bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural chronology, oroclinal deformation, and tectonic evolution of the southeastern Klamath Mountains, California
Authors: Renne Paul R. , Scott Gary R.
Published: February 2008

Devonian to Jurassic volcanic, sedimentary, and minor intrusive rocks of the Redding section (RS) accumulated in an intermittently active island arc setting. General stratigraphic concordancy and the downsection continuity of metamorphic facies series indicate that these rocks shared a common postdepositional geologic history. Burial metamorphism produced secondary mineral assemblages of the zeolite, prehnite‐pumpellyite, pumpellyite‐actinolite, and greenschist facies. Close correspondence between pressures calculated from existing stratigraphic overburden and those predicted from the observed metamorphic facies' series indicates that less than 3 km of strata have been eroded from the presently exposed top of the Early‐Middle Jurassic Potem Formation. Shortly after deposition of the Potem Formation, compressional deformation (F1) produced open to isoclinal folds, foliations, and small thrust faults. K‐Ar dates from post‐tectonic intrusions indicate that F1 deformation was completed before 168 Ma. This minimum age of F1 deformation is corroborated by a postfolding paleomagnetic overprint acquired at about 171 Ma. A second suite of intrusions dated at 120–130 Ma provided a thermal pulse that variably reset the K‐Ar system in F1 fabric‐defining celadonite and other relatively Ar‐unretentive materials. The Nevadan orogeny (Late Jurassic) did not significantly deform rocks of the RS, if at all, nor is evidence for Permo‐Triassic (Sonoman) deformation apparent. The geometry of F1 structures varies systematically with location: in the western Bollibokka Quadrangle (BQ), F1 folds are upright, with subvertical axial planes and associated foliations. In the eastern BQ, F1 folds are overturned to the east, with axial planes and associated foliations dipping as little as 40° to the west. Regional relationships suggest that F1 may have been a response to coeval tectonism in the western Klamaths, wherein crustal thickening during regional westward thrusting induced gravitational detachment of the RS from its partly serpentinized ophiolitic basement. F1 structures subsequently were broadly warped about vertical axes, producing variable paleomagnetic declination anomalies that range from 0 to greater than 100° clockwise. The timing and cause of this F2 deformation event are as yet relatively unconstrained but were almost certainly post‐Jurassic and may have been associated with development of the Columbia Embayment. Paleomagnetic data indicate North American paleolatitudes for the RS since at least the Permian, and apparently weigh against an allochthonous or “exotic” origin for this terrane. Paleontologic data indicate strong faunal similarities with North America and to a lesser extent with a Tethyan province. The significance of this apparent dual provinciality remains uncertain, but extreme endemism of Tethyan fusulinids, corals, brachiopods and pelecypods is questionable.
Ecors deep seismic data and balanced cross sections: Geometric constraints on the evolution of the Pyrenees
Authors: Roure F. , Choukroune P. , Berastegui X. , Munoz J. A. , Villien A. , Matheron P. , Bareyt M. , Seguret M. , Camara P. , Deramond J.
Published: February 2008

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.
Are systematic variations in thrust belt style related to plate boundary processes? (The western Alps versus the Carpathians)
Authors: Royden Leigh , Burchfiel B. C.
Published: February 2008

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.
Geometric and kinematic development of border faults and accommodation zones, Kivu‐Rusizi Rift, Africa
Authors: Ebinger Cynthia J.
Published: February 2008

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.
Brittle and ductile deformation in a zone of rapid uplift: Central Southern Alps, New Zealand
Authors: Holm Daniel K. , Norris Richard J. , Craw David
Published: February 2008

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.
Strike‐slip restoration of the Barents Sea Caledonides Terrane, Finnmark, north Norway
Authors: Rice A. H. N. , Gayer R. A. , Robinson D. , Bevins R. E.
Published: February 2008

The Trollfjord‐Komagelv (T‐K) fault is a major WNW–ESE trending dextral strike‐slip fault which cuts the Caledonides of northern Norway, with a palaeomagnetically determined displacement of 500–1000 km. Although earlier interpretations suggested a pre‐Caledonian age for the strike‐slip displacement, our analysis shows that the fault movement was intimately associated with late Caledonian thrusting. The epizone/high‐intermediate baric type metamorphism of the Barents Sea Caledonides has been used to constrain their restored position relative to other units in the Caledonides in northern Scandinavia. This indicates a postmetamorphic dextral displacement along the T‐K fault of ∼415 km. The Kalak Nappe Complex straddles the line of the T‐K fault but has not been cut by it. Despite this relationship, it is argued that the Kalak Nappe Complex was emplaced prior to strike‐slip fault movement. North of the fault, structures reflect blind thrusting and passive roof duplex development, refolded by en echelon strike‐slip structures, indicating that T‐K fault movement started after thrusting. South of the fault, structures in the Gaissa Thrust Belt have been affected by strike‐slip movement, although final thrust movements deformed the T‐K fault. The Kalak Nappe Complex north of the T‐K fault was carried passively by the strike‐slip block while to the south, major thrusting occurred.
The anatomy of a pull‐apart basin: Seismic reflection observations of the Dead Sea Basin
Authors: ten Brink Uri S. , Ben‐Avraham Zvi
Published: February 2008

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.
The Mojave Extensional Belt of southern California
Authors: Dokka Roy K.
Published: February 2008

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).
Nature and origin of seismic reflection fabric, Ruby‐East Humboldt Metamorphic Core Complex, Nevada
Authors: Valasek P. A. , Snoke A. W. , Hurich C. A. , Smithson S. B.
Published: February 2008

Seismic reflection profiling across exposed upper and middle crustal rocks of the Ruby‐East Humboldt metamorphic core complex delineates important characteristics of the crustal structure developed during Tertiary extensional deformation. The goals of this study were to trace a Tertiary extensional shear zone from mylonitic surface outcrops into a seismic section and to characterize the deeper crustal fabric associated with the polyphase deformational history of the complex. Reflections in the shallow surface correlate with a plastic to brittle shear zone that formed during the tectonic unroofing of the middle crustal rocks. Constructive interference from strong planar layering in the mylonitic shear zone is considered chiefly responsible for generating the reflections. East and southwest dipping reflectors in the seismic section appear to correlate with exposed, opposing dipping mylonitic foliation domains. The opposing dips of the mylonitic layering may reflect warping of the normal‐sense shear zone during tectonic exhumation coupled with an overall anastomosing character. Deeper in the section, a heterogeneous reflection character correlates with increases in velocity shown from wide‐angle measurements in this region and is interpreted as penetrative fabric originating from extensional flow facilitated by broad‐scale pure shear and localized simple shear. Lower crustal rocks apparently achieved granulite facies metamorphism during extension. A maximum of 6–8 km of mafic material could have been added to the crust during Cenozoic extension. The Moho reflection has such a high amplitude that it may be caused by partially molten rocks interleaved with peridotite. Reflections from the Moho show no significant upwarping of the base of the crust beneath the core complex. This suggests that the lower‐crustal configuration is at least as young as the period of extensional activity that lasted from approximately 40 to 20 Ma which was responsible for the exhumation of the metamorphic core complex.
The extension of Grenville Basement beneath the northern Appalachians: Results from the Quebec‐Maine seismic reflection and refraction surveys
Authors: Spencer Carl , Green Alan , Morel‐à‐l'Huissier Patrick , Milkereit Bernd , Luetgert James , Stewart David , Unger John , Phillips Jeffrey
Published: February 2008

From 1983 to 1984 the United States Geological Survey and the Geological Survey of Canada undertook a series of seismic experiments to determine the crustal structure of the Appalachian mountain belt in Maine and southeast Québec. These experiments included four deep crustal seismic reflection profiles perpendicular to the strike of regional geological trends and several high‐resolution refraction/wide‐angle reflection profiles both parallel and perpendicular to strike. Here the data recorded in Québec and the northwest part of Maine are interpreted in conjunction with existing reflection profiles from Québec. Synthetic seismogram calculations in laterally varying media are used to model the refraction/wide‐angle reflection data, while reprocessing, migration, and attribute plotting are used in the interpretation of the reflection data. A major zone of reflections extending over a distance exceeding 200 km can be traced from shallow depths beneath the St. Lawrence Lowlands southeast to about 25 km depth beneath the southeast edge of the Chain Lakes Massif. This zone of reflections is interpreted as a master décollement separating autochthonous Grenville basement from overlying allochthonous rocks of the Appalachian Orogen, including foreshortened miogeoclinal rocks, the remnants of one or more magmatic arcs, the Connecticut Valley ‐ Gaspé Synclinorium, and the Chain Lakes Massif. In the vicinity of the Chain Lakes Massif, extension has caused Grenville basement to be thinned by as much as 40%. Reflections from the décollement show strong layering and evidence for basement faulting associated with the Guadeloupe and other faults. Normal incidence and wide‐angle reflections are observed to be almost continuous beneath the Baie Verte ‐ Brompton Line and Connecticut Valley ‐ Gaspé Synclinorium, indicating that the Baie Verte ‐ Brompton Line is confined to the upper crust. The Connecticut Valley ‐ Gaspé Synclinorium is characterized by P‐wave velocities of about 5.3 km s−1 and is shown to be a shallow structure with a maximum depth of about 3 km. Rock velocities immediately below the Connecticut Valley ‐ Gaspé Synclinorium are typical of Chain Lakes material (6.1 km s−1). A new model for the tectonic development of the region is proposed in which all pre‐Silurian units between the Baie Verte ‐ Brompton Line and the southeastern edge of the Chain Lakes Massif are allochthonous and confined to the upper crust. We infer that the Chain Lakes Massif, volcanics of the Ascot‐Weedon Formation, and associated deeper crustal rocks were thrust over Grenvillian basement as a composite unit and may now underlie parts of the Connecticut Valley ‐ Gaspé Synclinorium.
K‐Ar and Ar‐Ar geochronology of the Himalayan collision in NW Pakistan: Constraints on the timing of suturing, deformation, metamorphism and uplift
Authors: Treloar Peter J. , Rex D. C. , Guise P. G. , Coward M. P. , Searle M. P. , Windley B. F. , Petterson M. G. , Jan M. Q. , Luff I. W.
Published: February 2008

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.
Structure and tectonics of an Early Mesozoic oceanic basement in the northern Sierra Nevada Metamorphic Belt, California: Evidence for transform faulting and ensimatic arc evolution
Authors: Dilek Yildirim
Published: February 2008

The N‐S trending structural grain of the Sierra Nevada metamorphic belt bends sharply to an E‐W orientation along the Big Bend fault zone at the northern end of the Smartville complex. Multiply deformed and metamorphosed, early Mesozoic and older (?) rocks of the Jarbo Gap ophiolite crop out within and north of the Big Bend fault zone. The Jarbo Gap ophiolite includes serpentinized harzburgite tectonite, dunite, metagabbro, amphibolite, and metavolcanic‐metasedimentary rocks which are commonly separated from each other by steeply dipping and along‐strike discontinuous fault contacts. Locally, serpentinite occurs as diapiric intrusions and/or fault‐bounded blocks in metavolcanic rocks or is stratigraphically overlain by metavolcanic and metasedimentary rocks. Metasedimentary rocks include hemipelagic sandstones and mafic talus breccias composed of ophiolitic detritus. Amphibolite and metagabbro display multiple deformation features associated with mineral assemblages, indicating retrograde metamorphism from amphibolite to greenschist facies. Raser gabbro within the metagabbro unit shows mylonitic bands and shear zones and is intruded by a leucodiorite, which has a U/Pb zircon age of 204±2 Ma. The internal structure and rock types of the Jarbo Gap ophiolite characterize a heterogeneous oceanic crust that is consistent with an oceanic fracture zone setting. The Jarbo Gap ophiolite is intruded by ∼200 Ma dikes that have a geochemical composition similar to high‐magnesium andesites or boninitic lavas of arc volcanoes. These intrusions are coeval with calc‐alkaline plutonic and hypabyssal rocks of the Smartville and Slate Creek complexes and are interpreted as feeders for an incipient early Jurassic arc. Both complexes include ∼200 Ma plutonic and volcanic rocks that are intruded by ∼160 Ma dikes and plutons. These relations suggest that the Jarbo Gap ophiolite may be part of the oceanic basement of a Jurassic island arc terrane, which includes the Smartville and Slate Creek complexes. The E‐W orientation of the Big Bend fault zone is interpreted to be a manifestation of a regional tectonic discontinuity separating the northern Sierra Nevada and the Klamath Mountains provinces. Regional reconstructions and correlations and the paleomagnetic data from both regions suggest that this discontinuity is either a sinistral transform boundary between the two provinces or the northern termination of the Sierra Nevada province, which was probably translated northward for nearly 500 km in and/or prior to Early Cretaceous time.
Deep seismic structure and plate tectonic evolution of the Canadian Appalachians
Authors: Stockmal Glen S. , Colman‐Sadd Stephen P. , Keen Charlotte E. , Marillier Francois , O'Brien Sean J. , Quinlan Garry M.
Published: February 2008

Three seismically defined lower crustal blocks (LCBs) have been recognized to underlie the familiar tectonic‐stratigraphic zones of the Canadian Appalachians. Tectonic development of the orogen is inferred to have been strongly influenced by collision of the two outboard LCBs (Central and Avalon LCBs) against the sharply irregular North American craton (Grenville LCB), as evidenced by an abrupt offset in the trend of the orogen localized at this irregular edge. The LCBs are restored to a precollisional configuration by (1) removing Carboniferous strike‐slip motion on major faults linking through the Magdalen pull‐apart basin; and (2) removing Devonian(?) offset along a hypothesized fault through the present‐day Strait of Canso (Canso fault) to realign the Central and Avalon LCBs, presumed to have been continuous prior to accretion to North America. The restoration leads to (1) a qualitative explanation for deep seismic reflection observations across the Magdalen basin which suggest that portions of the underlying crust do not correspond to any of the three principal LCBs; and (2) a plate tectonic model for the orogen which suggests the backarc basin separating the Taconian (Ordovician) arc from the eastern margin of the early Paleozoic Iapetus ocean was at least as wide (>450 km) as the offset in the irregular Grenville LCB, inherited from rifting of Iapetus. The present‐day Southwest Newfoundland Transform Margin may be a segment of the Canso fault reactivated during Mesozoic opening of the Atlantic ocean.
Plate tectonics and palinspastic reconstructions of the Anadyr‐Koryak Region, northeast USSR
Authors: Stavsky A. P. , Chekhovitch V. D. , Kononov M. V. , Zonenshain L. P.
Published: February 2008

The Anadyr‐Koryak region in northeastern USSR consists of a series of exotic terranes, most of which arrived from the Pacific and accreted to the Eurasian margin in Cretaceous and Tertiary times. Eight major terranes can be recognized: (1) The Kanchalan and (2) Pekulnei terranes include possible ancient blocks in their cores. The (3) Penzhina, (4) Ust' Belaya, and (5) Vaega terranes (which are parts of the Talovo‐Mainsky zone) have chaotic structure incorporating Paleozoic and early Mesozoic oceanic, island arc, continental rise, and other lithologie assemblages indicative of a subduction melange environment. The (6) Mainits and (7) Ekonai terranes (two parts of the Khatyrka megaterrane) contain late Paleozoic and Triassic island arc and back‐arc assemblages with faunas of Tethyan provenance. The (8) Olyutorsky terrane is composed mostly of Late Cretaceous island arc assemblages which accreted to the Eurasian margin in the middle Tertiary. Computed trajectories of Kula/Eurasia and Pacific/Eurasia relative motions suggest that some (or most) of these terranes traveled thousands of kilometers from the central Pacific until they eventually collided with, and were attached to, the Eurasian margin.
Thick‐skinned deformation observed on deep seismic reflection profiles in western Argentina
Authors: Snyder D. B. , Ramos V. A. , Allmendinger R. W.
Published: February 2008

One of the first deep seismic reflection profiles in South America has resulted from reprocessing nearly 90 km of 96 channel Vibroseis data that were originally collected by YPF (the Argentine National Oil Company) across the Sierra de la Huerta in the Sierras Pampeanas of western Argentina. Reprocessed sections show the following features in the upper 20 km of crust: (1) prominent layering within Carboniferous(?) to Quaternary strata; and (2) a complex suite of faint reflections which are interpreted to define both extensional and compressional faults. The Mesozoic strata are bounded by one or more west‐dipping normal faults. One or two east‐dipping thrust faults shallow with depth, a geometry similar to the Rocky Mountain foreland of the western U. S. A. One thrust has a clear surface trace. Although lacking evidence of surface scarps or clear fault plane reflections, panels of constant dip domain seen in the seismic data justify the application of fault‐bend folding techniques to suggest that a second, deeper fault becomes nearly flat at 15–20 km. Cumulative Cenozoic offset on the thrusts beneath the Sierra de la Huerta is about 13 km. A well‐located earthquake hypocenter lies within the footwall block of a major thrust and is presumed to represent susidiary thrust motion along a secondary high‐angle fault. Gravity models suggest that the thrust ramps may nucleate at an anomalous dense mass in the middle or lower crust. Interpretation of lower crustal structure is hampered by the lack of clear reflections, presumably due to small‐scale and low‐amplitude impedance contrast variations of lithologic units at those depths. Consideration of recorrelation limitations and signal penetration constraints provide likely rationale for the scarcity of clear reflections in the basement rocks of the region, but low reflectance of mafic metamorphic rocks remains a possible explanation.
Structural evolution of the Southern Baboquivari Mountains, south‐central Arizona and north‐central Sonora
Authors: Goodwin Laurel B. , Haxel Gordon B.
Published: February 2008

Structural analysis of metamorphic tectonites, low‐angle faults, and planar dilational dike swarms in the southern Baboquivari Mountains reveals a complex Mesozoic through middle Tertiary history comprising multiple periods of deformation. Locally preserved rootless folds probably represent Jurassic soft sediment deformation or tectonic deformation that pre‐dated the main episode of Late Cretaceous through early Tertiary tectonism. The latter involved thrust faulting, isoclinal folding, ductile strain, and transposition of bedding into foliation; greenschist to lower amphibolite facies regional metamorphism; and late‐ to post‐metamorphic intrusion of an early Tertiary peraluminous granite pluton. Intrusion of the pluton was accompanied by minor folding of foliation in the country rock and subsequent brittle extension with emplacement of a granitic dike swarm. Middle Tertiary metamorphic‐core‐complex deformation involved mylonitization accompanied and closely followed by detachment faulting and intrusion of microdiorite and lamprophyre dikes. The southern Baboquivari Mountains thus record two independent tectonic episodes, the first compressional and the second extensional. Both episodes are marked by a progression from ductile to brittle deformation. Orientations of late‐metamorphic extension fractures and the granitic dike swarm indicate NE‐SW crustal extension in Late Cretaceous through early Tertiary time. The middle Tertiary extension direction, NNE‐SSW, is indicated by the attitudes of post‐mylonite mafic dikes. Poles to these dikes are parallel to mylonitic lineation. These extension directions documented in the southern Baboquivari Mountains are typical of the southern Papago terrane of south‐central Arizona. In contrast, the rest of southern Arizona is characterized by NNW‐SSE extension during Late Cretaceous through early Tertiary time and ENE‐WSW extension during middle Tertiary time. The anomalous extension directions of the southern Papago terrane probably reflect deviations from regional strain patterns in a terrane with inherited anomalous crustal rheology and/or thickness.
The lower crust and the extension of the continental lithosphere: Kinematic analysis of Birps Deep Seismic Data
Authors: Reston T. J.
Published: February 2008

Deep seismic reflection profiles, recorded in the seas around the United Kingdom, image bands of mantle reflections, interpreted as shear zones, that are not aligned with the basin‐bounding faults in the upper crust. Instead, the lower crust, represented as a zone of sub‐horizontal reflectivity, appears to decouple deformation in the upper crust from that in the mantle. Simple modeling shows that the bulk deformation of the lithosphere, in terms of pure or simple shear, is controlled by the spatial distribution of upper crustal and mantle faults, and that deformation is transferred between the two by sub‐horizontal shear within the lower crust. The simplest form this lower crustal shear can take is a gently inclined zone of simple shear, both transferring deformation between the upper crust and the mantle, and extending the lower crust. This model predicts many of the features of the DRUM profile north of Scotland, including a distortion of the Moho in the vicinity of the Outer Isles Fault that has previously been interpreted as evidence for a Moho offset. In general, the predicted shear strain within the lower crust closely matches its observed reflectivity, so it seems likely that the latter can be related, at least in part, to shearing between the upper crust and the mantle. The model also show that the relative position of upper crustal faults and mantle faults can change during progressive deformation, causing the character of large‐scale deformation of the lithosphere to change with time. This result could have implications for the subsidence behavior of sedimentary basins during progressive extension.
Multiple strike‐slip fault sets: A case study from the Dead Sea Transform
Authors: Ron H. , Nur A. , Eyal Y.
Published: February 2008

In many strike‐slip tectonic settings, large rotations of crustal blocks about vertical axes have been inferred from paleomagnetic data. These blocks are bounded by sets of parallel faults which presumably accommodate the relative motion between the blocks as regional deformation progresses. A mechanical model suggests that rotations greater than ϕc = 25° to 45° must be accommodated by more than one set of faults, with the angle ϕc between their directions, consequently the sum of the angles between sets must be roughly equal to the total tectonic material rotation. To test this model we investigated the fault geometry and field relationship of fault sets in the Mt. Hermon area in northern Israel, where paleomagnetic declination data imply 69°±13° counterclockwise (CCW) block rotation. The statistical and field relationship analysis of over 315 faults shows that the faulting is predominantly right lateral strike slip consisting of three distinct sets. The oldest set strikes 254°, the second oldest set strikes 295° and the youngest strikes 331°. This last direction is consistent also with the current north‐south direction of the maximum principal stress axis. The angle ϕc between the first and second sets is 41° and between the second and third sets 36°, in good agreement with the ϕc angle predicted from mechanical considerations. The sum of the two angles is 77°CCW, in good agreement with the 69°±13°CCW paleomagnetically derived rotation. The results suggest specifically that the sequential development of multiple intersecting fault sets is responsible for the faulting in the Mt. Hermon area and generally that the model of block rotation with multiple faults provides very good simple rules for analyzing very complex fault patterns.
Formation and thrusting in some great valley rocks near the Franciscan Complex, California, and implications for the tectonic wedging hypothesis
Authors: Glen R. A.
Published: February 2008

On the western side of Sacramento Valley in northern California, the Upper Jurassic and Cretaceous Great Valley sequence dips east at 40° to 70° in what has been regarded as a tilted but otherwise undeformed east dipping homocline containing strata collectively up to 15 km in thickness. This paper reports the results of structural mapping in the Paskenta area which suggests that, in contrast, steep dips are caused by westward directed thrusting and folding which led to tectonic thickening of the sedimentary pile. In a mainly east dipping sequence, thrusts are recognized by changes in the structural facing and vergence of both folds and cleavage. Although mainly affecting undated rocks of the Great Valley sequence, one major thrust rides on a serpentine‐matrix melange which forms the substrate of the Great Valley sequence. Differential shortening occurring in different blocks during thrusting was accommodated by left‐lateral movement on the Paskenta and Elder Creek fault zones which acted as tear faults. Ductile and brittle faulting have also occurred throughout the sequence, in the melange, and on the tear faults. From work described in this paper and from other reports of thrusts and deformation in the Great Valley sequence in the foothills west of Sacramento Valley, the hypothesis is advanced that these strata have all been tectonically thickened by westward directed thrusting of possible early Tertiary age. While the regional applicability of this hypothesis awaits future detailed structural work, it is no longer sufficient to assume that east dipping strata have been merely tilted. The proposed thrusting model is compatible with the recent tectonic wedge model proposed for the Coast Ranges‐Sacramento Valley by Wentworth et al. (1984). With respect to such a crustal model in which a tectonic wedge of Franciscan Complex was thrust east above older continental basement but below rocks of the Coast Range ophiolite and Great Valley sequence, thrusts of this paper are back thrusts developed above a gently west dipping floor thrust. Back thrusting, shortening, and structural elevation of the Great Valley sequence are direct consequences of this wedging. The uplifted strata lie east of a poorly defined triangle zone and define the western limb of a regional synclinorium that has some analogies with the Alberta syncline of the Canadian foothills.
The present status of the theory of the earth's magnetism
Authors: Nippoldt A.
Published: February 2008

It is characteristic of Gauss's method of working that he published no investigation until it was either finally completed or had been developed to the highest stage attainable at the time with the material at hand. Were there any weak points, no opportunity to point these out was neglected by him. Herein lies the classic value of his works, and so, naturally, it came to pass that for many years the principal endeavor of his successors was to fill in the missing links, and that only recently attempts have been made to farther develop his methods.

In the field of terrestrial magnetism, especially, did further development of method take place but recently—hardly more than a decade ago. Since then, however, a wonderful zeal, enthusiasm, and singleness of purpose, as probably in no other branch of geophysics, have been displayed. This activity has disclosed a great number of new ideas and points of view, and has revealed a more intimate connection between terrestrial magnetism, meteorology, and atmospheric electricity than was at first suspected. This discovery, though but very recent, is leading future investigation into totally different paths from those of the earlier days.
Theory of the earth's magnetism
Authors: Sutherland William
Published: February 2008

In the previous issue of the Journal, page 90, Mr. Norman E. Gilbert, who had kindly verified the proofs of the abstract of my paper on the “Cause of the Earth's Magnetism,” makes two brief criticisms to the effect, first, that an electric field which does not revolve with the Earth is required; and second, that all evidence goes to show that under the conditions ruling inside the Earth the materials there ought to be good conductors rather than insulators, and, therefore, to make the existence of any electrostatic field impossible.

To the first criticism I would reply that I presume it means that, as all magnets on the surface of the Earth revolve with it, it is necessary that static charges which are to produce the Earth's magnetic field must be at rest, so as to allow of the supposed required relative motion between magnet and charge. In the original paper I touched upon this matter, omitting to do so in the abstract for the sake of brevity. But it is not necessary that a static charge should move relatively to a magnet in order to produce on it the effect of an element of current. The static charge by its motion relative to the ether sets up in it those whirls which we study as lines of magnetic force. Now, it is not necessary that, once these lines of force are generated in the ether, they should lose their magnetic effect on a magnet simply because it is moved with the same velocity in the same direction as the charge whose motion brought them into existence; for if this were so, then motion of a magnet in any magnetic field could deprive the field of its power to affect the magnet. The matter is one to be ultimately decided by experiment, and is a question of great interest,; but as our conceptions stand at present, it seems to me more reasonable to assume that the motion of static electricity in Rowland's experiment establishes a magnetic field which would still affect magnets moved with the electricity.
Résumé of magnetic observations made chiefly by the United States Coast and Geodetic Survey on the day of the total solar eclipse, May 28, 1900
Authors: Bauer L. A.
Published: February 2008

In order to ascertain over how large an area the variations as recorded at the magnetic observatories may apply, it has been arranged that on certain days throughout the year special simultaneous magnetic observations be made by the various parties in the in the employ of the Coast and Geodetic Survey.

This is a question of fundamental importance in conducting the magnetic survey of the United States and countries under its jurisdiction; namely, in the reduction of the field determinations of the magnetic elements to the common epoch of the survey, great dependence will have to be put upon the continuous records of the magnetic variations as obtained at the magnetic observatories.
Report on the magnetic observations made in North America during the total solar eclipse of May 17–18, 1901
Authors: Bauer L. A.
Published: February 2008

The following1 special directions were sent to the field observers of the united States Coast and Geodetic Survey, and to volunteers. It should be recalled, that for this region of the Earth the entire interval was during the night hours, and hence, special precautionary measures had to be taken. It was not deemed advisable to attempt intensity observations, except at the fixed magnetic observatories, for which special instructions fitting each special case were given.
Magnetic storm of October 31–November 1, 1903, in Japan
Authors: Okada T.
Published: February 2008

The magnetic storm which occurred October 31–November 1 was of very great energy. In this country the magnetic “Unruhe” was first noticeable on the magnetograms at six o'clock morning on the 31st October. The disturbances became very great from 3 P.M. (31st) to 4 A.M. on 1st November, and then gradually died away.

The maximum range of the disturbance was 38′ in the declination and 215γ in the vertical component. The trace of the horizontal magnet passed beyond the photographic record, and, hence, the range cannot be obtained.
Interruptions to telegraph lines in New South Wales, Australia, as observed from the chief office (Sydney) on October 31, 1903
Authors: Klotz Otto J.
Published: February 2008

At 4 p. m.,1 the current on the Melbourne quadruplex circuit fluctuated considerably, and at 5 p. m., southern quadruplex circuits suddenly disclosed a foreign element. Two copper wires were the first affected, the galvanometers indicating a varying current from 10 to 25 degrees, which was located between Mittagong and Goulburn.

Other southern circuits were unworkable. The disturbance appeared much stronger further south than Goulburn.
Proposed magnetic and electric observations during the total solar eclipse of August 30, 1905
Authors: Bauer L. A.
Published: February 2008

Preliminary information regarding the parties, which propose making magnetic and electric observations is herewith given.

Preliminary Information regarding Stations.

CANADA. Photographic registration of the magnetic declination and of the horizontal intensity will be made in the belt of totality by a Canadian expedition under the direction of Professor R. F. Stupart, Director of the Meteorological Service, at North‐West River, Labrador (latitude 53°30′, longitude 60°03′), where there will likewise be located a Canadian astronomical party, under the direction of the Chief Astronomer, Dr. W. F. King. Atmospheric electricity observations may also be attempted.
Magnetic and allied observations during the total solar eclipse of August 30, 1905
Authors: Bauer L. A.
Published: February 2008

In addition to the stations as already announced in the previous issues, information was received from General Rykatschew that a magnetic expedition would be sent to Assuan, Egypt, under the auspices of the St. Petersburg Academy of Sciences and under the direction of M. Dubinsky.

The stations finally decided upon by the Department Terrestrial Magnetism in order to provide for the proper distribution and successful study of the subject under investigation were as follows: