bluuebunny/crossref_metadata_2025_split huggingface dataset:
Tectonomagmatic evolution of Cenozoic extension in the North American Cordillera
Authors: Wernicke Brian P. , England Philip C. , Sonder Leslie J. , Christiansen Robert L. 
Published: April 2008

Summary
The spatial and temporal distributions of Cenozoic extension and magmatism in the Cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt (<100 km, pre-extension) and followed the onset of intermediate and silicic magmatism by no more than a few million years. Extension began in early Eocene time in southern British Columbia, northern Washington, Idaho and Montana. Farther S, extension began at about the Eocene-Oligocene boundary in the Great Basin and slightly later in the Mojave-Sonora Desert region. The intervening area, at the latitude of Las Vegas, remained quiescent until mid-Miocene time. Compositional and isotopic characteristics of most pre-Miocene magmas are consistent with their containing major components of melted continental crust.
In mid-Miocene time, two major changes occurred: widening of the area of extension and the widespread appearance of basaltic magmas. The area affected by extension, from southwestern Montana to the Lake Mead region, widened to several hundred kilometres. By this time extension in southern British Columbia, northern Washington and northern Montana had ceased (probably before the end of the Eocene), and extension S of Lake Mead (except in the Gulf of California) had waned. Regions affected by the broader belt of extension during late Miocene, Pliocene, and Quaternary time experienced basaltic magmatism, which began along a central rift zone in the northern part of the region, and which had within a few million years spread to include most of the region; later basaltic activity has tended to concentrate in restricted zones, especially near the margins of the extended area.

We recognize a correlation between the amount of earlier crustal thickening and Cenozoic extension, and between the length of time after shortening but before extension and the degree to which a given region was intruded by Late Cretaceous plutons. The localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal-mechanical model developed in a companion paper (Sonder
et al.
). This raises the possibility that stresses inherent in the North American Plate dominated over plate-interaction forces as controls of the Cenozoic tectonomagmatic evolution of the North American Cordillera, especially in its earlier stages.
A shear-zone model for the structural evolution of metamorphic core complexes in southeastern Arizona
Authors: Davis George H. 
Published: April 2008

Summary
Some of the world’s best exposed and most thickly developed mylonites, ultramylonites, and cataclasites are found in parts of a number of mountain ranges in southeastern Arizona. These fault rocks and associated structures occupy metamorphic core complexes, which I view as mountain-size geological exposures of regional, upward-tapering ductile-brittle shear zones. The fundamental characteristics of the core complexes were fashioned by regional crustal extension in the Tertiary. Normal-slip shearing served to ‘telescope’ fault rocks and structures which had formed originally at different depth levels in the crust. The result of normal-slip simple shear within the shear zones was the translation of hanging wall crust by tens of kilometres. The shear zones and linked fault zones are interpreted to have dipped at 45° or more initially, rotating progressively to more gentle inclinations and shallower depths as the crust thinned and stretched. Progressive simple-shear under conditions of a steadily decreasing confining pressure and temperature is dramatically recorded in suites of structures and fabrics: for example mylonite gneiss is converted to microbrecciated mylonite gneiss, which in turn is transformed to cataclasite and ultracataclasite. The physical nature of this progressive deformation is beautifully exposed in the gently dipping Catalina-Rincon shear zone, whose once-deep and once-shallow parts are now exposed to view over broad expanses E and NE of Tucson, Arizona.
Ductile strain and metamorphism in an extensional tectonic setting: a case study from the northern Snake Range, Nevada, USA
Authors: Lee Jeffrey , Miller Elizabeth L. , Sutter John F. 
Published: April 2008

SummaryIn recent years considerable attention has focused on metamorphic core complexes of the Basin and Range Province of the western US Cordillera. These highly extended areas are characterized by an upper plate that has been brittlely attenuated by normal faults separated by a sub-horizontal detachment surface from a lower plate that has been ductilely thinned and stretched. A study of mesoscopic structures, finite strain, microstructures, quartzc-axis fabrics and40Ar/39Ar geochronology was undertaken in order to characterize the nature, geometry, kinematic history and timing of ductile extension in the lower plate of the northern Snake Range metamorphic core complex in E-central Nevada. These data provide new insights into the processes of deep-seated ductile strain beneath supracrustal normal fault mosaics in highly extended regions. Mesoscopic structures and finite strain analyses indicate that the lower plate underwent plane strain with a sub-vertical Z-axis and a sub-horizontal WNW-ESEX-axis. The magnitude of strain increases dramatically from W-to-E, and is constant vertically. Bedding and foliation are everywhere parallel and bedding is thinned 30–90%. The nature and geometry of microstructures andc-axis fabrics changes progressively from W-to-E. Lower strain rocks on the W flank of the range are characterized by a single bedding-parallel foliation, defined by flattened detrital grains, globular grains which have theirc-axes parallel to theZ-axis and symmetrical cross-girdlec-axis fabrics. Further E to slightly higher strains, the rocks are characterized by asymmetrical cross-girdlec-axis fabrics, although globular grains with theirc-axes parallel toZare still present. The high-strain rocks on the E flank of the range are characterized byC-planes, asymmetrical mica ‘fish’ and oblique quartz foliations, and asymmetrical single-girdlec-axis fabrics. These data suggest that the lower plate has not deformed entirely by either coaxial or non-coaxial strain, and we propose an evolutionary model whereby the lower plate deformed during an early period of coaxial strain followed by a later component of non-coaxial strain accompanied by coaxial strain on the E flank of the range.40Ar/39Ar geochronology on lower-plate hornblendes and micas indicates that temperatures of deformation were >280°C, but <530°C, and increased with depth. Geochronological data suggest that lower-plate ductile strain is post-latest Cretaceous, and biotite and muscovite data indicate that ductile deformation was probably ongoing 22–26 Ma. Our data and regional geological relations suggest that ductile extensional deformation in the northern Snake Range occurred because of localized heat input to shallow levels of the crust, and together with seismic reflection data, indicate that lower-plate rocks probably represent the top of a regional metamorphic terrane of Tertiary age.
Geometry and evolution of a mid-crustal extensional fault system: Chemehuevi Mountains, southeastern California
Authors: John Barbara E. 
Published: April 2008

Summary

The extensional fault system exposed in the Chemehuevi Mountains area of the southern Cordillera provides data on the mode of mid-crustal accommodation to continental extension. A stacked sequence of three mid-Tertiary low-angle normal faults cut gently down-section through deformed Proterozoic and Mesozoic crystalline basement below Cenozoic strata. Hanging wall blocks are consistently displaced relatively NE across these three detachment faults, recording unidirectional extension of quartzofeldspathic crust at palaeodepths of 6–10 km. The two structurally deepest faults in the sequence are exposed over 22 km in a down-dip direction, across a total area in excess of 350 km
2
, and were initiated with a regional dip of less than 15° NE. Both of the structurally deepest faults are corrugated parallel to the direction of transport; wavelengths of the corrugations range between 200 m and 10 km, and amplitudes range from 30 to 400 m. These undulations are broad mullion structures that developed coeval with fault slip. Amplitude and wavelength vary with footwall rock type and pre-existing structural grain. Slip on the faults at the present level of exposure was accomplished by brittle deformation, with the generation of gouge, breccias, rocks of the cataclasite series, and rare pseudotachylites. Major mylonite zones in the Chemehuevi Mountains are older and unrelated to the extensional faulting. These data support the conclusion that mid-crustal extension in the Chemehuevi Mountains area was accommodated by an asymmetrical normal-slip shear system. Extension occurred along seismically active, gently NE-dipping, undulating surfaces. During their evolution they rose from middle- to upper-crustal depths.
Kinematics of Basin and Range intraplate extension
Authors: Eddington Paul K. , Smith Robert B. , Renggli C. 
Published: April 2008

Summary

Strain rates assessed from brittle fracture (associated with historic earthquakes) and total brittle-ductile deformation measured from geodetic data have been compared to estimates of palaeostrain, from Quaternary geology for the intraplate Great Basin part of the Basin and Range Province, western United States. These data provide an assessment of the kinematics and mode of lithospheric extension that the western U S Cordillera has experienced from the past few million years to the present. Strain and deformation rates were determined by the seismic-moment-tensor method using historic seismicity and fault-plane solutions for sub-regions of homogeneous strain. Contemporary deformation (with maximum-deformation rates) in the Great Basin occurs principally along the active seismic zones: (i) the southern Intermountain Seismic Belt (ISB), 4.7 mm a
−1
; (ii) along the western boundary, the Sierra Nevada front, 1.6 mm a
1
(28.0 mm a
−1
if the
M
8.3 1872 Owen Valley, California earthquake is included); and (iii) along the W-central Nevada seismic belt, 7.5 mm a
−1
. The integrated opening rate across the entire Great Basin is accommodated by E-W extension at 8–10 mm a
−1
to the N which diminishes to 3.5 mm a
−1
to the S. These results show 8–10 mm a
−1
contemporary extension across the entire Great Basin associated with earthquakes that compares to the ≤9 mm a
−1
value determined from intraplate tectonic models (constrained by satellite geodesy) implying that contemporary strain is generally released by earthquakes. Zones of maximum lithospheric extension correspond to belts of thin crust, high heat flow, and Quaternary basaltic volcanism, suggesting that these parameters are related through the mechanism of extension such as a stress relaxation, allowing buoyant uplift and ascension of magmas.

Contemporary strain and deformation rates have been determined from geodetic measurements yielding maximum deformation of 11.2 mm a
−1
in the Hebgen Lake, Montana portion of the Intermountain Seismic Belt; 3.6 mm a
−1
in the Walker Lane, Nevada area; and 2.5 mm a
−1
in the Owens Valley, California area adjacent to the Sierra Nevada front. Palaeostrain and deformation rates principally from Quaternary fault-displacement rates gave deformation rates of 7.4 mm a
−1
along the southern ISB. Geodetically determined deformation rates compare well with rates determined from seismic moments, whereas poorly constrained palaeo-strain rates from Quaternary geology are ∼10 times smaller than contemporary rates except in parts of central and southern California, Wyoming, parts of Utah, and along the Idaho-Wyoming border.
Footwall structural evolution of the Tucki Mountain detachment system, Death Valley region, southeastern California
Authors: Hodges K. V. , Walker J. D. , Wernicke B. P. 
Published: April 2008

Summary

Tucki Mountain lies within the central portion of the Death Valley extended terrain. Its structure is dominated by the Cenozoic Tucki Mountain detachment system, a complex and long-lived extensional feature which places weakly metamorphosed-unmetamorphosed strata on middle to upper greenschist facies metamorphic rocks. Detailed analysis of tectonites in the footwall of the detachment system has led to the identification of seven phases of ductile structures. The earliest are latest Mesozoic-earliest Tertiary(?) isoclinal folds and associated schistosity which may have been related to compressional deformation. All subsequent generations of structures developed during Cenozoic extension at intermediate-upper crustal levels. Structures of
D
3
and
D
5
age are markedly asymmetric and record simple-shear deformation along the detachment system. Kinematic analysis of these structures indicates consistent NNW movement of the hanging wall relative to the footwall throughout the history of the fault zone. This extension direction is inconsistent with the overall N45W-S45E direction indicated for the Death Valley region by the orientation of major transfer faults like the Death Valley and Furnace Creek fault zones. The discrepancy may reflect late-stage changes in the regional extension direction from N-S to NW-SE, or a partitioning of overall NW-SE extension into domains of variably orientated local extension.
A compilation and regional interpretation of the northern North Sea gravity map
Authors: Zervos F. 
Published: April 2008

Summary
A Bouguer anomaly map of the northern North Sea between the Shetland Islands and Norway has been compiled based on four different data sources. The gravity effect of the Devonian to recent sediments was calculated along six published seismic profiles crossing the area and a regional gravity anomaly map produced on the assumption that the sedimentary basin structure is two dimensional. Regional consideration of the gravity field revealed the presence of a mass excess beneath the low-density sediments within the Viking graben. This high-amplitude, long-wavelength anomaly is explained by thinning of the crust from 30 km beneath the flanks to 10 km beneath the centre of the graben, excluding the thickness of the sediments. Crustal thickness along the profiles was derived using the regional gravity field and from this a contour map of the Moho was produced. Predictions obtained from the gravity data confirm the crustal structure derived so far from seismic experiments in the North Sea. For comparison a three-dimensional model of crustal thickness was constructed and a second contour map of the Moho was produced. The Moho contour maps derived from the two-dimensional and three-dimensional models show close agreement, demonstrating that a two-dimensional approach is justified in this case.
Kinematics of the Sinai triple junction and a two-phase model of Arabia-Africa rifting
Authors: Courtillot Vincent , Armijo Rolando , Tapponnier Paul 
Published: April 2008

Summary

The breakup of Arabia from Africa started at 40–50 Ma in the East African Rift system and concentrated from about 20 Ma around the Red Sea. Presently, the most active system centres along the Gulf of Aden, Red Sea and Dead Sea shear. Two less active features, the Gulf of Suez and East African Rift meet the main boundary at what have often been considered to be active triple junctions (the southern tip of Sinai and Afar). This paper focuses on the continental Sinai triple junction near 27°30′N. Almost no reliable kinematic data can be found in the Red Sea. Only total motion based on a fit of coastlines is available. Yet, the mechanical basis for this is now known to be wrong. We propose a mechanical interpretation that retains the same pole of ARA-NUB motion, with a reduced amount of total opening. The best data come from the Dead Sea shear: 100 km of left-lateral displacement occurred over the last 20, or more likely 15 Ma (subsidence event in the Gulf of Suez). The pull-apart basins in the Gulf of Elat provide the best constraints on the azimuth of SIN-ARA motion. Four models of SIN-ARA-NUB kinematics are proposed depending on whether the Gulf of Suez is; (i) now extinct, (ii) still opening in the original direction, (iii) in a state of left-lateral shear, (iv) in a state of right-lateral shear. Total SIN-NUB motion is unlikely to exceed 1–2 mm yr
−1
. We propose a two-phase history of break-up, first along the East African Rift/Red Sea/Gulf of Suez, superseded at ∼ 15 Ma by the present Gulf of Aden/Red Sea/Dead Sea shear. Distributed extension now occurs in the Afar and northern Red Sea deforming zones towards which three localized rifts propagate. The Sinai and Afar ‘triple junctions’ are actually kinks where plate boundaries that were not active at the same time are superimposed.
Slump strain in the Tertiary of Cyprus and the Spanish Pyrenees. Definition of palaeoslopes and models of soft-sediment deformation
Authors: Farrell S. G. , Eaton S. 
Published: April 2008

Summary
Slumps exposed in Tertiary sediments in Cyprus and the Spanish Pyrenees vary in inferred translation distance from metres to hundreds and possibly thousands of metres. Contractional strain initiates open to tight upright to inclined folds with hinges that are sub-parallel to the strike of the palaeoslope. This strain is produced mainly by pure shear layer parallel shortening as the slump detachment propagates, but may involve a component of simple shear strain. Downslope translation of the failed unit commonly imparts a simple shear strain which tends to tighten folds and rotate fold hinges and fold axial surfaces. Fold hinges rotate towards the downslope direction, and fold axial surfaces rotate towards parallelism with the upper and lower surfaces of the slump. Coaxially refolded folds and sheath folds may also develop. Steeply plunging folds may initiate in slumps, due to the development of steeply dipping shear zones with margins sub-parallel to the downslope direction which accommodate differential downslope movements of segments of the slump. Fold axial surface attitude and fold profile may be used as a rough estimate of the degree of fold rotation.
Mass transfer in unmetamorphosed carbonates and during low-grade metamorphism of arenites
Authors: Clifford P. M. , Rice M. C. , Pryer L. L. , Fueten F. 
Published: April 2008

Summary
Study of stylolites in Silurian carbonates of southern Ontario, and of spaced cleavage in Lower Palaeozoic meta-arenites of Nova Scotia, shows that diffusional or infiltrational mass transfer has been the dominant mechanism for dimensional change.
The carbonates show a quite specific pattern of stylolite growth in, and normal to, the mean plane of the stylolite, and there is always a low-porosity halo about each stylolite. Implied shortening usually exceeds the amplitude of any teeth in stylolites and may reach 5% of the ‘local’ sequence.

In the meta-arenites, increased inequancy, decreased long-axis fluctuation of quartz grains, lithon to cleavage, coupled with virtually constant grain dimension parallel to cleavage, suggest gross loss of volume. Modal and chemical analyses suggest that an average cleavage zone represents a dilation of −50%, expressed as shortening normal to cleavage. Bulk loss is about 10%, almost all SiO
2
.

In both cases, mass removal begins at zero or low levels of overall deformation. For the carbonates, total deformation is near zero; the process shuts down by self-suffocation. In the meta-arenites the process was probably partly driven by the conditions associated with greenschist-level metamorphism. In both cases, collapse across the surfaces of dissolution is nicely adjusted to keep the gross shortening approximately homogeneous.
The Kleszczów Graben (central Poland): reconstruction of the deformational history and inventory of the resulting soft-sediment deformational structures
Authors: Brodzikowski Krzysztof , Gotowała Roman , Kasza Ludwik , Van Loon Antonius J. 
Published: April 2008

Summary
There is a deep tectonic graben near the village of Kleszczów, some 50 km south of Łódź. A Mesozoic substratum has subsided in the graben. The infilling consists of unconsolidated Tertiary and Quaternary deposits that reach thicknesses of up to 300 m; locally they may be even more than 400 m thick. The equivalent deposits outside the graben area are 5–7 (or more) times thinner.
The lowermost part of the graben contains the oldest infilling, probably indicating the very first moment of subsidence, and consists of late Oligocene deposits. The overlying sediments form an almost complete succession up to the Holocene and represent the best developed and most intensively studied Cenozoic section in Poland. The sections show a unique degree of exposure in continuous horizontal and vertical sections, well developed deformational structures and an exceptional thickness of the Cenozoic succession. There has been much interest in the geological history of the intensely deformed infilling of the graben. The reconstruction of the sedimentological and deformational history is the main topic of this contribution.
Endogenic processes as a cause of penecontemporaneous soft-sediment deformations in the fluviolacustrine Czyżów Series (Kleszczów Graben, central Poland)
Authors: Brodzikowski Krzysztof , Krzyszkowski Dariusz , Van Loon Antonius J. 
Published: April 2008

Summary
Thick fluvial and lacustrine deposits of Quaternary age are rare in Europe. One such succession is found in the Kleszczów Graben. The high content of organogenic material is a specific characteristic. One of the major fluviolacustrine units in the graben is the Czyżów Series, formed during a Pleistocene interstadial. The series is characterized by abundant penecontemporaneous deformation structures whose formation was favoured by both the grain-size distribution (with a relatively high silt content) and repeated tectonic displacements that served as a trigger mechanism. The analysis of these structures forms the main topic of the present contribution. Endogenic activity within the graben emerged as the main cause not only of ‘normal’ tectonic deformations, but also of a large number of deformations that are usually considered to have a sedimentary or non-tectonic diagenetic origin.
Flame structures and associated deformations in Quaternary glaciolacustrine and glaciodeltaic deposits: examples from central Poland
Authors: Brodzikowski Krzysztof , Hałuszczak Andrzej 
Published: April 2008

Summary
Glaciolacustrine and glaciodeltaic sediments in the Quaternary of the Kleszczów Graben show many types of penecontemporaneous deformations. Their formation was due to the existence of important vertical anisotropies of the mechanical properties. Flame structures and associated types of deformations occur frequently. These structures are described in this contribution and their origins analysed.
Alkaline rocks and their inclusions: a window on the Earth’s interior
Authors: Menzies Martin 
Published: April 2008

Summary
The majority of inclusion-bearing alkaline rocks have a source region identical with that of ocean island basalts regardless of whether they were erupted through oceanic or continental crust. Models have been proposed where oceanic alkaline rocks are derived from the asthenosphere and during passage to the surface there have occurred variable amounts of interaction with lithosphere of the mid-ocean ridge basalt type. A similar scenario would account for most inclusion-bearing continental alkaline rocks. Genesis of micaceous kimberlites and lamproites, however, may require that asthenospheric plumes have impinged on ‘nuggets’ of aged heterogeneous enriched sub-continental lithosphere. This would lead to mixing of isotopically distinct asthenospheric and lithospheric melts.