Pre-existing Shear Zones Research Articles

Paleoproterozoic structural frame of the Yetti domain (Eglab shield, Algeria): Emplacement conditions of the Tinguicht late pluton from magnetic fabric study

The Tinguicht pluton is part of the ∼2.07 Ga post-collisional magmatic suites that intruded the Yetti Paleoproterozoic volcano-sedimentary series of the western part of the Eglab Shield (West African Craton). It represents one of the most recent units of these suites. This pluton, with a NW–SE elliptic shape, is unfoliated, and its deformational structures are practically restricted to fracturing and faulting. New structural, microstructural and aeromagnetic data are presented in order to analyze in particular the relationship between the Tinguicht pluton emplacement and the related NNW-SSE major mega-shear zone, separating the Yetti and Eglab domains. To constrain the context of the regional post-collisional evolution of the Eglab shield, a structural analysis was performed by mapping the magnetic structures (foliation and lineation) using AMS. The combination of the results of all the used approaches leads to a new and enriched image of this granitic pluton and of its tectonic emplacement context. The elliptic shape of the granitic body and the AMS strain pattern are consistent with the presence of a NNW-SSE major structure. NNW-SSE is also one of the major directions highlighted by the aeromagnetic data. This study thus evidences the role of the pre-existing major shear zones in controlling emplacement of post-collisional Paleoproterozoic plutons like Tinguicht, as shown for Drissa pluton in the Eglab domain earlier.

Analog modeling of one-way gravitational spreading of hot orogens – A case study from the Svecofennian orogen, Fennoscandian Shield

Geodynamic models offer insight into deformation styles of syn- and post-collisional extensional processes. Such models often ignore the effect of lateral viscosity contrast between accreted terranes and inherited tectonic boundaries. This paper presents a set of crustal-scale analog experiments of one-way post-collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechanically contrasting blocks, pre-existing cuts with varying dips, and three layers with contrasting rheologies. Two blocks represent strong, mature, and weak, juvenile crustal parts. Both blocks have three horizontal crustal layers: brittle upper, weak partially molten middle and dry strong lower layer. Cuts represent pre-existing crustal-scale shear zones and tectonic boundaries.The experiments indicated that tectonic boundaries may facilitate exhumation and increase extensional strain, when the boundaries dip opposite to the flow direction. The tectonic boundaries may also split and divide the crust into separate blocks with independent deformation signatures: shortening, elongation or rotation. The tectonic boundaries rotated along vertical axes toward the flow direction. The modeling results suggest that in areas with inherited tectonic boundaries and compositional differences the amount of extension is unevenly distributed between the different parts during the widespread unilateral gravitational spreading and that the flow has different effects on the contrasting crustal units, in both lateral and vertical directions.To validate the modeling, the results are compared to geophysical and geological data from the Paleoproterozoic Svecofennian orogen in the Fennoscandian Shield. The comparison suggests that the orogen underwent 50% extension and was thinned by maximum of 20% via westward gravitational spreading. This spreading rotated the crustal-scale blocks, shear zones and tectonic contacts. It also brought exhumed, high grade middle crustal blocks into contact with subsided, low grade, upper crustal blocks.We suggest that results of the analog models can be used as proxies in both ancient and modern hot accreted orogens, which have undergone post-convergence continental mid-crustal weakening and have comprised of juxtaposed terranes with varying rheological compositions.

Evolution of the ductile shear zone of the Paishanlou gold deposits, western Liaoning, China

The combination of field surveys with analysis of microstructure of tectonite and Electron Backscatter Diffraction (EBSD) on quartz fabric indicated that three periods of ductile shear events developed in the Paishanlou gold deposits and the E-W and NE-striking ductile shear zones were formed during each event. The E-W-striking ductile shear zone, accompanied by compressional and dextral shear slip, was shear-cut by the NE-striking shear zones, accompanied by compressional-sinistral shear slip and sinistral-normal shear slip, successively. An E-W-striking ductile shear zone developed at a deeper tectonic level and at middle- to high-temperatures, accompanied by abundant microstructures, including microlayering between a polycrystal quartz belt and mica, and quartz deformation was depended on cylinder (10-10) 〈a〉 or 〈c〉 glide. The development of an E-W-striking shear zone can be seen as a tectonic pattern in the region of the Paishanlou gold deposits of the collision between the Mongolian tectonic belt and the North Archean Craton from Suolun to the Linxi suture zone during the Indosinian. The NE-striking ductile shear zone developed approximately 160 Ma during the early Yianshanian at middle to shallow tectonic levels and at middle- to low-temperatures, accompanied by typical microstructures, including polycrystal quartz aggregation and quartz subgrain rotation recrystallization, etc., and quartz deformation was depended on prismatic (1011) 〈a〉 glide. The last ductile shear event around the NE-striking shear zone developed at low temperatures and shallow tectonic levels, yielding to a pre-existing NE-striking shear zone, accompanied by abundant microstructures, including low-temperature quartz grain boundary migration and bulging recrystallization. The last ductile shear movement may be related to lithosphere thinning and the destruction of the North China Craton from approximately 130-120 Ma, and this shear event resulted directly in the mineralization in the Paishanlou region.

Scale-dependent influence of pre-existing basement shear zones on rift faulting: a case study from NE Brazil

Rifting of continental crust initiates faults that are commonly influenced by pre-existing structures. We document newly identified faults cutting Precambrian units in the interior of the NE Brazilian margin to assess the effects of structural inheritance on both rift geometry and fault architecture. Stratigraphic and structural data indicate that the faults were active in the main phase of rifting of Gondwana. The influence of pre-existing structures on the Mesozoic rift faulting is scale dependent. Regionally, the faults trend parallel to subvertical, crustal-scale Brasiliano ( c . 750–540 Ma) shear zones. Mylonitic foliations and broadly distributed low strain in the lower crust indicated by shear-wave splitting controlled the overall orientation and kinematics of the rift faults. However, outcrop observations of the faults show that at scales up to hundreds of metres, mylonitic foliations have little influence on fault architectures. Faults cross-cut shear zones and do not commonly utilize foliation planes as shear fractures. Instead, slip zones and fractures have a range of orientations that form acute angles to the local foliation orientation. This observation explains the range of focal mechanisms associated with seismicity that coincides with ancient shear zones in intra-continental areas.

Carboniferous appinitic intrusions from the northern North China craton: geochemistry, petrogenesis and tectonic implications

Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating and geochemical study document Carboniferous ( c . 320–317 Ma) appinitic intrusions from the northern North China craton. The rock suite mainly consists of hornblende diorites and monzodiorites, with an SiO 2 range from 46.8 to 55.4%. These rocks exhibit high alkali contents, strong enrichment in large ion lithophile elements and light rare earth elements, and depletion in high field strength elements, with radiogenic 87 Sr/ 86 Sr i ratios of 0.7058–0.7093, unradiogenic ϵ Nd (t) of −9.3 to −13.9 and zircon ϵ Hf (t) from −8.5 to −18.4. These geochemical features suggest that their generation may involve a distinctive two-stage process: (1) a precursory metasomatism stage of mantle peridotites by melts from subduction-related sediments; (2) a delayed partial melting stage probably initiated by post-subduction transcurrent movements along pre-existing lithospheric shear zones. These mafic to intermediate intrusions, plus other coeval mafic– ultramafic complexes and high Ba–Sr granites from neighbouring regions, not only witness a heterogeneously enriched subcontinental lithospheric mantle along the northern North China craton, but also attest to a reworking-dominated metacratonic process within a post-subduction transtensional regime. Supplementary material: Data on mineral composition, zircon dating and Hf isotopic composition are at http://www.geolsoc.org.uk/SUP18513 .

Post-rift reactivation of the onshore margin of southeast Brazil: Evidence from apatite (U–Th)/He and fission-track data

On the continental margin of southeast Brazil there is good geological evidence for post-rift tectonic reactivation, in the form of onshore Tertiary basins up to 800 m deep and Cretaceous to Paleocene alkaline intrusive bodies at the surface. With the aim of constraining the Mesozoic and Cenozoic exhumation history of this region, we present new apatite (U–Th)/He ages on basement samples, which one of us previously used for apatite fission track (AFT) analysis. The new He ages range from 115.5 ± 2.2 Ma to 47.0 ± 6.4 Ma, whereas the AFT ages range from 330.3 ± 22 Ma to 64.5 ± 3 Ma. Inverse thermal history modelling of all the data provides more detail on the post-rift cooling history than do the AFT data alone. Furthermore, models incorporating radiation damage effects on He diffusion produce thermal histories that are more consistent with independent geological evidence than models without these effects. The results imply that only a few samples record an early cooling phase, around the time of opening of the South Atlantic Ocean. The new (U–Th)/He data also imply a Late Cretaceous phase of exhumation between 90 and 60 Ma. Subsequently, the region underwent slow cooling (i.e. erosion rate). Two or three samples seem to indicate a period of rapid cooling during the Neogene, although this is at the limit of resolution of the methods. The post-rift cooling phases imply exhumation from depths between 2 and 5 km. This activity we attribute to compression in the South American plate, between the Andean subduction zone and the mid-Atlantic ridge. In southeast Brazil, compression acted on a crust that was thermally or mechanically susceptible to reactivation. During the Late Cretaceous, deformation in the study area was of regional extent, whereas during the Tertiary, it concentrated along pre-existing shear zones of the Precambrian Brasiliano orogeny.

Gravity and magnetic joint modeling of the Potiguar Rift Basin (NE Brazil): Basement control during Neocomian extension and deformation

Abstract A 2.5D gravity and magnetic investigation was conducted along five transects across the Potiguar Basin in the Borborema Province, NE Brazil. The objective of the study is to model the internal architecture of this intracontinental rift basin, which represents the interaction between the heterogeneous Precambrian basement and the Neocomian extensional tectonics, which led to the South Atlantic opening. Joint modeling of the gravity and magnetic data was constrained by Euler deconvolution results, seismic data, well logs and geologic mapping. This integrated approach allowed to determine the rift architecture that is inserted in a complex tectonic and structural framework. Results from joint modeling show that a series of asymmetric half-grabens is oriented in the NE–SW direction and controlled by a system of normal faults with throw greater than 5.5 km. High-density and low-magnetized material constitutes the footwall and intrarift horsts. These supracrustal heterogeneities in association with preexisting shear zones probably guided the Mesozoic rifting process in NE Brazil. Their composition seems to be related to metamorphic rocks of the Proterozoic basement, as suggested by gravity and magnetic anomalies and the geology of the exposed basement. Our interpretation is supported by geophysical studies carried out in the Benue Trough, the counterpart of the Potiguar Basin in West Africa.

Late Alpine brittle faulting in the Rotondo granite (Switzerland): deformation mechanisms and fault evolution

The unlined Bedretto tunnel in the Central Swiss Alps has been used to investigate in detail the fault architecture and late Alpine brittle faulting processes in the Rotondo granite on macroscopic and microscopic scales. Brittle faults in the late Variscan Rotondo granite preferentially are situated within the extent of preexisting ductile shear zones. Only in relatively few cases the damage zone extends into or develops in the previously undeformed granite. Slickensides suggest a predominant (dextral) strike-slip movement along these steeply dipping and NE–SW-striking faults. Microstructures of these fault rocks illustrate a multi-stage retrograde deformation history from ductile to brittle conditions up to the cessation of fault activity. In addition these fabrics allow identifying cataclastic flow, fluid-assisted brecciation and chemical corrosive wear as important deformation mechanisms during this retrogressive deformation path. Based on the analysis of zeolite microfabrics (laumontite and stilbite; hydrated Ca–Al- and Na–Ca–Al–silicate, respectively) in fault breccias, cataclasites and open fractures we conclude, that the main phase of active brittle faulting started below 280°C and ceased ca. 14 Ma ago at temperatures slightly above 200°C. This corresponds to a depth of approx. 7 km.

Upper crust reworking during gravitational collapse: the Bembibre–Pico Sacro detachment system (NW Iberia)

Abstract: The kinematics of the basal allochthon in the SW of the Órdenes Complex is analysed to constraint its evolution during collisional and postcollisional stages of the Variscan orogeny. Two distinct sequences have been identified in the basal allochthon of this sector: the upper and lower sequences, in close correlation with the subdivision of the basal allochthon in the Malpica–Tui Complex. Three main tectonic episodes have been established in the basal allochthon: a high-pressure event related to continental subduction, a mesozonal event of regional exhumation by thrusting and recumbent folding, and a regional-scale extensional episode that resulted in the development of the Bembibre–Pico Sacro detachment system. The Bembibre–Pico Sacro system rejuvenated pre-existing shear zones, whose weakness favoured the nucleation of the detachments. Shear zones associated with the detachments overprinted the previous tectonic fabrics under conditions ranging from lower amphibolite to greenschist facies, and with heterogeneously distributed ductile to brittle deformation. The detachment system is coeval with late orogenic collapse and widespread magmatism, and represents its upper crustal expression. It correlates with mid- and lower crustal flow and the development of gneiss domes at depth in such a way that the basal allochthon can be considered a rheological boundary between the more rigid allochthonous sequences above and the more viscous Schistose Domain and autochthon below.

Multiple accretion at the eastern margin of the Rio de la Plata craton: the prolonged Brasiliano orogeny in southernmost Brazil

The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300 million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The Sao Gabriel block in the west is characterized by c.760–690 Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880 Ma old igneous rocks. Both igneous and metasedimentary rocks have positive eNd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the Sao Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D3) yield an U–Pb zircon age (LA-ICP-MS) of 701 ± 10 Ma giving the approximate minimum age of the Sao Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789 ± 7 Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234 ± 28 Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100–2,000 Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800–700 Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and Sao Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the Rio de la Plata craton, (i) accretion of an intra-oceanic arc at c. 880 Ma (Passinho event) and (ii) accretion of the 760–700 Ma Cambai/Vila Nova magmatic arc (Sao Gabriel event). The latter event also includes the collision of the Encantadas block with the Rio de la Plata craton to the west. Collision and crustal thickening was followed by sinistral shear along SW–NE-trending orogen-parallel crustal-scale shear zones that can be traced from southern Brazil to Uruguay and have been active between 660 and 590 Ma. Voluminous granitic magmatism in the Pelotas batholith spatially related to shear zones is interpreted as late- to post-orogenic magmatism, possibly assisted by lithospheric delamination. It marks the transition to the post-orogenic molasse stage. Localized deformation by reactivation of preexisting shear zones continued until c. 530 Ma and can be assigned to final stages of the amalgamation of West Gondwana.

Structural analysis of the coseismic shear zone of the 2008 M w 7.9 Wenchuan earthquake, China

Field investigations reveal that the surface rupture of the 2008 M w 7.9 Wenchuan earthquake, China, occurred along a pre-existing shear zone in the Longmen Shan Thrust Belt. Structural analyses of the coseismic fault zone and fault rocks show that i) the main coseismic shear zone consists of a fault core that includes a narrow fault gouge zone of <15 cm in width (generally 1–2 cm) and a fault breccia zone of <∼3 m in width, and a wide damage zone of >5 m in width that is composed of cataclastic rocks including fractures and subsidiary faults; ii) the foliations developed in the fault core and damage zones indicate a dominantly thrust slip sense, consistent with that indicated by the coseismic surface rupture; and iii) coseismic slip was largely localized to within a narrow fault gouge zone of <2–3 mm in width. The structural characteristics of the coseismic shear zone and cataclastic rocks indicate that the location of coseismic slip zone associated with the 2008 Wenchuan earthquake was controlled by a pre-existing shear zone and that the main active fault of the Longmen Shan Thrust Belt has moved as a thrust since the formation of cataclastic rocks along the fault during the late Miocene or early Pliocene.

Finite element modelling of the pull-apart formation: implication for tectonics of Bengo Co pull-apart basin, southern Tibet

The tectonic deformation and state of stress are significant parameters to understand the active structure, seismic phenomenon and overall ongoing geodynamic condition of any region. In this paper, we have examined the state of stress and crustal deformation during the formation of the Beng Co pull-apart basins produced by an enéchelon strike-slip fault systems using 2D Finite Element Modelling (FEM) under plane stress condition. The numerical modelling technique used for the experiments is based on FEM which enables us to analyze the static behavior of a real and continues structures. We have used three sets of models to explore how the geometry of model (fault overlap and pre-existing weak shear zone) and applied boundary conditions (pure strike-slip, transpressional and transtensional) influence the development of state of stress and deformation during the formation of pull-apart basins. Modelling results presented here are based on five parameters: 1) distribution, orienttation, and magnitude of maximum (σH max) and minimum (σH max) horizontal compressive stress 2) magnitude and orientation of displacement vectors 3) distribution and concentration of strain 4) distribution of fault type and 5) distribution and concentration of maximum shear stress (σH max) contours. The modelling results demonstrate that the deformation pattern of the en-échelon strike-slip pull-apart formation is mainly dependent on the applied boundary conditions and amount of overlap between two master strike-slip faults. When the amount of overlap of the two master strike-slip faults increases, the surface deformation gets wider and longer but when the overlap between two master strike-slip faults is zero, block rotation observed significantly, and only narrow and small surface deform ation obtained. These results imply that overlap between two master strike-slip faults is a significant factor in controlling the shape, size and morphology of the pull-apart basin formation. Results of numerical modelling further show that the pattern of the distribution of maximum shear stress (τmax) contours are prominently depend on the amount of overlap between two master strike-slip faults and applied boundary conditions. In case of more overlap between two masters strike-slip faults, τ max mainly concentrated at two corners of the master faults and that reduces and finally reaches zero at the centre of the pull-apart basin, whereas in case of no overlap, τmax largely concentrated at two corners and tips of the master strike-slip faults. These results imply that the distribution and concentration of the maximum shear stress is mainly governed by amount of overlap between the master strike-slip faults in the en-échelon pull-apart formation. Numerical results further highlight that the distribution patterns of the displacement vectors are mostly dependent on the amount of overlap and applied boundary conditions in the en-échelon pull-apart formation.

Late Palaeozoic deformation of post-Variscan lower crust: shear zone widening due to strain localization during retrograde shearing

The Santa Lucia Shear Zone (SLSZ, Corsica) is a granulite-facies Permian shear zone that developed after the emplacement of a deep-seated gabbroic intrusion. New structural data shows that the SLSZ results from the juxtaposition of three spatially distinct mylonite belts, which are the product of the interaction between magmatism, metamorphism and shearing over a temperature range from ~800 to ~400°C. During the earlier high-grade deformation stage, which was accompanied by decompression from ~7 to ~5 kb at ~800°C, the SLSZ has accommodated high finite strain on a shear zone ≥1 km wide. Strain became increasingly localized as temperature decreased, but rather than reactivating pre-existing shear zones as commonly expected, younger mylonites expanded into previously unsheared rock, extending the total width of the shear zone. The zonation of different fabrics across the SLSZ suggests that pre-existing compositional and grain size heterogeneities in the starting material played a key role in governing superposed generations of shear zones.

The Benagerie Shear Zone: 1100 Myr of reactivation history and control over continental lithospheric deformation

A newly identified northwest–southeast oriented, deeply-rooted, steep to vertical, large-scale structural system within the Proterozoic Curnamona Province, Australia, which we term the “Benagerie Shear Zone”, is imaged in regional magnetic and gravity datasets. In this study, we use a combination of field analysis and quantitative geophysical methods, to establish a 1100 Myr history of activity along the Benagerie Shear Zone during which the location of younger geological structures are influenced by the pre-existing shear zone. This deformational system is interpreted to have 1) aided ascent and emplacement of the ca. 1600 Ma Ninnerie (magmatic) Supersuite; 2) controlled the loci of nucleation of normal faults during rifting and continental breakup at ca. 800 Ma; and 3) influenced the development of fold structures as well as acting as a plane co-linear to the rotation axis of pre-existing normal faults such that they were steepened and reactivated as strike slip structures during the ca. 500 Ma Delamerian Orogeny. We interpret that the Benagerie Shear Zone has not undergone uni-directional propagation during its evolution but rather through reactivation was a primary influence on controlling the nucleation of Neoproterozoic rift faults, thereby playing a major role in accommodating strain over a significant period of the evolution of the Curnamona Province. This study demonstrates that crustal-scale shear zones can evolve over hundreds of millions of years, have strike-lengths of hundreds to thousands of kilometers, and have vastly different surface expressions along strike.

Structural and provenance analyses of a newly defined major fault: The Homyeong Fault as a northern boundary fault of the Cretaceous Nagdong Trough, South Korea

The Homyeong Fault on the Yecheon Shear Zone is newly defined and described in the northwestern part of North Gyeongsang Province, South Korea. In the study area, the basement rocks consist, of the Precambrian rocks and Jurassic granitoid, and are dextrally deformed by the Jurassic Yecheon Shear Zone to the north of the Cretaceous Gyeongsang Supergroup in a ductile manner. These basement rocks are covered with unconformity by the sindong Group, the lowest part of the Gyeongsang Supergroup. The Homyeong Fault runs on the Yecheon Shear Zone, and is marked by cataclasite that contains mylonite fragments originating from the mylonitic rocks of the Yecheon Shear Zone and that shows sinistral sense of shear with normal-fault component dipping basinward, which indicates reactivation of the Yecheon Shear Zone. The conglomerates of the northern part of the Sindong Group usually contain the gravels of granite, quartzite, blackish quartzite, mylonite, ultramylonite, and mudstone. Petrological similarity between these gravels and the basement rocks suggests that most of the gravels were very likely derived from the Yecheon Shear Zone and its vicinity. Considering the Korean Cretaceous basins mostly formed by sinistral reactivation of the pre-existing shear zones, it can be judged that the reactivation of the Yecheon Shear Zone caused the basement rocks nearby the Homyeong Fault, or the Yecheon Shear Zone and its vicinity, to be deposited as the gravels to the Nagdong Trough where the Sindong Group was filled on the south of the fault. This gives new proposals that the Homyeong Fault had played a role as a northern boundary fault of the Nagdong Trough and also that the inversion tectonics from Jurassic dextral to Cretaceous sinistral motions, observed in the eastern margin of Asia, had an effect on the formation of the Nagdong Trough.

Acadian deformation in the shallow crust: an example from the Siluro-Devonian Arisaig Group, Avalon terrane, mainland Nova Scotia

The Silurian – Early Devonian Arisaig Group of the Avalon terrane in northern mainland Nova Scotia consists mainly of thinly bedded sandstones, siltstones, and shales deposited in a near shore environment. These strata were deformed in the middle Devonian to form regional northeast- to NNE-trending folds and record deformation processes in the shallow crust during the Acadian orogeny, one of the most regionally extensive orogenic events in the Canadian Appalachians. Structural features in the Arisaig Group are consistent with fold propagation associated with thrust fault geometry and coeval local extension recorded by a set of conjugate normal faults. Many outcrop-scale folds have sheared limbs and show evidence of a complex progressive deformation. Folding was predominantly accomplished by bulk rotation and flattening above thrust fault tips. Early structures (D1–D2) produced regional cylindrical folds, whereas later (D3a, D3b, D3c) structures produced conical folds. D1–D3 fold orientations show high variability, but are consistent with progressive deformation related to reactivation and coeval dextral strike-slip movement along the Hollow Fault. The style of deformation is compatible with models in which strain is partitioned into preexisting shear zones in the basement, with folds in the overlying Arisaig Group initiated above the tips of upward-propagating thrusts as second-order structures related to movement along those shear zones. Taken together, these data indicate that fold mechanisms and geometry in the shallow crust during the Acadian orogeny in mainland Nova Scotia may be related to dextral strike-slip along major faults in the basement and co-genetic upward-propagating thrusts that rotated and flattened overlying strata.

Delineation of subsurface Proterozoic Unkar and Chuar Group sedimentary basins in northern Arizona using gravity and magnetics: Implications for hydrocarbon source potential

Subsurface occurrences of Mesoproterozoic Unkar Group (∼1.25–1.1 Ga) and Neoproterozoic Chuar Group (∼0.9–0.7 Ga) rocks have been delineated in regions east and southeast of the Grand Canyon of northern Arizona. Gravity and aeromagnetic data, combined with gravity modeling, an analysis of seismic velocities, geologic cross sections, and geologic structures, have identified two major Proterozoic extensional regimes in the region: (1) a Mesoproterozoic northwest to southeast trend indicating northeast-southwest–directed extension throughout the study area and (2) a Neoproterozoic, mostly north-south–directed trend indicating predominant east to west extension, evident mostly in northern regions of the study area. A more ancient, northeast-southwest–aligned system of basement shear zones has also been identified. These shear zones correlate with many of the exposed major fault systems of the region such as the Bright Angel and Mesa Butte fault systems. Unkar Group graben and half-graben formation was constrained by the preexisting northeast-directed shear zones. Chuar Group basin formation was influenced by the reactivation of northwest to southeast Unkar Group basin structures, the more ancient northeast-trending shear zones, and by Neoproterozoic age north- to south-oriented extension. This superimposed network of deep-seated normal faults led to the development of a complex array of surface structures during the Laramide. By delineating Proterozoic basin structures in the region, this study helps to delineate the probable subsurface occurrences of hydrocarbon source rocks of the Walcott Member of the Neoproterozoic Chuar Group. John M. Seeley is a research geologist and assistant director for the Pan-American Center for Earth and Environmental Studies at the University of Texas at El Paso. He received his B.S. (1985) and M.S. (1990) degrees from Texas A&I University and his Ph.D. (1999) from the University of Texas at El Paso. His research interests include depositional systems, sequence stratigraphy, gravity and magnetics, and tectonics.G. Randy Keller holds the L. A. Nelson Professorship. He is also chief scientist of University of Texas at El Paso's NASA Earth Science Research Center. His research interests stress the geological applications of geophysics, and he is particularly interested in the integration of diverse data sets and the use of information technology. He is a longtime member of the AAPG, Society of Exploration Geophysicists, Geological Society of America, American Geophysical Union, and the European Association of Exploration Geophysicists.

A granite?gabbro complex from Madagascar: constraints on melting of the lower crust

The Ranomandry Complex is a Neoproterozoic, nested intrusion from central Madagascar composed of a gabbroic core within a coeval peraluminous granite ring intruding pelitic metasediments. Although xenocryst entrainment and magma mixing have both contributed to marginal phases of the granite, the primary melt is characterised by steep LREE/HREE ratios and negligible, or slightly positive, Eu anomalies. Both isotopic and trace element systematics preclude an origin from either partial melting of the metapelitic country rock or from fractional crystallisation of the gabbroic magma. However, trace-element modelling suggests an origin from the dehydration melting of a plagioclase-poor, garnet-bearing metagreywacke at temperatures of 850–900 °C and at lower crustal pressures (>10 kbar). Melting of an enriched subcontinental mantle generated gabbroic magmas that caused advective heating and anatexis at the base of thickened continental crust during their ascent. Transport of the resulting granite magma was facilitated by the pre-existing plumbing system that overcame thermal and mechanical problems associated with both diapirism and self-propagating dykes as mechanisms for long-distance transport of granite magmas. The nested geometry of the intrusions is an indication of a structurally homogeneous lower crust that contains no pre-existing shear zones or fault systems.

Crustal architecture and evolution of the Eastern Ghats Belt and adjacent regions of India

Abstract Extending along the east coast of peninsular India, the Eastern Ghats expose a deep section through a composite orogenic belt that once formed part of the Proterozoic mobile belt system within East Antarctica and East India. The critical evaluation of the existing geological and isotopic data strongly suggests that this orogenic belt includes not only the granulite facies Eastern Ghats Belt but also the Nellore-Khammam Schist Belt and lower grade units at the southern margin of the Singhbhum Craton. The present authors propose its subdivision into four crustal provinces with widely different geological evolutions. The Rengali and Jeypore Provinces formed at the margin of the Bhandara Craton in the Late-Archaean. In the Krishna Province, volcanosedimentary rocks equivalent to the Cuddapah Supergroup accumulated, probably on the Dharwar Craton in the Palaeoproterozoic, and the major tectonometamorphic event took place between 1.67 and 1.55 Ga, subsequent to a short-lived igneous activity. The Eastern Ghats Province, which shows considerable similarities with the Rayner Province of East Antarctica, was strongly affected by pervasive deformation, high-grade metamorphism and crustal-derived magmatism between 1.1 and 0.9 Ga, which extensively modified the crustal structure of present eastern peninsular India. Neoproterozoic and Early Phanerozoic tectonothermal activities were largely restricted to pre-existing shear zones, but the present configuration of the composite orogenic belt may have been achieved only during the Pan-African Orogeny.

Late Oligocene volcanism and extension in the southern Red Sea Hills, Sudan

The temporal and spatial relationships between Cenozoic extension in the Red Sea and flood basalt volcanism in the Ethiopian plateau region were poorly understood along the Red Sea Hills region of Sudan. Rift basins in the Red Sea area developed in terranes accreted in Pan-African time, and it is believed that some of these ancient sutures controlled later Red Sea rifting. The objectives of field, remote sensing and geochronological studies of the southern Red Sea Hills near the Sudan–Eritrea border were to constrain the geometry and kinematics of extension, the timing of volcanism, and the structural relationship between Cenozoic and Pan-African faults. Basaltic volcanism began at about 31 Ma in the Derudeb region, coincident with the widespread flood basaltic volcanism in Ethiopia and Yemen associated with the Afar plume, suggesting a causal link. Felsic lavas erupted onto a faulted erosional surface at about 29 Ma, based on 40 Ar/ 39 Ar step heating results on feldspar separates. The original form of these basins has been modified by subsequent faulting, Red Sea rift flank uplift, and erosion. Structural patterns indicate that extension increases eastward toward the Red Sea, with stratal dips of up to 35° in the Adar Ribad region, but most of this fault block rotation post-dates the rhyolite eruptions. The location of the Odi and Adar Ribad basins, and the reactivation of segments of Pan-African shear zones of the Baraka suture suggest that the development of these basins was influenced by pre-existing crustal shear zones, although some faults and dykes cross-cut other basement structures.