Differential Shortening Research Articles

Bimodal volcanism in a tectonic transfer zone: Evidence for tectonically controlled magmatism in the southern Central Andes, NW Argentina

This field-based and analytical laboratory study focuses on the genetic relationship between bimodal volcanic centres and fault types of an important tectonic transfer zone in the southern Central Andes, the NW–SE striking Calama–Olacapato–Toro (COT) volcanic belt. More specifically, tectono–magmatic relationships are examined for the 0.55 Ma Tocomar, the 0.78 Ma San Jerónimo and the 0.45 Ma Negro de Chorrillos volcanic centres in the Tocomar area (66°30 W–24°15 S). Structures of the COT volcanic belt, notably NW–SE striking strike-slip faults and NE–SW trending normal faults, accommodated differential shortening between major N–S striking thrust faults on the Puna Plateau. We present evidence that bimodal volcanism was contemporaneous with activity of these fault types in the COT volcanic belt, whereby eruption and composition of the volcanic rocks in the Tocomar and San Jerónimo–Negro de Chorrillos areas appear to have been controlled by the kinematics of individual faults. More specifically, rhyolitic centres such as the Tocomar are associated with normal faults, whereas shoshonitic–andesitic monogenetic volcanoes, e.g., the San Jerónimo and Negro de Chorrillos centres, formed at strike-slip dominated faults. Thus, the eruption of higher viscous rhyolite magmas appears to have been facilitated in tectonic settings characterized by horizontal dilation whereas ascent and effusive volcanic activity of less viscous and hot basaltic andesites to shoshonites were controlled by subvertical strike-slip faults. While the Tocomar rhyolites are interpreted to be derived from an anatectic crustal source, geochemical characteristics of the San Jerónimo and Negro de Chorrillos shoshonitic andesites are in agreement with a deeper source. This suggests that the composition of erupted volcanic rocks as well as their spatial distribution in the Tocomar area is controlled by the activity of specific fault types. Such volcano–tectonic relationships are also evident from older volcanic centres of the COT volcanic belt. This points to a close genetic relationship between bimodal volcanism and the upper-crustal deformation regime, whereby episodes of increased volcanic activity correlate with deformation episodes in the Central Andes.

Simplon fault zone in the western and central Alps: Mechanism of Neogene faulting and folding revisited

The Neogene Simplon fault zone is commonly either considered as a transfer zone for dextral strike-slip movements during oblique convergence or as a core complex–type normal fault leading to orogen-parallel extension in the Alps. There is, however, evidence that the Simplon fault zone lacks a southeastward continuation. On the basis of new structural and fission-track data, we propose a model of oblique indentation of a bent Adriatic indenter leading to the formation of a curved continuous wedge of backfolds in the south, not including a southern continuation of the Simplon fault zone. Updoming related to backfolding, followed by erosion, assisted exhumation of amphibolite facies rocks. In the north, indentation led to differential shortening in the footwall and hanging wall.

37층 초고층주상복합건물 SRC기둥의 기둥축소량 현장계측 및 보정법

Long-term axial shortening of the vertical elements of tail buildings results in differential movements between two elements and may lead to the additional moments of connection beam and slab elements, and other secondary effects, such as cracks of partitions or curtain walls. Accurate prediction of time-dependent column shortening is essential for tall buildings from both strength and serviceability aspects. The compensation method is different from reinforced concrete and SRC(Steel Reinforced Concrete) members. The SRC columns are usually compensated according to total differential shortening between two vertical elements. In this study, column shortenings of 37-story W building under construction are predicted and compensated. The SRC column shortenings are compared with the actual column shortening by field measurement and the column shortenings are reanalysed and recompensated.

Effect of Relative Humidity on Absolute and Differential Shortening of Columns and Walls in Multistory Reinforced Concrete Buildings

In tall, multistory reinforced concrete buildings, absolute and differential shortening between adjacent vertical members has become a significant problem during the recent past due to the use of high performance concrete and more advanced design techniques that resulted in smaller member sizes. The failure of nonstructural elements such as claddings has been observed in several structures constructed in the past few decades. The factors that cause these problems could be identified as time-dependent creep and shrinkage coupled with elastic shortening. Both creep and shrinkage depend on the relative humidity of the surrounding environment. This paper discusses the effect of relative humidity on absolute and differential shortening of vertical elements in reinforced concrete buildings.

The evolution of an exposed mid-lower crustal attachment zone in Fiordland, New Zealand

Abstract Studies of convergent margins suggest that large subhorizontal shear zones in the lower crust help regulate how displacements are transferred horizontally and vertically through the lithosphere. We present structural data from the Fiordland belt of SW New Zealand that illustrate the progressive evolution of a 25 km thick section of exhumed, Early Cretaceous middle and lower crust. The data show that the mechanisms by which displacements were relayed through the crust during a 25 Ma cycle of arc-related magmatism, high-grade metamorphism and contraction changed repeatedly. During the period 126–120 Ma, a ≥10 km thick batholith composed of gabbroic-dioritic magma was emplaced into the lower crust. Melt-enhanced shear zones evolved at the upper and lower contacts of the batholith where magma and steep temperature gradients created strength contrasts. By ∼ 120 Ma, partial melting of mafic-intermediate lower crust resulted in the formation of high-pressure (14–16 kbar) migmatite and steep, regionally extensive vein networks up to 10 km below the batholith. Melt segregation and transfer through and out of the lower crust were aided by melt-induced fracture arrays and ductile deformation in shear zones. During the period 116–105 Ma, differential shortening of the crust produced a network of subhorizontal and subvertical shear zones at different crustal depths. Near-vertical shear zones up to 15 km wide formed at the deepest part of the section. These shear zones cut upwards across the entire lower crust to merge with a gently dipping upper amphibolite facies fold-and-thrust zone that formed in the middle crust. A 1 km thick, subhorizontal shear zone underlies this mid-crustal fold-and-thrust zone and physically connected shear zones that formed at different crustal depths. Our data suggest that deformation above and below this mid-lower crustal attachment zone was coupled kinematically and accommodated subhorizontal arc-normal displacements in the middle crust and oblique sinistral displacements on steep shear zones in the lower crust. The steep lower crustal shear zones also record components of subhorizontal arc-normal shortening and vertical thickening. These results strongly suggest that large, kinematically coupled networks of flat and steep shear zones separated the Fiordland crust into distinctive structural domains and relayed displacements vertically and horizontally through the lithosphere during Early Cretaceous oblique convergence.

Optimal compensation of differential column shortening in high‐rise buildings

AbstractTo avoid unexpected damage in structural and nonstructural elements, differential shortening between vertical members resulting from differing stress levels, loading histories, volume‐to‐surface ratios and other factors in a high‐rise building must be properly considered in the design process. While research activity has been reported in the literature on the development of estimation algorithms or prediction procedures for the elastic and inelastic shortenings of vertical members, no algorithms or systematic methods for the compensation of the differential shortenings have been reported. In this paper, a compensation method for the differential column shortening in a high‐rise building is formulated into an optimization problem. A simulated annealing algorithm is used to find optimal solutions. The proposed method is applied to the compensation of the differential shortening of the vertical members in two high‐rise buildings, including one verifying example of a 70‐storey building and a practical example of a 63‐storey building. As demonstrated in the examples, the differential shortenings of the examples are effectively controlled by the optimal compensation method. Copyright © 2003 John Wiley & Sons, Ltd.

Paleomagnetism indicates no Neogene rotation of the Qaidam Basin in northern Tibet during Indo-Asian collision

Paleomagnetic data were obtained from Tertiary red sedimentary rocks at two locations separated by several hundred kilometers within the Qaidam Basin. In the east-central part of the basin, 30 sites from the lower Pliocene Youshashan Formation yielded characteristic remanent magnetization (ChRM) directions with intermediate unblocking temperatures (100‐600 8C); ChRM with high unblocking temperatures (to 680 8C) was isolated from 14 sites. In the same area, ChRM directions were obtained from six sites within the Oligocene Lower Gancaigou Formation. Characteristic magnetization was also determined from 16 sites within the Lower Gancaigou Formation exposed in the E Bo Liang range of the north-central Qaidam Basin. When compared with equivalentage expected directions for Eurasia, the mean paleomagnetic directions indicate no Neogene vertical-axis rotation of the Qaidam Basin or the Altyn Tagh fault. The Qaidam Basin may act as an indentor translating without rotation toward the North China block and driving clockwise vertical-axis rotations by differential shortening within the Nan Shan fold-and-thrust belt.

Enforced polarisation and locomotion of fibroblasts lacking microtubules.

The polarisation and locomotion of fibroblasts requires an intact microtubule cytoskeleton [1]. This has been attributed to an influence of microtubule-mediated signals on actin cytoskeleton dynamics, either through the generation of active Rac to promote protrusion of lamellipodia [2], or through the modulation of substrate adhesion via microtubule targeting events [3] [4]. We show here that the polarizing role of microtubules can be mimicked by externally imposing an asymmetric gradient of contractility by local application of the contractility inhibitor ML-7. Apolar fibroblasts lacking microtubules could be induced to polarize and to move by application of ML-7 by micropipette to one side of the cell and then to the trailing vertices that developed. The release and retraction of trailing adhesions could be correlated with a relaxation of traction on the substrate and a differential shortening of stress-fibre bundles, with their distal tips relaxed. Although retraction and protrusion in these conditions resembled control cell locomotion, the normal turnover of adhesion sites that form behind the protruding cell front was blocked. These findings show that microtubules are dispensable for fibroblast protrusion, but are required for the turnover of substrate adhesions that normally occurs during cell locomotion. We conclude that regional contractility is modulated by the interfacing of microtubule-linked events with focal adhesions and that microtubules determine cell polarity via this route.

Physical explanation of the formation and evolution of inversion zones and marginal troughs

Inversion zones are elongate structures, some tens of kilometers wide and up to hundreds of kilometers long, that have deformed in response to compression and produced topography. Inversion zones in the Alpine foreland are mainly associated with Mesozoic grabens and troughs, and although very important in the geologic picture, the conditions of their formation and evolution and their regional geologic significance are not entirely understood. The internal structure of inversion zones is variable and depends on details in the pre-inversion setting, the inversion-inducing stress field, and the sedimentary fill. However, on a larger scale, most inversion zones share certain principal observational features, which sample the physical structure and the rheologic properties of the lithosphere and thereby provide an opportunity to test hypotheses of lithospheric rheology and dynamics. The quantitative model presented in this paper explains how inversion zones and the associated marginal troughs are related to lithospheric zones of differential shortening and regional isostatic compensation of the induced topography.

On the mechanism of crustal block rotations in the Central Andes — Comment

In this paper Beck (1998)reviews the database of palaeomagnetically observed rotations in the Andean margin of South America, in particular with respect to the well known change in sense of rotation linked to the Arica deflection (Bolivian Orocline) in the orogen. The paper argues firstly for the latitudinal distribution of rotations supporting an interpretation that distributed shear combined with oroclinal bending is the best hypothesis to explain the observed pattern of rotations and secondly that this rotation/deformation is time-dependent. While we essentially agree with the former conclusion we would define it more tightly in that the over-arching control on the pattern of rotations is a combination of differential shortening, as opposed to oroclinal bending, along the orogen coupled with distributed shear across the orogen. Our main concern with this paper is that it fails to provide a convincing demonstration that there is a time-dependent element to the magnitude of rotations. Primarily we will argue that it is far from clear from the palaeomagnetic data presented that there is a case for a continuous rate of rotation applicable to the southern limb of the orocline, at least. Not withstanding this we would also argue that geologically the deformation cannot and should not be described by a continuous deformation rate over time, as postulated. We also make specific points about aspects of the paper dealing with the Atacama Fault Zone and its relation to the observed rotations.

SARCOMERE SHORTENING DURING FIXED-END CONTRACTION OF FROG SEMITENDINOSUS MUSCLES

1051 Activation of an intact muscle-tendon unit at a fixed length results in sarcomere shortening at the expense of tendon lengthening. Series elasticity permits muscle-tendon units to increase active range, absorb length changes, and to absorb energy thus increasing locomotion efficiency. However, sarcomere shortening at the expense of tendon elongation has not been quantified. The purpose of this study was to measure sarcomere shortening as a function of initial sarcomere length in the intact muscle-tendon-bone (MTB) complex of Rana pipiens semitendinosus (ST) muscles. MTB complexes (n=6) were dissected from R. pipiens after double pithing. Muscles were activated at nine lengths (−3 mm to +6 mm relative to optimal length in 1 mm intervals), while measuring sarcomere length by laser diffraction. Sarcomere shortening with muscles at long sarcomere lengths (∼3.3 μm) was significantly smaller (0.039±.014 μm) compared to sarcomere shortening at short (∼2.0 μm) sarcomere lengths (0.32±.024 μm). The functional consequence of this differential shortening as a function of sarcomere length was to skew the muscle length-tension relationship to longer lengths and to slightly underestimate maximum tetanic tension. The effect was relatively small for the ST muscle but simulations reveal the result would be much greater for muscles with long in-series tendons. This work was supported by NIH grant AR40050 the VA and the Mizuno International Sports Exchange Foundation.

Geometry and kinematics of vertical-axis rotations in fold and thrust belts

Vertical-axis rotations indicated by palaeomagnetic study are a common and important component of deformation in fold and thrust belts. The kinematics of rotating thrust sheets may be best described as a combination of a far-field rotation (or translation in a planar geometry) and a near-field, or local rotation, so that the instantaneous rotation axis lies outside the body of the sheet. This simplifies the thrust-sheet kinematics significantly, as it means that all the points on the sheet have a component of forward motion, and the bounding fault is a thrust over all its area. The consequences of a simple thrust-sheet rotation for the interpretation of fault lineation data are investigated. The lineations recorded in the footwall and the hangingwall are likely to be different, the hangingwall lineations being skewed towards the direction of rotation. The principal mechanism for local thrust-sheet rotation is differential shortening, either on folds or thrusts, or by a combination of these structures. The forward propagation of thrusting towards the foreland can accumulate large (>120°) rotations on the uppermost sheets. These rotational thrust sheets cannot extend indefinitely along strike; instead they terminate against lateral ramps or transfer faults. Sidewall duplexes may form as a consequence of rotation over a lateral ramp. Examples of rotational geometries from the Subbetic Zone of southern Spain are described. On a larger scale, obliquely convergent orogenic wedges can evolve by rotation of faults with an initial thrust geometry into progressively greater components of strike-slip motion. Reliable palaeomagnetic data are not always available for fold and thrust belts, so independent evidence for rotational deformation is of great importance. Fold and thrust geometries, particularly along-strike variability in fold geometries, highly variable axial traces of folds, evidence for near-synchronous refolding and the deviation of regional trends may all be a consequence of thrust-sheet rotation. Detailed mapping, and particularly the identification of the position of footwall and hangingwall cut-offs can also provide strong evidence for rotation.

Structure of the Alice anticline, Papua New Guinea: serial balanced cross-sections and their restoration

The structure of the western part of the Papuan Fold Belt is characterised by both thin-skinned thrusting and basement involved structures. Basement involvement is attributed to inversion of earlier formed extensional faults. The Alice anticline is a frontal structure of the Papuan Fold Belt and formed due to inversion of a Tertiary extensional fault system. Complex forelimb geometry of the Alice anticline changes markedly along strike and suggests varying amounts of shortening in this part of the structure. Balanced cross-sections quantify differential shortening along the anticline, whereas palinspastic restoration shows that the differential shortening is compatible between cross-sections. Three-dimensional restoration of the Alice anticline makes use of a series of balanced cross-sections and is based on a line-length method. Paradoxically, the restoration reveals non-plane strain in the balanced cross-sections upon which it relies. However, the restoration also reveals and quantifies a component of rotation about vertical axes which would not be detected by application of conventional methods of structural analysis. Rotations about vertical axes are attributed to pinning of progressive, foreland propagating deformation. The distribution of rotations about vertical axes suggest that zones of pinning are coincident with relay zones in the early extensional fault geometry. Two relay zones associated with the original geometry acted as obstructions to deformation and have effectively pinned contractional structures during their formation causing the rotations about vertical axes. Such rotations are not manifested in changing fold axis orientations and have only been derived from a consideration of material balance during deformation.

Major crustal rotations in the Andean margin: Paleomagnetic results from the Coastal Cordillera of northern Chile

Paleomagnetic analyses of Mesozoic lavas and dike swarms from the northern Chilean Coastal Cordillera, between 25.4°S and 26.4°S, reveal a clockwise rotation of about 42°. Magnetizations from lava flows of andesitic‐basaltic composition of the Middle Jurassic La Negra Formation pass both fold and reversal tests and are interpreted as prefolding remanences. Five dike swarms of Middle Jurassic to Early Cretaceous age yield similar directions to that obtained from the La Negra Formation. Four of the five swarms have mixed polarity, suggesting that they too carry a primary or very early remanence. The structural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the remanence. The clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid–Late Cretaceous age, post‐100 Ma, associated with abandonment of the Jurassic–Early Cretaceous magmatic arc in this region and its eastward migration to form a new mid–Late Cretaceous magmatic arc in the former back arc region. This younger arc is located east of the Coastal Cordillera and lies in the Central Valley region. The clockwise sense of rotation is consistent with other paleomagnetic data from northern Chile and southern Bolivia, south of the Arica Deflection in the Andean margin, although it is the largest yet reported. To the north of the Arica Deflection, paleomagnetic studies report counterclockwise rotations, and several large‐scale models have been proposed to explain the overall pattern of rotations. Models include oroclinal bending of an originally straight margin, differential shortening across the margin at a preexisting bend which is subsequently tightened by the passive rotation of the limbs of the bend, and distributed shear throughout the margin as a consequence of oblique convergence at a preexisting bend. In contrast to these models, several workers have argued that rotation is better explained in terms of localized in situ rotations. We review these models in light of our results and present a domino‐type model with blocks bounded by left‐lateral faults and rotating clockwise in response to mid‐Late Cretaceous transpression within a crustal scale shear zone. This is consistent with the observed strike‐slip fault systems identified in the Coastal Cordillera.

Geometry and state of stress of the Nazca plate beneath Bolivia and its implication for the evolution of the Bolivian orocline

Research Article| May 01, 1996 Geometry and state of stress of the Nazca plate beneath Bolivia and its implication for the evolution of the Bolivian orocline Mark Andrew Tinker; Mark Andrew Tinker 1Southern Arizona Seismic Observatory, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Terry C. Wallace; Terry C. Wallace 1Southern Arizona Seismic Observatory, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Susan L. Beck; Susan L. Beck 1Southern Arizona Seismic Observatory, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Stephen Myers; Stephen Myers 1Southern Arizona Seismic Observatory, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Andrew Papanikolas Andrew Papanikolas 1Southern Arizona Seismic Observatory, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Author and Article Information Mark Andrew Tinker 1Southern Arizona Seismic Observatory, University of Arizona, Tucson, Arizona 85721 Terry C. Wallace 1Southern Arizona Seismic Observatory, University of Arizona, Tucson, Arizona 85721 Susan L. Beck 1Southern Arizona Seismic Observatory, University of Arizona, Tucson, Arizona 85721 Stephen Myers 1Southern Arizona Seismic Observatory, University of Arizona, Tucson, Arizona 85721 Andrew Papanikolas 1Southern Arizona Seismic Observatory, University of Arizona, Tucson, Arizona 85721 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (5): 387–390. https://doi.org/10.1130/0091-7613(1996)024<0387:GASOSO>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Mark Andrew Tinker, Terry C. Wallace, Susan L. Beck, Stephen Myers, Andrew Papanikolas; Geometry and state of stress of the Nazca plate beneath Bolivia and its implication for the evolution of the Bolivian orocline. Geology 1996;; 24 (5): 387–390. doi: https://doi.org/10.1130/0091-7613(1996)024<0387:GASOSO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A series of deep earthquakes beneath Bolivia in 1994 provides valuable constraints on the geometry and state of stress of the subducting Nazca plate. These earthquakes occurred northeast of the Bolivian orocline, which is bracketed between two regions of a “flattened” Wadati-Benioff zone. Furthermore, these events occurred in an area that experienced no seismicity in at least the past 30 years. North and south of this zone, the deep seismicity is abundant and indicates down-dip compression. Collectively, the seismicity defines the geometry of the deep Nazca slab, which is continuous and bent about a sharp arc. This bending is a function of the dynamic evolution of the South America–Nazca plate system. Since the Tertiary, South America has incurred a maximum shortening along the Bolivian orocline. North and south of the orocline, the South American crust has shortened less; the differential shortening occurs in conjunction with the flattened and bent Wadati-Benioff zone. The upper Nazca slab has undergone a tremendous amount of horizontal deformation that is controlled by the convergence direction relative to the trend of the trench. In response, the lower slab has been slowly bent about an axis parallel to the dip orientation of the deep Nazca plate. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

New insights into the Appalachian Orogen from geology and geochronology along the Cape Ray fault zone, southwest Newfoundland

Reverse-oblique overthrusting of high-grade Port aux Basques Gneiss over the lower-grade, supracrustal Windsor Point Group along the Cape Ray fault zone occurred during late Silurian to early Devonian time and is attributed to major oblique collision between Gondwanan and Laurentian continental blocks. New U-Pb zircon, 40 Ar/ 39 Ar, and K/Ar data limit the timing of this orogenic event. The megacrystic granitic phase of the Cape Ray Igneous Complex and the Cape Ray tonalite have been dated at 488 ± 3 Ma and 469 ± 2 Ma, respectively. A black rhyolite from the Windsor Point Group yielded an Ordovician age (453 +5/−4 Ma). A pretectonic gabbro sill within the Windsor Point Group, dated at 424 +4/−3 Ma, is cut by the Windowglass Hill granite dated at 424 ± 2 Ma. Synkinematic hornblende and biotite from the Grand Bay Complex mylonites of the Port aux Basques Gneiss have 40 Ar/ 39 Ar ages of 407 ± 4 Ma and 403 ± 4 Ma, respectively, which is indicative of a late Silurian–early Devonian age for the amphibolite grade deformation. These ages are compatible with a previously determined 415 Ma monazite age, interpreted as the time of peak metamorphism in the Port aux Basques Gneiss, and suggests rapid exhumation. The east-west flexure of the Cape Ray fault zone represents a tear fault accommodating differential shortening between the Cape Ray fault zone and the correlative fault system in central Newfoundland. Mid-Devonian granites (386 ± 3, 384 ± 2 Ma) intervene between the thrusting events and a younger sinistral transcurrent movement. This deformation and magmatic history can be correlated throughout the northern Appalachians along other major fault boundaries.

Structure and Kinematic Genesis of the Quealy Wrench Duplex: Transpressional Reactivation of the Precambrian Cheyenne Belt in the Laramie Basin, Wyoming

Seismic and borehole data in the southern Laramie basin of southeastern Wyoming outline a fault-bordered pop-up structure that is central to the Quealy wrench duplex and the site of the Quealy Dome oil field. This contractional duplex formed at a restraining fault offset between two parallel, laterally extensive, northeast-trending fault zones--the South and North Quealy faults. Spaced about 2 mi (3.2 km) apart, these parallel fault zones are interpreted as wrench straights, equivalent to the floor and roof thrusts of a duplex in a fold and thrust belt, but described in map view rather than in cross section. The Quealy wrench duplex is comprised of three east-vergent, basement-involved thrust imbricates and a west-vergent, antithetic thrust, detached in Permian shales. Duplex thrusts strike at high angles to the bordering South and North Quealy fault zones. Measurements based on interpretive piercing-point evidence and fault-parallel, differential shortening indicate that net slip on the dominant South Quealy fault zone is dextral-oblique. A component of dextral slip is also indicated on the North Quealy fault zone. The South Quealy fault and North Quealy/Overland fault trends can be traced southwesterly into the footwall of the north-trending Arlington (basin-boundary) thrust and projected into the Precambrian Cheyenne belt of the Medicine Bow Mountains in the hanging wall of the Arlington thrust. It is proposed that the Quealy wrench duplex is a product of Precambrian shear zone reactivation along the Cheyenne belt under Permian and Laramide (Tertiary) transpression.

The Falkland Islands: A key element in Gondwana paleogeography

Stratigraphic and sedimentological fieldwork on the Falkland Islands confirms that they were originally part of South Africa. East and West Falkland have contrasting geological and deformation histories and are separated by the Falkland Sound fault, which marks a significant deformation edge to the Cape Fold Belt. The degree of structural deformation and metamorphism in West Falkland is substantially less than that of East Falkland, adjustments to differential shortening between the two islands being reflected by structures developed along the Falkland Sound fault. Prior to fragmentation of Gondwana, the Falkland Platform and Maurice Ewing Bank were positioned in the Natal Valley, adjacent to the east coast of South Africa. This predrift reconstruction shows that the Falkland Islands have subsequently been rotated by 180° relative to South Africa and complements recent paleomagnetic data. There was a complete overlap of the Maurice Ewing Bank and Falkland Platform to the exclusion of the Falkland Plateau Basin which is a small oceanic basin formed during the Jurassic fragmentation of Gondwana. Extension in this basin was accommodated by movements along the Aghulas Falkland Fracture Zone and the Gastre Fault System. Predrift restoration of these faults shows that Patagonia was positioned closer to both Africa and Antarctica. The Falkland Plateau Basin was an integral component of the West Gondwana Jurassic microplate system and provides an invaluable insight into the evolution of its western margin. At some time prior to the main opening of the South Atlantic, the Falkland Platform was rotated clockwise by 120° against the Agulhas Falkland Fracture Zone, such that the Falkland Islands were translated some 300 km to the south. The driving force for this rotation is not known but could possibly have been initiated by periodic stress buildups in the Falkland Plateau Basin as the Gastre Fault System, within continental crust, became unable to accommodate the continuous dextral shear in the Agulhas Falkland Fracture Zone. The Falkland Islands represent an outboard extension of the Eastern Cape and are significant for understanding the paleogeography of both the Cape and Karoo supergroups. East Falkland is closely similar to South Africa in its geology, while West Falkland shows significant contrasts with both the Devonian and Permian successions being more proximal in character but still demonstrate the eastward continuation of the Cape and Karoo basins.

Vertical-axis rotation associated with folding and thrusting: An example from the eastern Subbetic zone of southern Spain

The differential rotation about a vertical axis across a southeast-vergent recumbent fold and thrust fault within a single nappe from the eastern Subbetic zone of southern Spain has been determined using paleomagnetism. The upper limb has rotated clockwise by 68° ±22° relative to the lower limb. Mapping of the structure indicates that the syncline is an isoclinal fold in the northeast and an open fold in the southwest. The differential vertical-axis rotation across this structure appears to have been accommodated by a pro- gressive increase in along-strike horizontal shortening, reaching 8.5 km at 3.7 km from the pivot point. This differential shortening and the rotation are probably the result of increased drag on the underlying thrust close to the pivot point compared to the farthest extent of the structure. The lateral boundary of the rotating sheet is a sinistral strike-slip fault, which is consistent with the rotational geometry. Pinning is probably enhanced by lateral heterogeneities within the stratigraphy. This result ties rotational deformation to early-middle Miocene folding and thrusting, rather than to late Miocene- Holocene strike-slip deformation. Rotation by pinning of thrust sheets may prove to be an important aspect of deformation in many fold and thrust belts.

An ecomorphological review of the dodo(Raphus cucullatus)and solitaire(Pezophaps solitaria),flightless Columbiformes of the Mascarene Islands

This paper describes a morphological study of the dodoRaphus cucullatusand solitairePezophaps solitariaextinct, flightless Columbiformes of the Mascarene Islands, Indian Ocean—based on mensural data from 387 skeletal elements, comparative data from four flighted species of Columbidae, ancillary mensural data from other Columbiformes, and the literature.Raphus cucullatus and P. solitaria are characterized by great body size and substantially reduced pectoral limbs. Sexual size dimorphism is unusually great in bothR. cucullatusandP. solitaria,and sexual dimorphism ofP. solitariamay be the greatest of any carinate bird. Estimates of body mass (kg), based on femur lengths of flighted columbids and adjusted for flightlessness and seasonal deposition of fat, were: 21 and 17 for male and femaleR. cucullatus,and 28 and 17 for male and femaleP. solitaria,respectively. Pectoral reduction is greater in R. cucullatus than in P. solitaria, but both species are characterized by differential shortening of wing elements, changes in the sternum and scapulocoracoidal angle. R. cucullatus has disproportionately long femora, short tarsometatarsi and long digits, whereas P. solitaria has disproportionately long tibiotarsi, short tarsometatarsi and short digits; relative shaft widths of leg elements are substantially greater in bothR. cucullatusandP. solitariathan in flighted columbids.As is typical of other birds, giantism ofR. cucullatusandP. solitariaprobably was associated with physiological changes, increased longevity, enhanced thermodynamic efficiency and improved capacity for fasting. Evidently bothR. cucullatusandP. solitariawere primarily frugivorous, and foraging‐related morphological peculiarities include an enlarged crop and the retention of ‘gizzard stones’. Both species had clutch sizes of one, and egg masses of the flightless species approximated that predicted for a flighted columbid of equal size. Extreme sexual dimorphism and territoriality ofP. solitariasuggest that the species may have been polygynous and perhaps lek‐breeding. The description of ‘gigantic immaturity’ inR. cucullatusby Strickland &amp; Melville (1848) probably represents the first recognition of paedomorphosis in any species of bird. Pectoral underdevelopment and (in at leastR. cucullutus) comparatively ‘juvenile’ plumage in adults substantiate the role of paedomorphosis in the ontogeny of flightlessness inR. cucullatusandP. solitaria;both species also were characterized by peramorphic skulls, trunks and pelvic appendages. The common assumption of monophyly of the fligHtless species (it. Raphidae) lacks rigorous analytical support. In spite of the anthropogenous extinction of both species, bothR. cucullarusandP. solitariawere evolutionarily innovative in ontogeny, morphological characters and life‐history strategies.