatite fission track analysis
Authors: Dumitru Trevor A.
Published: February 2008

The Franciscan accretionary prism of California contains large belts of sediments that were subducted to depths of 10 to 30 km, metamorphosed there under high‐pressure, low‐temperature conditions, and later uplifted back to the surface. Reconnaissance apatite fission track data (40 samples) place constraints on the uplift histories of these and related lower‐pressure rocks. Franciscan metagreywackes from a few areas (parts of the Diablo Range, parts of the northern Coast Ranges' Eastern and Central belts) retain imprecise fission track evidence of cooling in the middle or Late Cretaceous(?). Using annealing temperatures of fission tracks in apatite and assuming 9°C/km subduction‐period geothermal gradients (estimated from blueschist mineralogy, paleogeothermal gradients in the Great Valley forearc basin, plate motion reconstructions, and other data), these rocks were uplifted through about the 12 km depth level at that time. Since some of these rocks were probably metamorphosed at depths of 15–25 km, substantial uplift had occurred by the middle or Late Cretaceous(?). Rocks from several areas retain fission track evidence of cooling in the early to middle Tertiary, including the Diablo Range (roughly 32 Ma), San Francisco (about 43 Ma), Chiles Creek in Napa County (about 45 Ma), and parts of the northern Coast Ranges' Eastern and Central belts (early to middle Tertiary). Great Valley forearc basin sandstones in the Clear Lake outlier retain evidence of cooling at roughly 35 Ma. Assuming 8.5°–10.5°C/km gradients, rocks from these areas were buried at least 8–10 km in the early or middle Tertiary and experienced at least a few kilometers of uplift at that time. In the northern Coast Ranges, two areas retain evidence of cooling in the late Tertiary (Coastal belt at roughly 20–10 Ma; Yolla Bolly Mountains in Eastern belt at roughly 20 Ma). Assuming 12°C/km gradients, they were buried at least 7 km in the late Tertiary and were uplifted at least a few kilometers at that time. The data thus suggest in general that uplift was occurring in various areas of the Franciscan forearc more or less continuously thoughout much of the period that subduction was occurring. This progressive uplift is most compatible with models where uplift is driven by synsubduction underplating of subducted sediments at the base of the accretionary prism, because those models predict continuing, long‐term uplift throughout the period of subduction but also allow episodicity in uplift as underplating rates and locations vary.

Modern geothermal gradients in most Franciscan outcrop areas are moderately high (about 28°–35°C/km), a consequence of latest Cenozoic termination of subduction and development of the San Andreas transform margin. Franciscan and Great Valley samples show no fission track evidence of latest Cenozoic exposure to high temperatures (>100°C). Therefore they were only shallowly buried during the postsubduction period, and postsubduction uplift has been relatively minor (maximum of about 3.2 km).
Origin of regional, rooted low‐angle normal faults: A mechanical model and its tectonic implications
Authors: Yin An
Published: February 2008

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.
Continental growth of northwest China
Authors: Coleman Robert G.
Published: February 2008

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.
Tectonic evolution of a part of the Tethyside orogenic collage: The Kargi Massif, northern Turkey
Authors: Tüysüz Okan
Published: February 2008

The central part of the Rhodope‐Pontide fragment, one of the major tectonic units in Turkey, provides critical data for evaluating the Cimmeride and Alpide evolution of the Mediterranean Tethysides. Tectonic events that affected the central part of the Rhodope‐Pontide fragment since the end of the Paleozoic, generated east‐west trending belts with the event of every episode redeforming and partly obliterating the structures of previous episodes. This evolution may be conveniently described in terms of three major episodes: (1) Two different realms of pre‐Dogger oceanic rocks are present in the area. The northern realm coincided with main branch of Paleo‐Tethys that was being actively destroyed by south dipping subduction. The southern realm, the Karakaya ocean, a back arc basin related to this subduction, began opening by rifting of a retroarc carbonate platform during the Permo‐Triassic. To the west a continental domain with sparse magmatism seperated the two oceanic areas. Toward the east the two oceans become united by the wedging out of the continental domain. These two pre‐Dogger oceans closed during the Lias, and their remnants were emplaced between the southern margin of Laurasia and the fragments of the Cimmerian continent. (2) The second episode partly overlapped the first with rifting south of the Cimmerian continent fragment during the Lias. This rifting was followed by a transgression which covered the ruins of the Cimmeride orogenic belt by the Malm. This rifting concurrently led to the development of the northern branch of the Neo‐Tethys and a south facing Atlantic‐type continental margin. A southerly thickening sedimentary prism developed on this margin during the Lias to early Cretaceous interval. (3) The floor of the northern branch of Neo‐Tethys began to be consumed along the north dipping subduction zone beneath the previosly constructed continental margin. This convergent margin generated a magmatic arc to the north and to the south a subduction accretion complex, present width of which approaches 80 km. To the south of the subduction accretion complex yet another convergent margin is indicated by the presence of the remnants of an ensimatic arc. The northern branch of the Neo‐Tethys was totally consumed in the late Cretaceous to Eocene interval.
Core complex geometries and regional scale flow in the lower crust
Authors: Block Lisa , Royden Leigh H.
Published: February 2008

A simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the Basin and Range Province. However this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation (3–5 km) above core complex structures. Because topographic lows of this magnitude are not observed above core complexes today, it requires mountain ranges 3–5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat Moho as the core complexes developed. An alternate mode of isostatic compensation, whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust, provides an equally good fit to the observed detachment fault geometries, while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat Moho beneath these zones of upper crustal doming. If this interpretation is correct, then the effective elastic plate thickness below the Basin and Range Province probably cannot exceed a maximum of about 4 km, with a best guess estimate of about 0.5 to 1 km. Computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the Moho of about 100°C follows doming by several million years, perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks. Subsequent cooling of the mid to lower crust beneath the core complex could result in “freezing” of the initially ductile lower crust within the core of the dome, effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults.
Displaced terranes and crustal evolution of the Levant and the eastern Mediterranean
Authors: Ben‐Avraham Zvi , Ginzburg Avihu
Published: February 2008

Geophysical data from the Levant and the eastern Mediterranean suggest that this area can be divided into distinct crustal units of different origins. For example, a marked difference in crustal structure and structural style exists in the Levant between Samaria and Judea, south of the Carmel structure, and the Galilee and Lebanon, north of it. On the other hand, several microcontinental fragments such as Cyprus, and the Eratosthenes and Anaximander Seamounts are embedded within the oceanic crust of the eastern Mediterranean. The present location of these units has resulted from complex tectonic processes which were active during the mid Paleozoic‐Mesozoic evolution of this area. During this period several crustal units were accreted to the northern margin of Africa while others were detached away from it. The crustal evolution of the eastern Mediterranean and the Levant is intimately linked with the closure of the Paleo‐Tethys, the formation of the Neo‐Tethys, and its subsequent closure.
Stress and timing relationships of a fault‐related, paleohydrothermal system in central New Hampshire: Record of a Mesozoic stress change in New England?
Authors: Hardcastle Kenneth C. , Albaugh Dennis S.
Published: February 2008

A mineral assemblage comprised of chalcedony, drusy quartz, and very fine grained iron hydroxide (limonite), with or without blocky calcite (QFC), coats joint surfaces, fills fractures, and commonly occurs as pressure shadow encrustations on brittle fault surfaces in central New Hampshire but only occurs sporadically in other areas of New England. Locally, QFC includes silicified breccias of highly altered host and euhedral quartz crystal fragments suspended in a matrix of chalcedony and limonite. One hundred and four brittle faults from central New Hampshire, mineralized with QFC, can be separated into two families. Set N is composed of primarily north–northeast to northeast trending, steeply dipping normal faults (n=61), and set S is composed of north‐northwest to north‐northeast trending, steeply dipping, primarily right‐lateral strike‐ to normal oblique‐slip faults (n=43). These two sets of faults are thought to record two paleostress conditions. The extension direction for both tensors plunges gently east‐southeast, but the compression direction is near vertical for set N and trends roughly northeast, plunging 30° for set S. Fault‐slip inversion suggests that set N faults are best fit with σ1 most deviatoric (σ2 ≈ σ3). Set S is best fit with σ3 most deviatoric (σ1 ≈ σ2) and low values for fault plane coefficient of friction and/or high fluid pressure. QFC occurs in the 158 Ma Belknap Mountain Complex of the White Mountain Magma Series and is thought to have developed at relatively shallow crustal levels based on the assemblage, nature of fluid inclusions, and association of QFC as brittle fault and fracture filling mineralization. Occurrence of QFC in the 158 Ma Belknap Mountain complex, paleostress implications of QFC‐mineralized brittle faults, and range and character of fault fabrics in the Belknap units suggests that QFC developed around the time of Belknap intrusion and cooling and that Late Jurassic to Early Cretaceous is the approximate time at which a regional change in paleostress conditions occurred in New England. This age and nature of the paleostress change is in accord with a change postulated by de Boer et al. (1988).