Brittle Extension Research Articles

Late Palaeozoic NE–SW ductile–brittle extension in the La Serre horst, eastern France

Abstract In the La Serre horst of the Alpine foreland, the pre-Triassic La Serre median fault zone separates a Late Devonian–Early Carboniferous granite from an ignimbrite of unknown age and from Permian deposits. Motion along this fault zone took place first in ductile conditions and then evolved in brittle conditions. Both ductile and brittle shear criteria indicate a top-NE normal-dextral displacement. Similar motions are reported along faults bounding Late Palaeozoic intramontane coal basins located in the Massif Central and correspond to a widespread NE–SW Late to Post-Orogenic extension that affected the Variscan basement during Late Carboniferous to Early Permian times. To cite this article: G. Coromina, O. Fabbri, C. R. Geoscience 336 (2004).

Brittle deformation between the Ambin and Vanoise domes in the frame of the structural evolution of the internal Alpine belt

New geophysical data and geological observations emphasize the importance of brittle tectonics in the Neogene to present day evolution of the Western Alps. The oldest Neogene stress state (F1) is characterized by extension direction parallel to the orogen and shortening direction vertical or perpendicular to the Alpine belt whereas the youngest tectonic event (F2) is characterized by brittle extension perpendicular to the mountain belt. According to new brittle microtectonics data from the High Maurienne valley near Modane and a synthesis of the available geochronological and microtectonics data at the scale of the internal Alps, we discuss a timing of the brittle tectonics phases and their role in the formation of basement domes (Internal Crystalline massifs and Brianconnais domes). We propose that the F1 event spans from 32-30 Ma to 22 Ma south of the Simplon fault and was probably active up to 5 Ma on the Simplon fault. The F2 event started at about 6-5 Ma and is still active. In this scheme, basement domes observed in the Western Alps are the results of interference between an early E-W extension along NE-SW ductile to brittle faults and the two F1-F2 brittle tectonic events documented here.

Neogene to ongoing normal faulting in the inner western Alps: A major evolution of the late alpine tectonics

Widespread brittle extension occurs in the internal zones of the southwestern Alps, forming a dense fault network, which overprints the compressional structures. The extension has been followed by transcurrent motions. In terms of paleostress this extension is radial to the belt close to the Crustal Penninic Front and multidirectional in the eastern part of the internal zones. The paleostress field of the strike‐slip phase is coherent with the extensional one and compatible with dextral shear along longitudinal faults. Globally, we propose that the internal zones of the southwestern Alps underwent a single Neogene transtensive tectonic regime. To strengthen our structural results, the current tectonics has been analyzed using seismotectonic tools. This approach allowed the recognition of several active faults. Moreover, the ongoing tectonics is coherent with the observed brittle deformation. The fault network controls the present‐day seismicity. The combined seismotectonic and structural approaches give coherency and continuity from the Neogene to ongoing extensional tectonics in the southwestern Alps. In the framework of the whole western Alps a synthesis of our results with neotectonic‐related data allowed us to provide an accurate map of the Neogene to present kinematics of the belt. Extension, which appears as a major feature of the internal zones, has been coeval with the propagation of thrusts in the outer zones during Neogene times. The geodynamic processes, which rule this major tectonic evolution of the belt, remain a matter of debate.

Post‐nappe brittle extension in the inner Western Alps (Schistes Lustrés) following late ductile exhumation: a record of synextension block rotation?

AbstractFault data collected from the Schistes Lustrés domain point to the existence of successive steps of deformation and indicate that extension is not multidirectional. This study underlines the continuity between the patterns of late brittle/ductile exhumation tectonics and brittle deformation, and strenghtens the view that extensional movements dominate in shallow levels of the inner Western Alps since at least 35–30 Ma. The progressive clockwise rotation of the earliest directions of extension with time is compatible with the amount of anticlockwise rotation from c. 35 Ma determined by recent palaeomagnetic studies, whereas the last documented N–S extension may reflect a short‐lived stage of orogen‐parallel extension.

Geodynamic evolution of the Vulcan Sub‐basin, Timor Sea, northwest Australia: A pre‐compression New Guinea analogue?

The Vulcan Sub‐basin lies immediately inboard of the incipient arc‐continent collision in the Timor Sea and comprises part of the Bonaparte Basin system, the northernmost basin on Australia's North West Shelf. Given the high level of preservation of its extensional fabric, the region can provide important analogues for the likely pre‐orogeny architecture of New Guinea, which enables a better understanding of the onset of, and response to, orogenesis. Structural restoration of regional, depth‐converted 2–D seismic lines shows that although the Late Jurassic Swan Graben is significant and contains a thick source‐rock section, the principal phase of crustal extension took place in the Triassic to Middle Jurassic. Within the Vulcan Sub‐basin, the southern Tilted Fault Block Domain records ∼10% Triassic to Middle Jurassic extension, whereas <5% upper crustal extension has been measured in the northern Hourglass Domain. Similarly, while Jurassic extension in the Tilted Fault Block Domain is both deep and focused, the Hourglass Domain is expressed as a broad sag to the northeast, indicating a strong underlying basement influence on compartmentalisation. The Vulcan Sub‐basin shows four principal stages of evolution: (i) regional, evenly spaced crustal faulting and subsidence in the Triassic ‐ Middle Jurassic; (ii) focused faulting in the Late Jurassic that created grabens with uplift of the shoulders; (iii) regional subsidence from the Middle Valanginian; and (iv) minor extensional and contractional reactivation in the Mio‐Pliocene. The measured brittle extension is much less than that suggested by modelling of lithospheric subsidence, which suggests long wavelength distribution of strain in the ductile lower crust, with upper crustal extension mainly focused along the continent‐ocean boundary. Along the North West Shelf and on a smaller scale within the Vulcan Sub‐basin per se, the obvious, basement‐involved, rectilinear compartments defined by prominent offsetting of both extensional fault systems and abyssal plains have important implications for the development of the New Guinea orogen. Similar scale compartments are recognised in New Guinea and display different structural styles and hydrocarbon prospectivity. The transfer zones separating the compartments are the sites of the major copper‐gold deposits in New Guinea. Using the Vulcan Sub‐basin ‐ Timor area as an analogue, it can be seen that an arc could originally collide with a promontory, such as what is now Timor, and reactivate the lineaments allowing local extension and mineralisation. In addition, interpretation of the structure of the New Guinea Fold Belt may be aided by considering the effects of compression on the geometry of the Vulcan Sub‐basin and of the similar Carnarvon Basin and adjacent extended and broken Exmouth Plateau.

Widespread post-nappe normal faulting in the Internal Western Alps:a new constraint on arc dynamics

Abstract: Field data collected in the Briançonnais and Piémont zones at the centre of the western Alpine arc demonstrate a high density of normal faults. Normal displacements are pervasive from the millimetric to the kilometric scales. They occur both parallel and at high angles to the arc, overprinting all the Alpine folds and thrusts and the associated schistosities. The orientation of fault striae implies that the pile of metamorphic nappes SE of the Pelvoux massif has undergone a general extension. Extension tends to be multidirectional in the Briançonnais zone. Orogen-parallel extension prevails in most of the Piémont Schistes lustrés. To the west, in Briançonnais zone, brittle extension is associated with a reversal of movement along the Briançonnais Frontal Thrust that started before the end of the Oligocene. In the Piémont Schistes lustrés, the onset of normal faulting occurred early in the Miocene and continued the ductile extension that accompanied the exhumation of HP–LT metamorphic thrust sheets during the Palaeogene. In both zones, extension remains currently active, resulting in the regional seismicity. Since more than 20 Ma ago, this normal faulting in the internal arc coexists with an outward propagation of folds and thrusts in the external arc. We discuss its possible significance in the context to the dynamic evolution of the Alps.

Strike-slip and tectonics granitoid emplacement: an AMS fabric study from the Odenwald Crystalline Complex, SW Germany

AMS fabric studies supported by field and microscopic work were applied to identify the internal structure and possible emplacement processes of the Variscan late-tectonic granodiorite-granite intrusions of the Unit III in the Odenwald Crystalline Complex. This Unit is bounded towards NW and NNE by steeply inclined shear zones, the southern part is unexposed. The magnetic susceptibility ranges between 10−3 and 10−6 SI units and is caused by paramagnetic and subordinately by ferromagnetic components. AMS ellipsoids are typically oblate with gently plunging long axes (lineations). AMS foliations and lineations trend mainly WSW-ENE and NNW-SSE, parallel with the NNW and ENE trending marginal shear zones of Unit III, respectively. As revealed by microstructural studies, a penetrative foliation in the plutons is related to emplacement processes. Therefore the observed AMS foliation and lineation are also interpreted as the result of syn-emplacement deformation which is dominantly strike-slip. Weakly inclined foliations around pluton roof xenoliths point to a component of buoyant rise of magma. It is suggested that the granitoid magma was generated in a low level anatectic zone along a left-lateral transpressive shear zone during local extension at releasing bends. During successive fault movements magma ascended through extensional parts of the shear zone. Local normal faults and the Otzberg zone at the eastern margin of Unit III document mostly brittle extension, which overprinted the strike-slip fabrics after the emplacement of the plutons.

Faulting and fault scaling on the median valley floor of the trans‐Atlantic geotraverse (TAG) segment, ∼26°N on the Mid‐Atlantic Ridge

A quantitative assessment of faulting on the median valley floor of a slow spreading ridge is accomplished through the analysis of high‐resolution DSL‐120 sidescan sonar and coregistered bathymetric data from the TAG segment near 26°N on the Mid‐Atlantic Ridge. At this location, faulting is exposed within a 3–5 km wide ridge‐parallel zone lying asymmetrically on the eastern half of the median valley floor. Mapped faults have a normal sense of displacement, are &lt;2 km in length, and accommodate ∼1.5% brittle extension. Evidence of fault linkage within the fault population includes kinked and bent fault traces in map view, the development of overlapping fault segments or relay ramps, and the presence of multiple local maxima in the displacement‐distance profiles of some faults. Faults have a slight tendency to dip to the east, or outward relative to the valley axis, and exhibit little symmetry of fault dip about the axis of the faulted zone or any other ridge‐parallel line. Faults exhibit a roughly linear relationship between maximum fault throw and fault length, with a mean ratio of 0.030 for the population. Regression of length‐frequency data indicates a power‐law distribution, with an exponent of 1.64–1.96, demonstrating that fractal populations can exist in the mid‐ocean ridge environment. The fractal nature of this length‐frequency distribution and the ratio of maximum fault throw to fault length differ significantly from those described previously for populations of larger abyssal hill faults in the fast spreading environment, where the distribution is exponential and the throw‐to‐length ratio is ∼5 times lower. These results suggest that the scaling of fault populations in the midocean ridge setting may vary as a function of spreading rate and/or fault size.

Ductile and brittle extension in the southern Lofoten Archipelago, North Norway; implications for differences in tectonic style along an ancient collisional margin

The Lofoten archipelago, north Norway, occupies the most internal position of the Caledonian belt in northern Scandinavia, and rocks and structures exposed there are crucial to understanding processes of how the Baltic basement and its cover allochthons responded to continental lithospheric subduction and subsequent continental separation. Relatively little is published about the structural and metamorphic development and especially the timing of these events; consequently, it is unknown how features exposed on these spatially isolated islands relate to those of the adjacent mainland. Rocks in Lofoten were affected by Caledonian regional metamorphism, and structures record tops-east contraction and later extension related to late- to post-Caledonian basement exhumation. Tops-west extension is preferentially developed in meter to km-scale ductile shear zones containing west-dipping extensional shear bands, west-verging rootless folds, and asymmetric feldspar porphyroclasts. West-plunging, sinistral-oblique elongation lineations in the mylonitic foliation are interpreted to indicate the line of transport. Mesoscopic backfolds are locally developed within the tops-west shear zones and further document west-directed transport of structurally higher rocks. the ductile extensional shear zones locally are cross cut by low-angle, tops-west cataclastic normal faults, reflecting progressive unroofing of the shear zones to shallower crustal levels during late- and post-Caledonian extensional events. Northeast-striking, high-angle, brittle normal faults truncate all other fabrics and structures and juxtapose structurally deep undeformed Precambrian basement with the Caledonian nappe sequence. ^40^ Ar/ ^39^ Ar data indicate that ductile extension in Lofoten likely initiated soon after the Caledonian metamorphic peak (at approximately 430 Ma) and continued episodically until at least Permian time. Rocks passed through the brittle/ductile transition rapidly at approximately 275 Ma. Recent paleomagnetic data also point to later phases of brittle faulting in Lofoten, one in the Jurassic/Cretaceous and one related to Tertiary opening of the Norwegian Sea. The magnitude and style of late- to post-Caledonian extension and basement exhumation in Lofoten differs markedly from the formation of phenomenal Devonian basins and exposures of Caledonian ultra-high pressure assemblages in the Western Gneiss Region (WGR) of southern Norway. Differences in the styles of extension between northern and southern Norway are interpreted to reflect inherited differences in crustal architecture that may have been enhanced during the Caledonian orogeny.

Miocene high-pressure metamorphic rocks of Crete, Greece: rapid exhumation by buoyant escape

Abstract The pre-Neogene thrust sheets of Crete, Greece, accreted during Oligocene and Early Miocene time, can be divided into two main groups juxtaposed by a Miocene extensional detachment. Oligo-Miocene high pressure-low temperature (HP-LT) metamorphic rocks crop out in the lower plate to the detachment, and rocks that show no evidence of Tertiary metamorphism in the upper plate. Detailed pressure, temperature and structural information from the HP-LT metamorphic rocks combined with new fission-track data from the upper plate reveal that the lower plate rocks were subducted, then pervasively deformed and metamorphosed at their maximum depth of burial (30–35 km, 300–400°C) between c. 24 and 19 Ma and then exhumed to less than 10 km depth at rates &gt;4 km Ma −1 before c. 17 Ma. Microstructural studies reveal that during exhumation the lower plate acted as a coherent block, with deformation and retrograde metamorphism localized along the extensional detachment. The rocks of the upper plate can be shown to have remained in the upper 4–7 km of the crust since at least Eocene time. After cessation of movement along the main extensional detachment, both the upper and lower plates were subjected to brittle extension and increased erosion initiated at c. 16–17 Ma. These boundary conditions imply continuous subduction retreat since at least Eocene time along the Aegean segment of the Hellenic convergent plate boundary. A tectonic model is presented where exhumation is driven by positive buoyancy of the subducted continental crust following lithospheric delamination. We propose that the subducted microcontinent was exhumed by a process of ‘oblique buoyant escape’ and entered the space created by the retreating subducting oceanic slab.

Stratigraphical evidence supporting the rifting, drifting and collision of the Laurentian Precordillera terrane of western Argentina

Abstract Major stratigraphical evidence points toward initial rifting of the Precordillera terrane of western Argentina during the Early Cambrian from the Ouachita embayment in southeastern Laurentia. A first fully developed rifting episode is suggested from subsidence analysis and the sedimentary record. Restricted graben-fill red-beds and evaporites in the eastern tectofacies, together with coarse arkose and quartz conglomerate preserved as blocks in the western tectofacies, are interpreted as deposits of the initial rift-related brittle extension. Stratigraphical similarities with localities surrounding the Ouachita embayment leads to a model in which the Precordillera was the southern part of the pre-Appalachian Laurentia margin and the conjugate margin of the Texas promontory, separated along the Alabama-Oklahoma transform and the Ouachita rift, respectively. Overstepping of syn-rift facies by a thick succession of Lower Cambrian to Lower Ordovician carbonates indicates completion of rift-to-drift transition and initiation of thermo-tectonic subsidence characteristic of continental terrace passive-margin development. Slope facies, marginal faunal belts and recently dated mafic rocks indicate the development of an ocean basin separating the Precordillera from Laurentia by the Mid-Cambrian. A 20–30 Ma period of drifting and isolation is indicated from faunal and sedimentological evidence. Subsidence-induced incipient drowning and abundant K-bentonites suggest the influence of subduction and the progressive approach of the terrane to western Gondwana. Continuous Mid- and Late Ordovician post-collisional extension brackets the collision, although no clear evidence of shortening has been detected in the preserved former sedimentary bank. Post-collisional extension is separated by &gt;70 Ma from the initial rifting.

Listric normal faulting on the Cascadia continental margin

Analysis of multichannel seismic reflection profiles reveals that listric normal faulting is widespread on the northern Oregon and Washington continental shelf and upper slope, suggesting E‐W extension in this region. Fault activity began in the late Miocene and, in some cases, has continued into the Holocene. Most listric faults sole out into a subhorizontal décollement coincident with the upper contact of an Eocene to middle Miocene mélange and broken formation (MBF), known as the Hoh rock assemblage onshore, whereas other faults penetrate and offset the top of the MBF. The areal distribution of extensional faulting on the shelf and upper slope is similar to the subsurface distribution of the MBF. Evidence onshore and on the continental shelf suggests that the MBF is overpressured and mobile. For listric faults which become subhorizontal at depth, these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the overlying stratigraphic section along a low‐angle (0.1°–2.5°) detachment coincident with the upper MBF contact. Mobilization, extension, and unconstrained westward movement of the MBF may also contribute to brittle extension of the overlying sediments. No Pliocene or Quaternary extensional faults have been identified off the central Oregon or northernmost Washington coast, where the shelf is underlain by the rigid basaltic basement of the Siletzia terrane. Quaternary extension of the shelf and upper slope is contemporaneous with active accretion and thrust faulting on the lower slope, suggesting that the shelf and upper slope are decoupled from subduction‐related compression.

Discussion on Late Precambrian tectonothermal evolution of the Malverns Complex

T. C. Pharaoh &amp; W. J. Barclay write: Strachan et al. 1996 are to be congratulated for providing further details on the tectonothermal evolution of the Malverns Complex, and for constraining the provenance of detrital grains in the basal Cambrian sequence. The new data add substance to the evolution outlined by Barclay et al. in press in the forthcoming BGS memoir, the manuscript of which was supplied to the authors during their study. As the authors have demonstrated, the &gt;600 Ma ductile deformation of the plutonic igneous rocks of the Malverns Complex predates, and is unrelated to, the development of the latest Proterozoic (&lt;565 Ma) terrane boundary invoked by Pharaoh et al. 1987. The ductile fabrics of the complex are not only discordant to the brittle structures of the Malvern Lineament, as recognized by Baker (1971) , but are cross-cut by mafic dykes testifying to late brittle extension of the complex at about 566 Ma (Barclay et al. in press). Contrary to the authors' inference, Pharaoh et al. 1987 did not invoke ductile deformation of the Malverns Complex during the accretion of the 566 Ma Warren House Formation marginal-basin lithosphere and somewhat older Charnwood Terrane (Pharaoh &amp; Gibbons 1994). The latter event occurred in a more brittle tectonic regime subsequent to 566 Ma. The cause of the tectonism between 650 and 600 Ma is not clear at present, but presumably reflects compressional or transpressional deformation of the cooling Malverns calc-alkaline magmatic arc and is manifestly not related to the accretion of

The Stratigraphy and Structure of the Madamar-Salakh-Qusaybah Range and Natif-Fahud Area in the Oman Mountains

Melanges and debris flows with clasts derived from the top of the Natih Formation found in shales in the base of the Aruma Group indicate that a period of Structural growth on the platform took place during Aruma deposition in the Late Cretaceous. In this respect the platform in the Jebel Salakh area may have undergone a similar period of structural growth in the Late Cretaceous to the Fahud area where a syn-Aruma normal fault down throwing to the South accounts for a difference in the stratigraphic thickness of the Aruma of 1 km. A younger series of debris flows in the Aruma of the Sufrat al Khays area to the South of Jehel Salakh is dated as Campanian/Maastrichtian. The clasts in these flows were derived exclusively from the Simsima limestones. Natih-derived elasts are conspicuously absent. This is taken to indicate that the Madamar-Salakh Qusaybah range was covered by Aruma sediments at this time and did not form the distinctive positive feature seen at present - i.e. Madamar-Salakh-Qusaybah range folding though partly Late Cretaceous is mainly Post-Manslrichtian in age. This Post Maastrichtian event in the Madamar-Salakh-Qusaybah range produced a series of doubly-plunging anticlines in the Cretaceous strata- These folds show a high degree of brittle extension in the form of normal faults and extensional fractures, The faults are delineated by fault gouge with visibly interconnected void space. In the subsurface, if such fractures were developed in a fold closure similar to those seen at the surface in the Madamar-Salakh-Qusaybah range. then they could provide preferred conduits for oil flow and the harrier to fluid flow provided by the Aruma shale seal could lead to a hydrocarbon accumulation.

ABSTRACTS POSTER PRESENTATIONS

S OF POSTER PRESENTATIONS SOURCES OF SILVER IN THE EAST LOOE RIVER NATURAL OR ANTHROPOGENIC? C.J. Moon and B. Dickie Department of Geology, University of Leicester Both the East and West Looe Rivers have very high concentrations of silver in their lower reaches. Enhancement in silver in the West Looe river is caused by contamination from, and natural enrichment around, the disused lead mines at Herodsfoot. The source in the East Looe river is of comparable intensity but of enigmatic origin. We have traced the silver back to the storm water outfall from Liskeard where sediment values are 25 ppm Ag, 200 ppm Cu and 500 ppm Pb. The poster will present mineralogical data from the outfall to discriminate between a natural vein source and an anthropogenic origin. EXTENSIONAL COLLAPSE IN THE FOOTWALL REGION OF THE LIZARD OPHIOLITE, EASTERN MOUNTS BAY: A METAMORPHIC CORE-COMPLEX ANALOGUE? A.J. Styles, A.C. Alexander, R.K. Shail and R.E. Holdsworth Department of Geological Sciences, University of Durham Camborne School of Mines, University of Exeter Detailed structural field-mapping along the Mounts Bay coastalsection has revealed a group of late-Variscan extensional structures which have, until recently, largely been overlooked by previous workers. Although the extensional style is variable and partitioned, it is likely that they represent broadly coeval deformation which is related to gravity collapse. • Structures verge down the dip of pre-existing regional foliations. • They are dominated by low-angle detachments or shear zones in which the sense of displacement is consistently directed top to the south-east, which show local cross-cutting relationships with ductile features. • The low-angle detachments are associated with a suite of folds and domino faults which do not show overprinting relationships and hence suggest synchronous brittle and quasi-ductile behaviour causing strain hardening. • The late ductile deformation is cut by granite sheets, making it pre-final emplacement in age. • They are cut by moderate to steep post-orogenic normal-faults. • Many stages of quartz and, more rarely, calcite veins are observed, indicating high pore-fluid pressure throughout orogenesis. The features documented along the section form a distinct and continuous group which shows increasingly brittle deformation styles towards the south-east. There are also strong geometric similarities between these structures and soft-sediment slump and slide features, suggesting that in bulk rheological terms, the overthickened crust is acting as a viscous wedge spreading under the influence of gravity. The extensional structures appear to systematically become more brittle away from the Tregonning-Godolphin Granite and towards the Lizard Ophiolite. Kinematic indicators suggest extensional collapse directed away from a ductile region, through a zone of ductile folds, shears and transposing cleavages, and into a region deforming through brittle faulting. The ductile structure in the northwest transposes all earlier structures, but as the extension becomes more brittle to the southeast, earlier compressional features are still preserved. The ductile-brittle transition appears to be analagous to the core, carapace and cover of core complex models. Early brittle extension estimates

Late orogenic, plastic to brittle extension along the Robertson Lake shear zone: implications for the style of deep-crustal extension in the Grenville orogen, Canada

Rocks regionally metamorphosed and deformed at middle- to lower-crustal levels in the contraction-dominated Mesoproterozoic Grenville orogen are exposed in southern Ontario, Canada. Investigation of the Robertson Lake shear zone (RLSZ) indicates that extension is a significant component in the late tectonic evolution of this deeply eroded orogen. The RLSZ is a discrete zone 0.5–1.0 km thick that can be traced for nearly 100 km, cross-cutting regional structures and lithologic units. The zone is composed of a thick mylonite zone and a narrow overlying brecciated zone. Mylonite foliation shallowly dips east-southeast and contains a down-dip lineation. Mylonites are characterized by microstructures indicative of crystal-plastic deformation and contain a variety of shear-sense indicators, including SC and CC′ composite fabrics, sigma porphyroclasts, mica fish and other shape fabrics oblique to shear planes, consistently indicating a normal (down-to-the-east) sense of shear. A zone of cataclastic structures overlies the mylonite zone, and brittle deformation overprints the mylonitic fabrics. Slickensides in the zone generally strike parallel to foliation in the mylonites but have steeper dips. Sense of slip indicators and brittle fault orientations also indicate down-to-the-east displacement during east-west extension. Juxtaposition of brittle and crystal-plastic structures is resolved with a model of displacement during exhumation whereby localization of cataclasis occurred along a previously active mylonite zone. Metamorphic facies are upper greenschist in the hanging wall and upper amphibolite in the footwall and metamorphic grade increases from west to east in both the hanging wall and footwall. The regional variations in metamorphic grade and the low-angle shear zone geometry are a result of isostatic flexural rotations that accompanied extension. Combined with the crystal-plastic to cataclastic nature of the RLSZ, new 40Ar/ 39Ar isotope data from biotite constrain the timing of shear zone displacement until at least 901 ± 1 Ma, late in the evolution of the Grenville orogen.

Testing the Concept of Diapiric Rise and Fall Using 3D Seismic Data from the U.K. Southern Gas Basin: ABSTRACT

The U.K. Southern Gas Basin experienced a long and complicated developmental history-involving extension, salt tectonics, inversion, and subsidence-that began in the early Triassic and continued into the middle Tertiary. In our study areas, faulted Paleozoic rocks (Rotliegendes Fm. and older) are separated from a faulted overburden sequence (Bacton Grp. to Chalk) by the salt-dominated Zechstein Grp. Overburden structure is dominated by a series of broadly NW-SE trending graben and associated salt diapirs and walls. New insights into salt tectonics derived from scaled analog and numerical model studies, reveal that in areas with originally thick salt, regional extension of the overburden is important in triggering and controlling diapirism and graben growth. Diapiric piercement proceeds in a predictable series of modes-reactive, active, and passive. As extension continues, if for some reason the salt supply becomes restricted, the diapir can no longer grow and begins to fall. The diapir can then no longer support the graben floor and the graben collapses into the falling diapir. The change from diapiric rise to diapiric collapse is often marked by fundamental changes in sediment distribution patterns. We interpret graben and diapiric growth to be initiated by gravity-driven thin-skinned brittle extension of the overburden. The supply ofmore » salt to the developing diapirs eventually became restricted and continued extension lead to diapiric collapse. Collapse is marked by fundamental changes in sediment distribution patterns. While all graben continued to act as depocentres, many graben flanks, areas that were originally high, became syn-collapse depocentres; many integraben troughs, areas that had acted as depocentres during diapiric rise, became starved and received little sedimentation during diapiric collapse.« less

Comparison of seafloor tectonic fabric at intermediate, fast, and super fast spreading ridges: Influence of spreading rate, plate motions, and ridge segmentation on fault patterns

We have conducted a comparative study of the tectonic morphology of young seafloor using SeaMARC II side scan sonar surveys of the intermediate spreading Ecuador Rift, the fast spreading East Pacific Rise (EPR) (8°30′–10°N), and the super fast spreading EPR (18°–19°S). We find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments (i.e., with proximity to large‐ and short‐offset discontinuities). We also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone. Most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer. For example, regions of super fast spreading are characterized by the largest numbers of short faults, the smallest average fault spacing and throw, and the highest fault density. In addition, clusters of short, closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area, presumably the result of a thinner, weaker brittle layer. Faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates. Rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates. Outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets. These differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge. Fault lengths and spacings in all areas approximate exponential distributions. The extensional strain represented by fault populations is calculated from the displacement and length distributions of faults, and strain estimates of ∼4% are obtained for each area. Assuming that fault spacing reflects fracture depth extent where faults initiate, we infer a brittle layer thickness of ∼1 km when faulting begins. Fault populations are examined for ridge segment scale variations in amagmatic extension. We see evidence for greater amagmatic extension associated with long‐term reduced magma supply along the eastern third of the Ecuador Rift. Evidence for local increased brittle extension is also found within 15 km of transform faults. Discordant zones left by overlapping spreading centers (OSCs) are characterized by low fault abundances. At OSCs, discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting. Fault azimuths do appear to be useful indicators of plate motion. Within the EPR 8°30′–10°N area, fault trends record a recent change in Pacific‐Cocos plate motion (3°–6° at ∼1 m.y.) consistent with magnetic anomaly and fault lineation data from elsewhere along the northern EPR. Within the Ecuador Rift, fault azimuths scatter within 3° of predicted trends and are consistent with constant spreading about one pole for the past 1.5 m.y.

The Dog Bay Line: a major Silurian tectonic boundary in northeast Newfoundland

The Dog Bay Line separates different Silurian rock groups in northeast Newfoundland. West of the line, terrestrial volcanic rocks and sandstones (Botwood Group) overlie marine greywackes and conglomerates (Badger Group). East of the line, red sandstones overlie shallow marine shales and limestones (Indian Islands Group). Throughout Dog Bay, the line is marked by a disrupted zone of dark grey to black shales, volcanic rocks, and gabbros. Pervasive dextral, transpressive ductile deformation followed by successively more brittle extension with renewed dextral movements mark the northwest side of the line on the coast.The Dog Bay Line is traceable for 100 km and it is open-ended. Dextral offset is deduced to be many tens of kilometres. The line trends northeast, parallel to outcrop belts, and both the line and outcrop belts are curved eastward at the coast. The Mount Peyton Batholith, dated at 420 ± 8 Ma, apparently cuts the line.The Dog Bay Line occurs within the Dunnage Zone whose Cambrian–Ordovician rocks represent vestiges of the Iapetus Ocean. Northwest of the line, the Silurian rocks were deposited on Ordovician rocks already accreted to Laurentia. Southeast of the line, the Silurian rocks were deposited on Ordovician rocks already amalgamated with the continental Gander Zone. Timing of major movement and a Silurian marine to terrestrial depositional change recorded on both sides of the line agree within error with isotopic ages for the onset of plutonism, regional deformation, and metamorphism in central Newfoundland. The Dog Bay Line may mark the terminal Iapetus Ocean.

A Concept Matrix for Geological Structures

Concept maps for six current structural geology texts show a diversity of approaches in ordering the content of the subject. Further complications result from the interrelationships of topics, for example, en echelon vein arrays were found to be covered under folds, shear zones, regional geology, field techniques, igneous geology and strain theory. This is a consequence of many geological structures forming as composites of two broad “types” of structures on different scales, for example, en echelon vein arrays are brittle extension fractures on a small scale within a shear zone on a larger scale. A matrix with six important structural “types” on each axis with the horizontal axis representing the smaller scale and the vertical axis representing the larger scale provides a framework for 36 “types” of composite structures. This concept matrix can be used as an introduction to structures, a summary of structures or a guide for student investigations of structures. By using the concept matrix, a student can ...