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The gold-endowed Larder Lake Cadillac deformation zone (LLCDZ) in the Abitibi greenstone belt of the Archean Superior Province generally trends east but swings to the southeast along the Malartic segment in Quebec. Immediately north of the LLCDZ along this segment, deformation of the less than ca. 2687 Ma metasedimentary Cadillac Group began with the formation of isoclinal [F.sub.1] folds, which are overprinted by post-2676 [+ ou -] 2.0 Ma regional [F.sub.2] folds with an axial planar [S.sub.2] cleavage oriented parallel to the trend of the Malartic segment. Auriferous quartz veins strike east-west (085[degrees]) anticlockwise to bedding and [S.sub.2]. They are folded into sigmoidal S-shaped folds due to sinistral shear parallel to bedding. Later dextral shearing segmented the veins and folded the [F.sub.2] folds into Z-shaped [F.sub.3] folds. The veins are similar in structural chronology, fluid isotopic composition (calculated [Please download the PDF to view the formula] value between-9.5[per thousand] and-3.4[per thousand] and [Please download the PDF to view the formula] values of 12[per thousand]), and isotopic re-equilibration temperatures (469 and 498 [degrees]C, quartz-biotite oxygen thermometer) to the veins at the nearby Canadian Malartic deposit to the east. They possess comparable Au-As-Sb association and sericite-arsenopyrite alteration halo, as other Cadillac-Group-hosted veins at the Lapa mine to the west. Collectively, they constitute a vein field that extends across the entire length of the Malartic segment. Contrary to recent interpretations of the Malartic segment as an early accretionary structure that controlled the emplacement of gold deposits, its southeast trend is interpreted as the short limb of a broad Z-shaped flexure that formed during later [F.sub.3] folding of the LLCDZ. Key words: gold deposit, Malartic, Cadillac goup, Larder Lake Cadillac deformation Zone, Abitibi greenstone belt La zone de déformation aurifère de Larder Lake Cadillac (ZDLLC) dans la ceinture de roches vertes de l'Abitibi de la province archéenne du Supérieur s'étend généralement d'ouest en est, mais bifurque vers le sud-est le long de son tronçon de Malartic, au Québec. Immédiatement au nord de la ZDLLC le long de ce tronçon, la déformation du Groupe métasédimentaire de Cadillac, de < 2687 Ma environ, a commencé avec la formation de plis isoclinaux [F.sub.1], sur lesquels se sont surimposés, après 2676 [+ ou -] 2,0 Ma, des plis régionaux [F.sub.2] associés à un clivage de plan axial [S.sub.2] de direction parallèle à l'orientation du tronçon de Malartic. Des filons de quartz aurifères sont orientés est-ouest (085[degrees]), antihoraires par rapport au litage et à [S.sub.2]. Ils sont déformés en des plis sigmoïdaux en S résultant d'un cisaillement senestre parallèle au litage. Un cisaillement dextre subséquent a segmenté les filons et replié les plis [F.sub.2] en des plis [F.sub.3] en Z. Les filons sont semblables sur le plan de la chronologie structurale, de la composition isotopique des fluides (valeur calculée de [Please download the PDF to view the formula] de-9,5 à-3,4 [per thousand] et valeurs de [Please download the PDF to view the formula] de 12 [per thousand]) et des températures de rééquilibration isotopique (469 et 498 [degrees]C, thermomètre basé sur l'oxygène dans le quartz et la biotite) aux filons dans le gisement Canadian Malartic non loin à l'est. Ils présentent une association Au-As-Sb et un halo d'altération à séricite-arsénopyrite semblables à d'autres filons dans le Groupe de Cadillac observés à la mine Lapa à l'ouest. Collectivement, ils constituent un champ filonien qui s'étend sur toute la longueur du tronçon de Malartic. Contrairement à des interprétations récentes du tronçon de Malartic comme étant une structure accrétionnaire précoce ayant contrôlé la mise en place de gisements aurifères, son orientation sud-est nous amène à l'interpréter comme étant le flan court d'une large flexure en Z qui s'est formée durant le plissement [F.sub.3] tardif de la ZDLLC. [Traduit par la Rédaction] Mots-clés: gisement aurifère, Malartic, Groupe de Cadillac, zone de déformation de Larder Lake Cadillac, ceinture de roches vertes de l'Abitibi, Introduction The Larder Lake Cadillac deformation zone (LLCDZ) is one of two major deformation zones in the southern Abitibi greenstone belt of the Archean Superior Province. It has long been [...]

Core-mantle differentiation is the largest event experienced by a growing planet during its early history. Terrestrial core segregation imprinted the residual mantle composition by scavenging siderophile (iron-loving) elements such as tungsten, cobalt and sulphur. Cosmochemical constraints suggest that about 97% of Earth's sulphur should at present reside in the core (1), which implies that the residual silicate mantle should exhibit fractionated [sup.34]S/[sup.32]S ratios according to the relevant metal-silicate partition coefficients (2), together with fractionated siderophile element abundances. However, Earth's mantle has long been thought to be both homogeneous and chondritic for [sup.34]S/[sup.32]S, similar to Canyon Diablo troilite (3-6), as it is for most siderophile elements. This belief was consistent with a mantle sulphur budget dominated by late-accreted chondritic components. Here we show that the mantle, as sampled by mid-ocean ridge basalts from the south Atlantic ridge, displays heterogeneous [sup.34]S/[sup.32]S ratios, directly correlated to the strontium and neodymium isotope ratios [sup.87]Sr/[sup.86]Sr and [sup.143]Nd/[sup.144]Nd. These isotope trends are compatible with binary mixing between a low-[sup.34]S/[sup.32]S ambient mantle and a high-[sup.34]S/[sup.32]S recycled component that we infer to be subducted sediments. The depleted end-member is characterized by a significantly negative δ[sup.34]S of -1.28 ± 0.33‰ that cannot reach a chondritic value even when surface sulphur (from continents, altered oceanic crust, sediments and oceans) is added. Such a non-chondritic [sup.34]S/[sup.32]S ratio for the silicate Earth could be accounted for by a core-mantle differentiation record in which the core has a [sup.34]S/[sup.32]S ratio slightly higher than that of chondrites (δ[sup.34]S = +0.07‰). Despite evidence for late-veneer addition of siderophile elements (and therefore sulphur) after core formation, our results imply that the mantle sulphur budget retains fingerprints of core-mantle differentiation., Earlier reports of mid-ocean ridge basalts (MORBs) δ[sup.34]S (where δ[sup.34]S = [[([sup.34]S/[sup.32]S).sub.sample]/[([sup.34]S/[sup.32]S).sub.CDT] - 1] X 1,000 and CDT is Canyon Diablo troilite) yielded values statistically indistinguishable from the reported chondrite [...]

The core-mantle boundary of Earth is a region where iron-rich liquids interact with oxides and silicates in the mantle (1). Iron enrichment may occur at the bottom of the mantle, [...]

The opening of back-arc basins behind subduction zones progresses from initial rifting near the volcanic arc to seafloor spreading (1). During this process, the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced (with high volatile contents derived from the subducting plate) (2-5). Current models (4,6-8) predict gradational, rather than abrupt, changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction-influenced mantle. In contrast, here we show that across-strike and along-strike changes in crustal properties at the Eastern Lau spreading centre are large and abrupt, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. With incremental separation of the ridge axis from the volcanic front of as little as 5 km, seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills, upper crustal seismic velocities abruptly increase by over 20%, and gravity anomalies and isostasy indicate crustal thinning of more than 1.9 km. We infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge, such that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges (9,10), cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which that material is rapidly released from the upwelling zone, resulting in rapid changes in the character of the crust formed at the ridge., The Lau back-arc basin is a triangular-shaped extensional basin bordered by the Lau Ridge remnant arc to the west and the active Tofua arc to the east (1,2,11). Within the [...]

We present the first recorded global positioning system (GPS) data from Myanmar measured at the northern tip of the Sagaing fault. This area is in a very complex geodynamic context, where rigid and semirigid plates interact. The 12 GPS sites measured in 2005 and 2008 in northern Myanmar show that the slip rate is 18 mm/yr and is localized along a single active narrow fault trace ( doi: 10.1130/G30872.1

A new onshore-offshore three-dimensional seismic velocity model for the Island of Hawaii reveals a massive buried rift zone within Manna Loa's southeast flank, introduced here as the Ninole rift zone. This feature extends more than 60 km south of Manna Loa's summit, spans a depth range of ~2-14 km below sea level, and is the probable source of the 100-200 ka Ninole volcanics in several prominent erosional hills. The ancient rift zone may stabilize Manna Loa's southeast flank, focusing recent volcanic activity and deformation onto the unbuttressed west flank. The upper portion of the Ninole rift zone appears to have migrated westward over time, possibly triggered by landsliding, causing its eventual abandonment in preference to Manna Loa's present-day southwest rift zone. Subsequently, the lower southwest rift zone broke away, tracking rift intrusions along the trace of the Kahuku detachment fault. Similar rift zone migration is thought to be under way at Kilauea volcano, and may one day lead to the abandonment of the east rift zone. Such rift zone reconfiguration is a reflection of changing stress conditions within growing volcanoes; it is probably much more common than previously assumed, and may enable the growth of large volcanic edifices such as Manna Loa. doi: 10.1130/G30626.1

The importance of rock type and tectonic history on rates of glacial erosion, and the relative roles of glacial quarrying an abrasion, are poorly understood. We use concentrations of cosmogenic [sup.10]Be in glacial polish and measurements of bedrock fracture spacing to explore the relationship between erosion rates and rock fracturing at 23 sites along Tuolumne River valley and five sites i a Tenaya Canyon in Yosemite National Park, California, USA. Most sites yield [sup.10]Be concentrations that can be best explained as reflecting solely postglacial nuclide accumulation. Six sites, however, display anomalously high concentrations, implying incomplete removal of the pre-glacial nuclide inventory during the last glaciation; these require that erosion in the last glacial cycle was doi: 10.1130/G30576.1

Relict majorite, recognized petrographically and confirmed by in situ laser-ablation microprobe analyses, is reported from eclogites of the Tso Morari Complex, India, and represents the first record of relict majorite in the Himalaya. Previous studies have established that the Tso Morari Complex eclogites represent continental metamorphosed basic rocks within the sequence of paragneisses and metapelites interbedded with granitic augen gneisses. Our study provides evidence for subduction of the Indian continental crust to majorite-forming depths (~200 km), and even more significant, its exhumation from such depth without being completely overprinted. This estimated depth is almost double the previous depth approximations and hence has important implications on the estimation of the angle of subduction and average rates of exhumation of the Indian plate in the Himalayan collisional zone. doi: 10.1130/G30584.1

Domes of the Sumisu volcanic complex (western Pacific), having summits at ocean depths of 1100, 600, 245, and 95 m, are mantled with compositionally identical rhyolitic pumice that has similar vesicularity, but that varies systematically in distribution, size, and surface texture--suggesting that facies and morphology can serve as useful indicators of eruption depth. At depths >500 m, the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant (meters to tens of meters) polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin. Vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace. At doi: 10.1130/G30500.1

We investigated the interaction among basin-bounding faults, basin fill, and geomorphic features of the southern Gar Basin, one of only two known releasing double-bend basins along the Karakoram fault, to better understand their structural evolution and role in basin development. The southern Gar Basin is bounded by an ~44-kin-long, N20[degrees]W-striking central fault segment flanked by two N40[degrees]W-striking segments that parallel the regional strike of the Karakoram fault system. The central fault segment is composed of a system of strike-slip and normal faults that young basinward and incorporate basin fill in their uplifted footwalls. The oldest faults along the extensional portion of the bend are dominantly strike-slip, and they strike ~15[degrees]W from the main strike of the Karakoram fault. Basin fill is broadly folded about a NNW-trending axis and can be explained by E-SE-directed slip along a listric normal fault. Cross sections across the basin and associated faults suggest the geometry is best described as an extensional flower structure. Forward structural modeling of the intrabasinal faults shows that the system has accommodated ~8 km of east-west extension. We interpret the bend to have formed from linking of R and P shears into a through-going principal displacement zone. At shallow levels in the crust (low confining pressures), R shears are exploited; at deeper levels, these faults merge with the principal displacement zone, forming the extensional flower structure geometry. We estimate that the shear zone is 50-35 km wide based on the aerial distribution of P and R shears. Restoration of R shears on the west and east sides of the Gar Valley indicates ~55 km of right-lateral separation along the Karakoram fault, which is a mininmm slip estimate for the Karakoram fault system. doi: 10.1130/B26566.1

The stratigraphic architecture of the James Ross Basin, NW Weddell Sea continental shelf, shows three major phases of deposition: preglacial, ice-sheet growth, and ice-sheet dominated. The preglacial phase shows stratigraphic sequences that are similar to low-latitude margins. During the ice-sheet growth phase, there is evidence for initial ice-sheet grounding in the southern part of the basin, which migrated in steps northward. Sediment supply was also high during tile ice-sheet growth phase. During the ice sheet--dominated phase, both north and south parts of the basin inherited all the classic features indicative of glacial strata: large glacial troughs hundreds of meters deep and tens of kilometers wide, truncation of sequences overlain by chaotic seismic facies and topped with coherent laminated reflections, and an oversteepened continental slope. The James Ross Basin strata are divided into five units (S5-S1). Each unit is characterized based upon seismic facies and stratigraphic architecture, and the age of each unit is inferred from a seismic stratigraphic age model. End-member strata (S5 and S1) were drilled and dated during SHALDRIL and help to constrain this age model. In total, 34 grounding events of the Antarctic Peninsula Ice Sheet (APIS) are recorded on the continental shelf. This is similar to the 31 grounding events imaged on the Pacific side of the peninsula. Ice grounding on the continental shelf is shown to have occurred as early as late middle Miocene to early late Miocene in the southern part of the basin. The ice sheet advanced northward to encompass the northern peninsula by late Miocene time. The seven oldest glacial unconformities are believed to predate all previously identified unconformities on the peninsula continental shelf. An expanded section of late Pliocene Pleistocene deposits shows a minimum of 10 grounding events; this record more closely matches the deep-sea oxygen isotope record than previous seismic studies of Antarctic margins have shown. doi: 10.1130/B26486.1

[sup.40]Ar/[sup.39]Ar dating of muscovite, biotite, and K-feldspar combined with microstructural analysis of lower greenschist-facies, polymetamorphic, phyllitic rocks, and marbles were successfully used to decipher the thermal and tectonic histories of the Westminster and adjacent terranes in western Maryland. The presence of unreset detrital muscovite in some samples demonstrates that temperatures in these rocks never exceeded the closure temperature for argon diffusion in muscovite, ~350 [+ or -] 50[degrees]C. Minor biotite in some arkoses constrains the minimum metamorphic temperatures to [greater than or equal to] ~320[degrees]C. These data show an Early Silurian (ca. 430 Ma) cleavage in the western part of the Westminster terrane and a Late Devonian event (ca. 370 Ma) in the eastern Westminster and adjacent Potomac terranes. These two cleavage domains are separated by the NE-trending, newly identified Parrs Ridge fault zone. We propose that the sinistral transpressive collision of the Carolina terrane with Laurentia emplaced the western portion of the Westminster terrane in the Pennsylvania embayment along the Martic fault where it was folded and cleaved at ca. 430 Ma but otherwise largely sheltered from later deformation. The later Late Devonian dextral transpressive accretion of the outboard Potomac terrane thrust rocks of the eastern Westminster and Potomac terranes to the west, causing Late Devonian (360-370 Ma) [S.sub.2] cleavage in these rocks, but only minimal discrete overprinting [S.sub.3] cleavages in rocks farther west. Final juxtaposition and thermal convergence of these terranes occurred along reactivated dextral strike-slip faults in the Alleghanian at ca. 300 Ma. doi: 10.1130/B30030.1

Offshore observations at modern ocean-margin subduction zones (OMSZs) reveal that bodies of accreted material are commonly volumetrically small or missing, that crustal thinning and subsidence (3-5 km) has occurred, and that most trench axes lie close (5-30 km) to the seaward tapering edge of coastal basement rock. Onshore mapping commonly documents missing or only narrow terranes of former forearc rock and the inboard migration of the arc magmatic front. These observations are evidence that subduction is accompanied by the removal of sediment and crustal material from the submerged forearc by the kindred tectonic processes, respectively, of sediment subduction and subduction erosion. Subduction erosion truncates the margin (migrates the trench inboard) at ~2.5 km/Ma. Onshore observations at ancient crust-suturing subduction zones (CSSZs) imply that collisional suturing is accompanied by sediment subduction and truncation of both upper and lower plates. During a protracted period of suturing (20-50 million years), a 100-200 km wide (or wider) band of the seaward edge of each plate can be removed subductively. Truncation of the upper plate is effected by subduction erosion, and that of the lower plate by the necking and break-off of its subducted edge. The average linear rate of crustal loss for each plate is estimated at ~1.5 km/Ma, or ~3 km/Ma combined. Because significant crustal loss occurs before and during tectonic fusing of colliding crustal blocks, structures and rock bodies that might be expected to record a former OMSZ and the formation of a CSSZ may be absent, unimpressively small, or preserved only as exhumed masses of once deeply subducted material. Des observations au large a des zones modernes de subduction de marge oceanique revelent que des amas de materiel accrete; ont frequemment une petite taille volumetrique ou sont manquants, qu'il s'est produit un amincissement de la croute et de la subsidence (3-5 km) et que la plupart des axes des tranchees reposent a proximite (5-30 km) de la bordure, a eepaisseur deecroissante, du socle rocheux de la cote. La cartographie du continent signale souvent des terranes manquants ou seulement etroits, d'anciennes roches d'avant-arc et la migration vers le continent du front magmatique de l'arc. Ces observations constituent des preuves que la subduction est accompagneee du retrait de materiel sedimentaire et de la croute de l'avant-arc submerge par des processus tectoniques apparentes, respectivement de subduction de sediments et d'erosion de la subduction. L'eerosion de la subduction tronque la bordure (fait migrer la trancheee vers le continent) a un taux d'environ 2,5 km/Ma. Les observations sur le continent a des anciennes zones de subduction de suture de la croute impliquent que les sutures de collisions sont accompagneees de subduction de sediments et d'une troncation des plaques inferieures et superieures. Au cours d'une periode prolongee de suture (20-50 Ma), une bande de 100 a 200 km (ou plus large) de la bordure oceanique de chaque plaque peut etre retiree par subduction. La troncation de la plaque superieure est causee par une erosion de subduction, c'est-a-dire de la plaque inferieure par etranglement et par rupture de la bordure subductee. Le taux moyen de perte lineeaire de la croute pour chaque plaque est estimee a ~1,5 km/Ma ou ~3 km/Ma pour les deux ensemble. Puisqu'une importante perte de la croute a lieu avant et durant la fusion tectonique de blocs de croute en collision, les structures et les amas rocheux qui pourraient enregistrer une ancienne zone de subduction de marge oceanique et la formation de zones de subduction de suture de la croute peuvent etre manquants, tres petits ou preserves uniquement en tant que masses exhumees de materiel jadis subduit a de grandes profondeurs. [Traduit par la Redaction], Introduction Continents are constructed of a collection of thick sequences of altered and unaltered mantle-derived igneous bodies, and mass wasting and sedimentary material derived from them. Volumetric growth of the [...]

The Abitibi and Wawa subprovinces of the southern Superior Province differ in terms of the extent of preexisting 2750 Ma sialic crust and relationships between mantle plume type (tholeiitic basalt--komatiite) and arc type (tholeiite to calc-alkaline basalt--andesite--dacite--rhyolite) volcanic successions but evolved in close proximity to each other. Isotopic data, evidence from the Kapuskasing uplift, continuation of major structures associated with large gold deposits from the Abitibi into the Wawa subprovince and the related occurrence of diamonds in lamprophyric rocks in both subprovinces point to a common evolution prior to and during orogeny. Differences preserved in supracrustal sequences of the two subprovinces suggest that the main petrogenetic controls on orogenic gold deposits and lamprophyre-hosted diamond deposits lay in the lower crust and upper mantle. Similar processes must also have been active where gold and diamonds are associated on other Archean cratons, such as the Slave and possibly the Kaapvaal craton. Based on evidence preserved in the Abitibi-Wawa orogen, rapid crustal growth at ~2.7 Ga was linked to the interplay between plate tectonics and mantle plumes. Key indicators in the model developed for the Abitibi-Wawa arc are the co-existence of plume-related rock types, modern-style adakites, major gold deposits, and lamprophyre-hosted diamond occurrences, at least in cases where shoshonitic host magmas can ascend rapidly through the crust. All of these indicators are now identified on the Kaapvaal craton by 3.1 Ga and many recur together in Paleoproterzoic and younger terranes, suggesting a common mechanism for rapid crustal growth through much of Earth's history. The variety of granitoid types found within the Abitibi-Wawa orogen demonstrates that local tectonic factors, rather than a hotter average upper mantle, were important in controlling the type of magmas formed. Based on the geodynamic history of the subprovince, mantle plume interaction with an existing volcanic arc and the subduction of oceanic plateau crust played an important role in the formation of magmas similar to Cenozoic adakites. Flat subduction beneath a mantle wedge was probably responsible for the generation of the adakites and also promoted diamond stability at shallow depths while enhancing the reservoirs for subsequent orogenic gold deposits. The history of magmatism and mineralization in the Abitibi and Wawa subprovinces precludes an early or gradual development of a cratonic keel, which instead must have coupled with crust during the latest stages of orogeny. Les sous-provinces d'Abitibi et de Wawa du sud de la province du Superieur different en termes de l'etendue de la croute sialique preexistante (2750 Ma) et des relations entre les suites volcaniques de type de panache de manteau (basalte tholeiitique--komatiite) et de type d'arc (tholeiitique a basalte--andesite--dacite--rhyolite calco-alcalin) mais elles ont evolue a proximite l'une de l'autre. Les donnees isotopiques, les observations du soulevement de Kapuskasing, la prolongation des structures principales associees a de grands gisements auriferes de l'Abitibi dans la sous-province de Wawa et la presence associee de diamants dans les roches lamprophyriques dans les deux sous-provinces signalent une evolution commune avant et apres l'orogenese. Les differences preservees dans les sequences supracrustales des deux sous-provinces suggerent que les principaux controles petrogenetiques sur les gisements auriferes orogeniques et les gisements de diamants dans les lamprophyres se situent dans la croute inferieure et le manteau superieur. Des processus similaires devaient aussi avoir ete actifs la ou l'or et les diamants sont associes dans d'autres cratons archeens, tels que le craton des Esclaves et possiblement le craton Kaapvaal. Selon les indices preserves dans l'orogene Abitibi-Wawa, la croissance rapide de la croute il y a ~2,7 Ga etait reliee a l'influence reciproque entre la tectonique des plaques et les panaches du manteau. Les principaux indicateurs dans le modele developpe pour l'arc Abitibi-Wawa sont la coexistence de types de roches reliees aux panaches, des adakites de style moderne, de grands gisements auriferes et les occurrences de diamants dans des lamprophyres, au moins dans les cas ou les magmas shoshonitiques hotes peuvent monter rapidement a travers la croute. Tous ces indicateurs sont maintenant identifies dans le craton Kaapvaal vers 3,1 Ga et plusieurs peuvent se retrouver ensemble dans des terranes datant du Paleoproterozoique ou plus recents, suggerant un mecanisme commun pour une croissance rapide de la croute durant une grande partie de l'histoire de la Terre. La diversite des types de granitoi des trouves dans l'orogene Abitibi-Wawa fait valoir que les facteurs tectoniques locaux, plutot qu'une croute superieure plus chaude que la moyenne, etaient importants pour le controle du type de magma forme. En se basant sur l'historique geodynamique de la sous-province, l'interaction du panache du manteau avec un arc volcanique existant et la subduction d'une croute de plateau oceanique ont joue un role important dans la formation de magmas semblables aux adakites du Cenozoique. Une subduction plane sous un coin de manteau a probablement ete responsable de la genese des adakites et a aussi favorise la stabilite des diamants a de faibles profondeurs tout en facilitant le developpement de reservoirs pour de futurs gisements auriferes orogeniques. L'historique du magmatisme et de la mineralisation dans les sous-provinces d'Abitibi et de Wawa ecarte la possibilite d'un developpement precoce ou graduel d'une quille cratonique, laquelle a du avoir ete jointe a la croute durant les dernieres etapes de l'orogenese. [Traduit par la Redaction], Introduction Although there is widespread acceptance of a role for plate tectonics from at least 3.2 Ga, there remain considerable uncertainties concerning the details of how Archean subduction might have [...]

Carbonate-associated sulfate (CAS) has the potential to generate high-resolution records of seawater sulfate ([delta][sup.34][S.sub.sw]) that improve upon existing evaporite-based records. With improved resolution, however, significant isotopic offsets have become apparent between the evaporite and CAS proxies. Here we present high-resolution [delta][sup.34]S measurements of both these proxies from the Ara Group, Sultanate of Oman, a series of six carbonate-evaporite sequences deposited ca. 547-540 Ma. The [delta][sup.34][S.sub.CAS] from Ara carbonates show little scatter and provide our estimate of [delta][sup.34][S.sub.SW]. Repeated enrichments (as much as 4 [per thousand]. relative to [delta][sup.34][S.sub.CAS]) were observed in floor and roof anhydrite units ([delta][.sup.34][S.sub.EVAP]) bounding the Ara carbonates. These enrichments cannot be explained by secular variation or from isotopic fractionation during evaporite deposition and require the existence of an additional [sup.34]S-depleted sink, which we attribute to [H.sub.2]S production via ongoing bacterial sulfate reduction during evaporite deposition. To preserve this isotope signature, the resulting sulfide must be sequestered as pyrite. The magnitude of the resulting [delta][sup.34][S.sub.EVAP] offset (relative to CAS) is a function of local pyrite burial ([f.sub.pyr]). In periods of low pyrite burial, it is possible for [delta][sup.34][S.sub.EVAP] to be depleted relative to [delta][sup.34][S.sub.SW], whereas during episodes of substantial pyrite burial ([f.sub.pyr] > 0.05), [delta][sup.34][S.sub.EVAP], can be strongly enriched relative to [delta][sup.34][S.sub.SW]. Our data suggest that local fpy, of ~0.13 is consistent with the observed [delta][sup.34][S.sub.EVAP] enrichmerits found in the Ara Group anhydrites. Such elevated pyrite burial in evaporitic settings requires a substantial iron source, supporting possible ferruginous ocean conditions across the Ediacaran-Cambrian boundary. Isotopic offsets between paired [delta][sup.34][S.sub.CAS]- [delta][sup.34][S.sub.anhydrite] data may thus serve as an independent proxy for marine redox through time, in addition to quantifying the importance of microbial activity in a setting where direct evidence (e.g., total organic carbon or biomarkers) may be scarce and physical processes are thought to dominate. doi: 10.1130/G30230.1

The upward transfer of partially molten crust and the formation of gneiss domes and metamorphic core complexes commonly take place by localization of normal or oblique extension in the middle and upper crust. In Marie Byrd Land, Antarctica, a transition from wrench to oblique extension occurred during oblique plate divergence along the East Gondwana margin and intracontinental crustal extension associated with the West Antarctic Rift System in mid-Cretaceous time. Migmatites in the Fosdick dome record steep fabrics formed during wrenching, and associated granite networks display crystallization ages of 117-115 Ma. These steep fabrics are overprinted by subhorizontal foliation and leucogranite sheets with crystallization ages in the 109-102 Ma range. Syntectonic emplacement of granite sheets in the South Fosdick detachment zone indicates that detachment tectonics led to rapid exhumation of the terrain by 100 Ma. This study has implications for understanding melt transport, magma accumulation, and the formation of detachments in an oblique tectonic setting. doi: 10.1130/G30493.1

Mechanisms proposed to explain efficient melt transport away from hot, ductile source regions are problematic. Brittle-elastic fracturing is a well-known mechanism that allows fast magma migration as dikes through cold crust. Ductile fractures have been proposed as an alternative for ductile environments, where brittle-elastic diking is inhibited. Ductile fracturing results from rock creep and growth of microscale voids that become interconnected, leading to rock failure. In this paper, we present observations and numerical models supporting the hypothesis that ductile fracture controls early steps in magma migration. We postulate that once developed, ductile fracture dikes may reach a critical length where magma stresses at dike tips overcome fracture toughness and lead to brittle-elastic diking, which subsequently controls magma migration. doi: 10.1130/G30482.1

Salt 'withdrawal' sedimentary minibasins, common features of salt tectonic provinces, are typically subcircular or polygonal, 10-30 km in diameter, and contain as much as 10 km of sediment above evaporite that has been expelled into surrounding diapiric structures. The early development of minibasins typically is not buoyancy driven (Rayleigh-Taylor instability) because thin sediment overburden is usually less dense than salt (strictly, halite) or other evaporites. We analyze an alternative mechanism that applies in compressional regimes involving sedimentation onto salt, lateral flow and shortening of sediment and salt to form viscous pressure ridges (VPRs) (the key process). Loading by sediment ponded between VPRs provides a positive feedback that grows the pressure ridges. We show using lubrication theory and numerical models that this mechanism quantitatively solves the enigma and grows minibasins until the compacting sediment is sufficiently thick and dense for Rayleigh-Taylor instabilities to take over. doi: 10.1130/G30520.1

The continental crust is the archive of the geological history of the Earth. Only 7% of the crust is older than 2.5 Ga, and yet significantly more crust was generated before 2.5 Ga than subsequently. Zircons offer robust records of the magmatic and crust-forming events preserved in the continental crust. They yield marked peaks of ages of crystallization and of crust formation. The latter might reflect periods of high rates of crust generation, and as such be due to magmatism associated with deep-seated mantle plumes. Alternatively the peaks are artefacts of preservation, they mark the times of supercontinent formation, and magmas generated in some tectonic settings may be preferentially preserved. There is increasing evidence that depletion of the upper mantle was in response to early planetary differentiation events. Arguments in favour of large volumes of continental crust before the end of the Archaean, and the thickness of felsic and mafic crust, therefore rely on thermal models for the progressively cooling Earth. They are consistent with recent estimates that the rates of crust generation and destruction along modern subduction zones are strikingly similar. The implication is that the present volume of continental crust was established 2-3 Ga ago. doi: 10.1144/0016-76492009-072.

At the northeastern edge of the Moldanubian Zone, a large body of felsic to intermediate granulite with relics of high-pressure mineral assemblage is exposed within medium-grade paragneisses, micaschists and metagranites. A polyphase tectonic evolution in the study area resulted in three deformation phases ([D.sub.1][D.sub.3]). Peak metamorphic conditions (860-1000[degrees]C and 16 kbar) in granulites are interpreted as reflecting an early stage of the high-grade evolution of the orogenic lower crust. These conditions were followed by exhumation of the orogenic lower crustal block to mid-crustal levels (6-8 kbar). In contrast to this, the study of metamorphic conditions in the surrounding micaschists (660[degrees]C and 8 kbar) shows that these rocks never experienced pressures and temperatures of the orogenic lower crust. Laser ablation inductively coupled plasma mass spectrometry (LA-1CP-MS) U-Pb dating of zircon from the granulite yielded a concordia age of 354 [+ or -] 7 Ma, whereas the conventional isotope dilution thermal ionization mass spectrometry (ID-TIMS) and LA-ICP-MS dating of xenotime gave identical concordia ages of 336.2 [+ or -] 1.2 and 339 [+ or -] 3 Ma, respectively. Combination of new structural and petrological results with LA-1CP-MS zircon and xenotime dating leads to development of a well-constrained exhumation model and brings new insights into the behaviour of lower crustal rocks during orogenesis. doi: 10.1144/0016-76492009-086.

Anticlines of the Lurestan Province in the Zagros fold--thrust belt have been studied by integrating field-based analysis with the use of high-resolution satellite images and data available from the literature. The distribution of folds in the southeastern Lurestan Province, expressed in terms of axial length and wavelength distribution, shows a direct link with the characteristics of the sedimentary multilayer in which the folds developed. Within the carbonate deposits of the Late Cretaceous Bangestan Group the transition from pelagic to neritic facies determines a threefold increase in anticline spacing and promotes the development of thrust structures in the forelimb of anticlines. The Oligocene--Miocene Shahbazan--Asmari unit folds harmonically with the Bangestan Group, except in the areas where the Palaeogene deposits interposed between the two units exceed 1300 m of thickness. In these areas the Shahbazan--Asmari carbonates display short-wavelength folds indicating a complete decoupling from the underlying folds of the Bangestan Group. It is suggested that this decoupling occurs because the summed thickness of the incompetent units separating the two carbonate units exceeds the extension of the zone of effective contact strain of the Bangestan Group folds.

Pockmarks are one of the most obvious and abundant structural features of the North Sea seabed, yet their influence on fauna is virtually unknown. We report the distribution of benthic megafaunal assemblages in and around four 'pockmark complexes' in the North Sea to determine the structure of megafaunal communities inside pockmarks and whether these ubiquitous topographical features acted as refuges against trawling and other disturbances. The study focuses on the large central pockmarks in each of the pockmark complexes. These large pockmarks had depths of around 10 m and diameters of 160-235 m. Remotely operated vehicle video transects showed that megafauna increased in abundance, species richness, and diversity from outside (background seabed) toward the center of the pockmarks. The number of taxa present in the center of pockmarks was approximately double those of similar surrounding areas, and the centers had almost an order of magnitude more individuals than outside. Carbonate rocks were found in the centers of all the pockmarks and may be indicative of their formation (past methane seeps). These rocks also provide novel habitat to fauna: a complex hard substrate for colonization and shelter in an otherwise homogeneous soft sediment environment. Habitat enrichment and morphological protection are suggested to be the main reasons for the increased faunal abundance and species richness. Indeed, despite fishing data showing the area to be intensively disturbed, large slow-growing (old) and vulnerable species, such as gorgonian corals, were found in the center of the pockmarks. Pockmarks may offer important refuges from trawling activity.

The 2500-km Altai Range is located in the central part of the Central Asia Orogenic System, a tectonic collage comprising oceanic and continental fragments that were assembled during the Paleozoic continental growth of Eurasia. We conducted field mapping, [sup.40]Ar/[sup.39]Ar thermochronology, metamorphic petrology, and Th/Pb ion-microprobe monazite dating in the southern Chinese Altai Range. This study demonstrates the presence of a south-vergent, Permo-Triassic thrust belt active across the region. Metamorphic conditions of 610 [+ or -] 35[degrees]C and 5.7 [+ or -] 1.8 kbar were reached by schists with Permo-Triassic monazite ages. Mica [sup.40]Ar/[sup.39]Ar ages range from Late Permian to Jurassic, and cooling in these rocks is correlated with thrust faulting. This shortening was synchronous with localized left-lateral, strike-slip shear deformation. Our work suggests that the high-grade schists of the Altai orogen were buried to depths of more than 18-20 km and were exhumed in the Permian to Jurassic. The Permo-Triassic Altai thrust belt was reactivated locally by Late Jurassic contraction after ca. 160 Ma, which may result from the final closure of the Mongol-Okhotsk Ocean or the collision of the Lhasa block onto the southern Asian margin.

Accreted terranes of the Appalachian lapetan and peri-Gondwanan realms display structural trends that are mainly concordant with promontories and embayments in the Laurentian margin, indicating that during accretion, the shape of the continental margin acted as a template around which accreted terranes were molded. In North Carolina and Newfoundland, post-accretion transcurrent motion appears to be recorded by displaced outboard portions of promontories, no longer concordant with those in Laurentia. A bend in structural trends confined to the peri-Gondwanan realm at the North Carolina--South Carolina state line is interpreted to represent the dextrally displaced outboard portion of the Virginia promontory. In Newfoundland, the Hermitage flexure is interpreted as a dextrally displaced Laurentian promontory that originated along strike to the northeast. In both places, promontories were truncated and dextrally translated for ~220-250 km by a Devonian--Mississippian orogen-parallel transcurrent system, which may well have extended for the length of the eastern Laurentian margin. South of the nearby St. Lawrence promontory, extreme thinning of Appalachian crust beneath the Maritimes Basin is consistent with extension at a releasing stepover. Estimates of extension are consistent with those obtained from promontory offsets.

The Balcones Escarpment of south-central Texas is the eroded topographic expression of an en echelon fault zone where Cretaceous carbonates have been modified by karst processes influenced by structural and stratigraphic controls. While the modern confined Edwards Aquifer flows through cavernous voids at the base of the escarpment, air-filled caves perched in the escarpment are relicts of paleoaquifer hydrology. The structural geology of the Balcones Escarpment and the phylogeography of its endemic cave spiders provide mutually informative frameworks from which to establish relative dates for the activation of discrete groundwater recharge areas. The mitochondrial genetic variability of troglobitic spiders is correlated with the structural elements of the Balcones fault zone in and around the San Antonio relay ramp. Older (basal) genetic lineages occur in structurally high, mature karst terrains, while the younger (derived) lineages occur in structurally low, emergent karst terrains. Based on mitochondrial deoxyribonucleic acid (mtDNA) data, Cicurina diversity is interpreted as the product of the progressive availability of vadose zone habitat as discrete recharge areas have developed.

Lineament identification is a standard but controversial hydrogeologic practice. In this study, we present a rigorous examination of the hydrology of lineaments in a crystalline bedrock setting. Lineaments are reinterpreted as watershed-scale hydraulic barriers, in contrast to previous interpretations as fractured conduits that focus recharge and flow. In the study area, an ~900 [km.sup.2] watershed underlain by the granitic and gneissic terrain of the Canadian Shield, bedrock lineaments are associated with linear lake shores and perennial wetland complexes. Lineaments were identified using a robust multi-image method and characterized by remote sensing, fracture mapping, drilling, hydraulic characterization, and numerical simulation of a coupled groundwater--surface-water system. Results indicate that two principal lineament sets are oriented parallel to fracture and fault orientations, and thus lineaments are interpreted as structural features, either fault zones or fracture zones with limited displacement. Faulted lineaments are more effectively identified by digital elevation model (DEM) topographic data rather than Landsat tonal imagery. Hydrogeological characterization and geomatic data indicate that the fractured bedrock underlying lineaments is composed of poorly connected zones of reduced permeability due to fault zone and/or fluid flow processes. Field data and numerical simulations suggest that lineament areas are barriers to recharge and flow in this setting as a result of permeability reduction. Integrated data sets and models of lineament permeability that are geologically realistic result in a better understanding of fractured bedrock aquifers and patterns of fluid flow in the brittle uppermost crust. Keywords: hydrogeology, structural geology, environmental geology, remote sensing, lineaments, faults, hydraulic barriers.

The origin of gullies on Mars is controversial (e.g., catastrophic groundwater release, debris flows, dry granular flows, or meltwater from surface ice and snow) and their ages are difficult to determine due to their small size. We describe a gully depositional fan that contains a unique chronostratigraphic marker (secondary crater clusters) between episodes of gully activity during fan development. This marker can be traced to its source, a 7-km-diameter rayed crater that we have dated as ca. 1.25 Ma. This age links gully activity to the emplacement of dust-ice mantling deposits interpreted to represent recent ice ages on Mars. This association, together with multiple episodes of depositional fan formation, favors an origin for these gullies from top-down melting of snow and ice during multiple favorable spin-axis and orbital variations. This melting mechanism is consistent with the occurrence of gullies in unique steep-sloped, poleward-facing insolation microenvironments that favor the melting of small amounts of surficial snow and ice.

The manner in which calderas develop is a key question in volcanology and has important implications for associated volcanogenic risk and geothermal and ore exploitation. To better understand the structural evolution of calderas, I use a discrete element model of a frictional cover undergoing piston-like subsidence at its base, simulating magma chamber deflation and cover collapse. These novel simulations capture not only the initiation of calderas but also much of the complexity of faulting during their later development. In all models, both normal and reverse faults accommodate deeper subsidence at higher structural levels. Curved, outward-dipping reverse faults are consistently the first structures to develop; subsequent caldera growth is mainly the result of movement on vertical or steeply inward-dipping normal faults. This may be followed by a later phase of lateral collapse on shallower normal faults. The roof aspect ratio (R = cover thickness/piston width) is seen to be an important factor: calderas with low aspect ratios (R [less than or equal to] 1) exhibit a coherent central subsiding block, while those with higher aspect ratios have a complexly faulted internal structure.

Relative to the CI chondrite class of meteorites (widely thought to be the 'building blocks' of the terrestrial planets), the Earth is depleted in volatile elements. For most elements this [...]

Analogue models are used to study the mechanical relationship between basement strike-slip faults and salt diapirs. Displacement along a strike-slip fault in 1 g models resulted in extension along pre-existing jogs and the formation of oblique extensional faults, where reactive diapirs were triggered in some models. In the centrifuge models, prescribed cuts, simulating pre-existing structures, were reactivated during simple shear deformation of the models, resulting in formation of pull-apart basins, which were intruded by diapirs. The models show that because of the low ratio of salt to overburden thickness in the Zagros (0.15-0.35), it is unlikely that diapirs have formed solely, if at all, as a result of movement along basement strike-slip faults. Two mechanisms are suggested. First, pre-kinematic thin overburden and continuous movement along a releasing bend in the cover units may have triggered some of the diapirs in the Zagros, which were later downbuilt to their current geometry by additional sedimentation. Second, movement along the strike-slip faults (e.g. the Kazerun and Mangarak faults) induced oblique movement along NW--SE Zagros structures (folds or thrusts) resulting in the formation of pull-apart basins where diapirs were eventually intruded. Fault plane solution of shallow earthquakes supports the second scenario, which is also in agreement with previous interpretations that some of the salt diapirs associated with basement faults are younger than Zagros shortening and young southwestwards.

Cratons are old, stable parts of the continental crust that have survived at least since Proterozoic rime and have not undergone strong magmatism or tectonism since their stabilization. Traditionally, the growth of continental crust in the North China craton is considered to have been complete by Paleoproterozoic time, Phanerozoic events being largely restricted to surrounding orogenic belts. However, the eastern part of the North China craton contains large volumes of Mesozoic igneous rocks, with widespread metamorphic core complexes and pull-apart basins. Hf isotope compositions of magmatic zircon grains from igneous rocks in the Liaodong Peninsula indicate that widespread late Mesozoic granitoids formed by partial melting of ancient crust, but with significant input of a mantle component via magma mixing and crustal assimilation. This magmatism has resulted from removal and modification of lithospheric mantle, accompanied by asthenospheric upwelling. The Hf isotopic signatures thus record the addition of juvenile crust beneath the eastern part of the North China craton, which appears related to major extension and possibly slab rollback of the Pacific plate. Whatever the mechanism, it is apparent that since ca. 200 Ma, ancient lithosphere beneath the eastern North China craton has been progressively reactivated and replaced, resulting in 'decratonization.' Tertiary to Holocene volcanism in the area and major subsidence around the Gulf of Bohai suggest that the effects of the process are continuing. Similar processes probably operated in the geological past, leading to significant modification of continental crust and requiring reconsideration of mechanisms for continental breakup and dispersal and of continental growth rates. Keywords: decratonization, Phanerozoic crustal growth, North China craton, zircon, Hf isotope, magmatism.

Porphyroblastic garnet-bearing metagabbros from the base of the lower crust section of the Serre (southern Italy) exhibit multi-stage dehydration and decompression after the Panafrican emplacement of their protoliths. The first dehydration event produced Am-Opx-Cpx-Pl-Grt as the peak assemblage. Two decompression stages are documented by: (1) coronas of Opx-Pl and Opx-Am, and symplectites of Opx-Am-Pl around clinopyroxene within the porphyroblastic garnet as well as in the matrix and (2) symplectites of Pl-Am-Opx-Grt having different textures around the porphyroblastic garnet. During the second decompression stage, a new local, somewhat intense, dehydration occurred and produced rims of Opx + Pl around the porphyroblastic amphibole, or lenses of Pl-Opx-Am-Spl [+ or -] Bt between layers of dominant amphibole. A deformation stage separates older from younger reaction textures. The porphyroblastic garnet, its inclusions and the matrix are affected by fractures, which have been overgrown by coronas and symplectites around the porphyroblastic garnet and the amphibole of the matrix. Preferred P-T estimates are: ~ 900[degrees]C and ~ 1.1 GPa at the metamorphic peak; ~ 850[degrees]C and 0.8-0.9 GPa during the formation of corona around clinopyroxene; 750-650[degrees]C and 0.7-0.8 GPa during the formation of corona around garnet. All these textures formed under granulite-facies conditions. The subsequent metamorphic evolution consists of rehydration under amphibolite-facies conditions. The P-T-t path agrees with the path shown by the uppermost migmatites of the Serre section, and the P-T estimates at the top and the bottom of the section are consistent with the thickness (7-8 km) of the lower crustal segment. A contractional regime, which caused a crustal thickening of about 35 km, was followed by an extensional one producing significant crustal thinning; the change of tectonic regime probably occurred about 300 Ma ago when the emplacement of voluminous granitoids and the initial stages of exhumation of the lower crustal section had taken place. Keywords: metagabbro, lower crust, granulite facies, P-T conditions, Calabria Italy.

An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault, the Main Central Thrust, has been recognized along the Himalaya for over 100 years. A major problem in Himalayan structural geology is that recent workers have mapped the Main Central Thrust within the Greater Himalayan Sequence high-grade metamorphic sequence along several different structural levels. Some workers map the Main Central Thrust as coinciding with a lithological contact, others as coincident with the kyanite isograd, up to 1-3 km structurally up-section into the Tertiary metamorphic sequence, without supporting structural data. Some workers recognize a Main Central Thrust zone of high ductile strain up to 2-3 km thick, bounded by an upper thrust, MCT-2 (= Vaikrita thrust), and a lower thrust, MCT-1 (= Munsiari thrust). Some workers define an 'upper Lesser Himalaya' thrust sheet that shows similar P-T conditions to the Greater Himalayan Sequence. Others define the Main Central Thrust either on isotopic (Nd, Sr) differences, differences in detrital zircon ages, or as being coincident with a zone of young (

Several depocentres have developed along the NNW-SSE-striking Strouma (Strymon) Lineament since the Miocene, forming the Strouma/Strymon graben system. Some of these basins, including the Dzherman and Simitli, strike at high angles to the lineament. The rhomboid-shaped Simitli basin is limited in the south by the NE-SW-striking, seismically active Kroupnik fault and contains terrestrial clastic sediments, which have been deposited since the Sarmatian. Its formation is attributed to 'wrench-dominated transtension' ([D.sub.B] event) which activated the basin's NE-SW-striking boundary faults as left-lateral strike-slip structures during the Early-Middle Miocene. This deformation was associated with NNE-SSW contraction and WNW-ESE extension and succeeded the NNE-SSW contraction caused by 'pure shear-dominated transpression' ([D.sub.A] event), which governed the region in Late Oligocene Early Miocene times. During the Middle Late Miocene, the [D.sub.B] event was followed by WNW-ESE 'pure shear-dominated extension' ([D.sub.1] event), which caused further widening of the basin. The transtensional origin of the Simitli basin is explained by lateral extrusion of the crustal mass squeezed between the Apulian-Adriatic microplate and the European foreland towards the North Aegean Sea. The subduction retreat of the Hellenic orogen possibly enhanced this lateral extrusion, and from the Late Miocene onwards the NE-SW-oriented back-arc extension balanced the continuing retreat.

The right-stepping Dobe and Hanle grabens display a variety of structures that serve to transfer extensional displacement. These structures range from relay zones between overlapping fault segments to accommodation zones between interacting rift segments. The study reveals the presence of three examples of displacement transfer structures: a breached relay zone, a large-scale accommodation zone that is partially breached, and a composite zone that combines elements of both. All three examples exhibit common structural elements. First, dipping ramps develop between horizontal horst blocks and graben floors. Second, these ramps are cut by numerous faults, most of which are antithetic to the ramps and the graben boundary faults. The antithetic faults bound elongate blocks that are rotated into the grabens. Third, crosscutting faults partially or completely link the en echelon or overlapping graben boundary faults. The identification of precursory structures (mode I fractures) at the leading edge of the Dobe-Hanle accommodation zone breaching faults suggest that the breaching process may be continuing. The spatial alignment from north to south of the crosscutting faults, open fractures and lineaments indicates that the breaching process is progressing from the Dobe graben towards the Hanle graben.

[1] The study of fabric development and juvenile batholith emplacement across the tilted crustal section of the Eastern Dharwar craton shows that horizontal, constrictional deformation affected large volumes of the midcrust and lower crust at the time of regional partial melting and magmatic accretion. Constriction is achieved by a combination of coeval shallow and steep planar fabrics sharing a common horizontal elongation direction, two sets of conjugate strike-slip shears, and extensional shear zones. The Eastern Dharwar craton illustrates an end-member deformation mode by which a particularly weakened lithosphere responds to shortening by developing distributed, horizontal plane strain on a crustal scale, resulting from the combination of crustal shortening and lateral gravity-driven flow. Thinning accompanying constrictional deformation is interpreted as compensating for juvenile magmatic accretion and thickening of greenstone belts and as acting to maintain a stable crustal thickness. Such a midcrustal to lower crustal deformation process may provide a resolution of the batholithic room problem in a softened crust submitted to lateral shortening and may explain nearly isobaric retrograde pressuretemperature-time paths of high temperature--low pressure high-grade terrains. Citation: Chardon, D., and M. Jayananda (2008), Three-dimensional field perspective on deformation, flow, and growth of the lower continental crust (Dharwar craton, India), Tectonics, 27, TC1014, doi:10.1029/ 2007TC002120.

Wave barriers (open or in-filled trenches) are often used in engineering practice to reduce ground vibrations caused by industrial activities or transportation systems. Typically, most of the energy is transmitted in the form of Rayleigh waves so that information related to their propagation can be very useful in the design process. This technical note presents a case study in which both active and passive surface wave tests have been used in the design of a vibration mitigation barrier. It is shown that several pieces of information can be inferred from these tests for specific applications: source characterization in terms of dominant frequencies and direction of propagation; dominant wavelength for the preliminary design; and soil characterization for the construction of a numerical model aimed at analyzing the performance of the wave barrier. DOI: 10.1061/(ASCE)1090-0241(2007)133:10(1320) CE Database subject headings: Vibration; Soil dynamics; Surface waves; Rayleigh waves; Signal processing; Remedial action; Ground motion.

The Teide and Pico Viejo stratocones and the Northwest and Northeast Rifts are products of the latest eruptive phase of the island of Tenerife, initiated with the lateral collapse of its northern flank that formed the Las Canadas Caldera and the Icod--La Guancha Valley ca. 200 ka. The eruptive and structural evolution of this volcanic complex has been reconstructed after detailed geological mapping and radioisotopic dating of the significant eruptive events. A set of 54 new [sup.14]C and K/Ar ages provides precise age control of the recent eruptive history of Tenerife, particularly Teide Volcano, the third-highest volcanic feature on Earth (3718 m above sea level, >7 km high), and unique in terms of its intraplate setting. The development of the Teide--Pico Viejo Volcanoes may be related to the activity of the Northwest and Northeast Rifts. Volcanic and intrusive activity along both rift zones may have played an important role in activating the gravitational landslide and in the subsequent growth, nested within the collapse embayment, of an increasingly higher central volcano with progressively differentiated magmas. The coeval growth of the central volcano with sustained activity along the rifts led to a clear bimodal distribution in composition of eruptive products, with the basaltic eruptions in the distal part of the rifts and phonolitic and more explosive eruptions in the central area, where the differentiated stratocones developed. Current volcanic hazard in Tenerife is considered to be moderate, because eruptive frequency is low, explosivity is modest, and the eruptive activity of the Teide stratocone seems to have declined over the past 30 k.y., with only one eruption in this period (1150 yr B.P.). Keywords: radioisotopic ages, volcanic history, structural history, rift volcanism, nested central volcanoes, Teide Volcano, Tenerife, Canary Islands.

The Nordfjord-Sogn Detachment Zone of western Norway represents an archetype for crustal-scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahigh-pressure (UHP) terranes. In this paper, we investigate the role of normal-sense shear zones with respect to UHP exhumation using structural geology, thermobarometry, and geochronology of the Hornelen segment of the Nordfjord-Sogn Detachment Zone. The Hornelen segment of the zone is a 2-6 km thick, top-W shear zone, primarily developed within amphibolite-grade allochthonous rocks, that juxtaposes the UHP rocks of the Western Gneiss Complex in its footwall with lower-grade allochthons and Carboniferous-Devonian Basins in its hanging wall. New thermobarometry and Sm/Nd garnet geochronology show that these top-W fabrics were initiated at lower crustal depths of 30-40 km between 410 Ma and 400 Ma. Structural geology and quartz petrofabrics indicate that top-W shear was initially relatively evenly distributed across the shear zone, and then overprinted by discrete ductile-brittle detachment faults at slower strain rates during progressive deformation and exhumation. These results require a three-stage model for UHP exhumation in which normal-sense shear zones exhumed UHP rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile-brittle structures. Keywords: ultrahigh-pressure rocks, exhumation, Nordfjord-Sogn Detachment Zone, low-angle detachment, western Norway, Hornelen Region.

Relay ramps form at different scales in extensional terrains to accommodate the differential stretching among the overlapping bounding-normal fault segments. The major boundary structure of the Gediz Graben (SW Turkey) is segmented, with formation, evolution and active breaching of relay ramps. The Akcapinar relay ramp is an example of these processes that forms within a 2 km wide overlap zone between ~ E--W-trending segments on the southern margin of the Gediz Graben. Change of structural style at the ramp area is clearly evident from variations in the orientation of fractures and faults of the relay ramp. As a site of local stress field anomaly, ~ N--S-trending structures in the Akcapinar relay ramp are conformable with the extension that is parallel to the bounding faults of the relay ramp. These structures are also superimposed onto the ~ E--W-trending regional structural style by an obvious cross-cutting relationship, suggesting both spatial and temporal variation of the state of stress at the ramp area during the relay ramp formation. The regional stress field and the resulting regional structural style characterize the earlier stages of ramp evolution, during which segment interaction is limited. As segment interaction and relay ramp formation advances towards the breaching phase, an anomalous local stress field arises and the relay ramp area experiences extension parallel to the bounding normal faults, that is, transverse to the regional direction of extension. This creates a new structural style at the ramp area. Keywords: continental extension, normal faulting, relay ramp, palaeostress, Gediz Graben, southwest Turkey.