Crustal-scale Ramp Research Articles

Miocene cave sediments record topographic, erosional and drainage development in the Western European Alps

The dynamic Neogene evolution of the Western European Alps included exhumation of the external crystalline massifs, thrust propagation to the foreland, drainage network reorganization, and major climatic variations. To constrain possible interactions between those factors, accurate geomorphological and sedimentological archives are required. However, intra-orogenic areas are subject to erosion, and extensive glacial cover during the Quaternary erased most of the geomorphic markers in the Alps. For these reasons, the genesis of the main features of the modern landscape, such as the major valleys and the drainage network, remains poorly understood. This study highlights how recently discovered karstic archives from the perched paleo-karst of the Obiou peak (Dévoluy massif, SE France) record the tectonic and drainage-network evolution of this part of the Alps during the Neogene. The Obiou caves are located at 2250-2380 m elevation, ∼1600 m above the modern Drac valley; they contain fluvial deposits including sand-clay units and rounded crystalline cobbles derived from the adjacent Ecrins-Pelvoux massif. As the Dévoluy and Ecrins-Pelvoux massifs are currently separated by the axial Drac valley (a major tributary of the Isère River), these cave sediments must have been deposited by a radial drainage system before incision of the modern Drac. We report new multi-method results from these sediments, including cosmogenic-nuclide burial dating (21Ne, 10Be, 26Al in quartz), provenance analysis (clast petrography and heavy-mineral analysis), and detrital thermochronology (apatite fission-track and (U-Th)/He) combined with a paleo-environmental reconstruction from palynology. 21Ne/10Be dating of cobbles and sand constrains the burial age to 11.5 ± 1.5 Ma, providing a maximum age for the modern axial drainage system and a minimum long-term incision rate of ∼140 m/Myr for the Drac valley. Comparison of the combined data to both modern rivers and nearby Oligocene foreland-basin deposits provides evidence for two successive drainage reorganizations. Early Miocene exhumation and development of high topography in the Ecrins-Pelvoux massif, linked to localized thrusting on a crustal-scale ramp, led to initial deflection of the antecedent radial drainage network, beheading its headwaters by establishment of the axial upper Durance valley. Subsequent propagation of thrusting into the subalpine Dévoluy massif and associated uplift during the mid to late Miocene led to establishment of the modern drainage system.

Fold interference pattern and crustal decoupling in northern Tanzania

High-temperature granulite facies metamorphism and deformation evolved through Neoproterozoic tectonic processes in large parts of southeastern Africa. The Eastern Granulites of Tanzania, one example for these extreme conditions, show progressive decoupling between lower and middle crust together with progressive cooling and rheology stratification. Coupled deformation during earliest stages of deformation arises from extremely small viscosity contrast of all minerals and rocks at granulite facies metamorphic conditions around 850 °C. With cooling of the orogen rheology stratification increases and deformation decouples into horizontal stretching through gravity driven channel flow in the lower crust and thickening and horizontal west-east shortening in the middle/upper crust. As soon as a stiff and cold indenter, represented by external parts of the Archean Tanzania Craton (Western Granulites) enters the system, the indenter acts as lower crustal ramp along which deep crustal units are exhumed and displaced towards the foreland. During this orogenic period the crust was fully stratified and a second phase of decoupling, detachment over crustal scale ramp, occurs. Superposition with late orogenic north-south shortening structures resulted in type 1 fold interference pattern and formation a of dome-and-basin geometry in northeastern Tanzania. Structural modelling of fold superposition helps to explain the complex structural pattern and to predict sites of decollement horizons. By comparison with fold interference pattern exposed in southern Madagascar we define different types of dome-and-basin formation. Those with neglectable rheology contrast are viscosity determined where mixing of upper and lower crustal material is enabled by vertical flow folding. Those with strong viscosity contrast are mechanically determined where domes and basins are formed by changes in regional principal stress directions.

Investigating possible gravity change rates expected from long-term deep crustal processes in Taiwan

We propose to test if gravimetry can prove useful in discriminating different models of long-term deep crustal processes in the case of the Taiwan mountain belt. We discuss two existing tectonic models that differ in the deep processes proposed to sustain the long-term growth of the orogen. One model assumes underplating of the uppermost Eurasian crust with subduction of the deeper part of the crust into the mantle. The other one suggests the accretion of the whole Eurasian crust above crustal-scale ramps, the lower crust being accreted into the collisional orogen. We compute the temporal gravity changes caused only by long-term rock mass transfers at depth for each of them. We show that the underplating model implies a rate of gravity change of −6 × 10−2 μGal yr−1, a value that increases to 2 × 10−2 μGal yr−1 if crustal subduction is neglected. If the accretion of the whole Eurasian crust occurs, a rate of 7 × 10−2 μGal yr−1 is obtained. The two models tested differ both in signal amplitude and spatial distribution. The yearly gravity changes expected by long-term deep crustal mass processes in Taiwan are two orders of magnitude below the present-day uncertainty of land-based gravity measurements. Assuming that these annually averaged long-term gravity changes will linearly accumulate with ongoing mountain building, multidecadal time-series are needed to identify comparable rates of gravity change. However, as gravity is sensitive to any mass redistribution, effects of short-term processes such as seismicity and surface mass transfers (erosion, sedimentation, ground-water) may prevent from detecting any long-term deep signal. This study indicates that temporal gravity is not appropriate for deciphering the long-term deep crustal processes involved in the Taiwan mountain belt.

Nd isotope mapping of allochthonous Grenvillian klippen: Evidence for widespread ‘ramp-flat’ thrust geometry in the SW Grenville Province

Over sixty new Nd isotope analyses are used to constrain the extent of several small Grenvillian lithotectonic domains that form erosional outliers of the Grenvillian Allochthonous Polycyclic Belt. The identification of these tectonic klippen, when combined with previous mapping of large nappes, shows that the Allochthonous Polycyclic Belt forms a mid-upper crustal sub-horizontal thrust sheet that was transported to the NW over the Archean-Proterozoic parautochthon. This thrust sheet was corrugated by folding and dissected by erosion, leaving the klippen as synformal remnants. Nd isotope mapping of these remnants, when coupled with seismic reflection data from the Lithoprobe programme, suggests that these thrust sheets were exhumed on relatively steep crustal-scale ramps before transportation to the NW on sub-horizontal shear zones. This type of thrust geometry, previously observed in the central—eastern Grenville Province was termed ‘ramp-flat geometry’ by Rivers et al. (2012). Re-examination of Lithoprobe seismic data for the Georgian Bay transect shows that reflectors under the Parry Sound klippe have a synformal character in the mid crust, suggesting that this is also part of an undulose sub-horizontal thrust sheet exhumed on a steep crustal-scale ramp, rather than the previous model of a very long ramp with low inclination. Hence we argue that the Georgian Bay section has a geometry resembling the ramp-flat trajectory of the Allochthon Boundary Thrust previously discerned elsewhere in the Grenville Province.

Control of detachment geometry on lateral variations in exhumation rates in the Himalaya: Insights from low-temperature thermochronology and numerical modeling

[1] The Himalayan range is commonly presented as largely laterally uniform from west to east. However, geological structures, topography, precipitation rate, convergence rates, and low-temperature thermochronological ages all vary significantly along strike. Here, we focus on the interpretation of thermochronological data sets in terms of along-strike variations in geometry and kinematics of the main crustal detachment underlying the Himalaya: the Main Himalayan Thrust (MHT). We report new apatite fission track (AFT) ages collected along north-south transects in western and eastern central Nepal (at the latitudes of the Annapurna and Langtang massifs, respectively). AFT ages are consistently young (<3 Ma) along both N-S transects in the high-relief zone of the Higher Himalaya and increase (4 to 6 Ma) toward the south in the Lesser Himalaya. We compare our new data to published low-temperature thermochronological data sets for Nepal and the Bhutan Himalaya. We use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the Pecube code in order to constrain potential along-strike variations in the kinematics of the Himalayan range. Our results show that lateral variations in the geometry of the MHT (in particular the presence or absence of a major crustal-scale ramp) strongly control the kinematics and exhumation history of the orogen.

A reappraisal of the tectonic significance of early Tertiary low-angle shear zones exposed in the Vernon map area (82 L), Shuswap metamorphic complex, southeastern Canadian Cordillera

Detailed geological mapping across the Shuswap metamorphic complex between latitudes 50°00′N and 50°45′N reveals that superstructure forms a semicontinuous carapace across the complex, with minimal evidence of internal thinning. Near the western margin of the complex, superstructure and infrastructure are juxtaposed across low-angle, ~2 km thick, ductile shear zones spatially associated with Paleocene to Early Eocene syn-kinematic granitic rocks. The shear zones, which yield upper plate to the west shear-sense indicators, are interpreted as the northern extension of the Okanagan Valley fault. Farther east, near the north–south axis of the complex, superstructure and infrastructure are separated by an attenuated metamorphic section, but evidence of noncoaxial strain is lacking. Discrete detachments were not found. Steeply dipping normal faults cut low-angle shear zones and do not merge with them at depth. Middle Eocene volcanic and sedimentary rocks rest unconformably on metamorphic basement. The continuity of superstructure indicates that infrastructure was not exhumed by crustal-scale detachments. The results provide the basis for a complete reinterpretation of the tectonic significance of low-angle shear zones exposed in the Vernon area. It is proposed that Late Cretaceous to early Tertiary partial melting of the middle crust resulted in the development of a zone of channel flow. As the channel was underthrust by a crustal-scale ramp in underlying, more competent Paleo proterozoic basement, it was exhumed from depths of 20–30 km and thinned vertically. Shear zones between infrastructure and superstructure are interpreted as being a transient rheological interface at the upper boundary of the channel.

U–Pb zircon ages as a sediment mixing tracer in the Nepal Himalaya

This paper presents a new approach to quantify sediment mixing based on the mixing of U–Pb zircon age distributions within sediment. Two statistical techniques are presented to determine the proportion in which two known age distributions combine to create a known mixed age distribution. These techniques are then used to determine relative erosion rates between adjacent drainage basins above and below the Main Central Thrust (MCT) in the central Nepal Himalaya. The MCT region is coincident with an abrupt north–south change in geomorphic character and mineral cooling ages that are thought to represent an erosional response to higher rock uplift rates north of the MCT zone. However, it is unclear whether the ongoing deformation responsible for the differential uplift rates is: (1) focused on the MCT; (2) at depth along a crustal scale ramp; or (3) along newly mapped thrust faults south of the MCT. Our study explores this issue by comparing modern erosion rates with longer-term erosion rates determined from mineral cooling ages. Zircons were separated from modern river sand and dated by LA-MC-ICPMS before the measured isotopic ratios and ages were used in 1-d and 2-d mixing calculations. The 1-d technique creates probability density functions of zircon ages for each sample and then uses both an iterative and inverse approach to estimate mixing between samples. In contrast, the 2-d technique estimates mixing between probability “fields” defined by the measured 238U/ 206Pb and 207Pb/ 206Pb ratios. Given a finite mixture with perfect sample representation, both techniques produce perfect mixing estimates across a range of mixing proportions. Modeling results demonstrate that given imperfect subsample representation of the complex parent age distribution, differing degrees of subsample smoothing may be required to achieve an accurate mixing estimate. Using mixing of zircon ages as a quantitative proxy for sediment mixing requires a correction for the concentration of zircon in the river sediment. Two new methods for establishing zircon concentration in river sediment are presented demonstrating the existence of 2- to 5-fold differences in zircon concentration between adjacent drainages. Relative erosion rates are estimated by determining the zircon mixing ratio between adjacent drainages which are then normalized by the ratio of zircon concentrations and the ratio of drainage areas. Results show ∼ 3 times higher modern erosion rates south of the MCT in the northernmost Lesser Himalaya. Future applications of this new technique may include reach-scale sediment transport dynamics, improved sedimentary basin analysis, and better interpretation of foreland mineral cooling ages.

CONTRASTING SEQUENCES OF METAPELITIC MINERAL-ASSEMBLAGES IN THE AUREOLE OF THE TILTED NELSON BATHOLITH, BRITISH COLUMBIA: IMPLICATIONS FOR PHASE EQUILIBRIA AND PRESSURE DETERMINATION IN ANDALUSITE SILLIMANITE-TYPE SETTINGS

The Nelson Batholith is a ca. 1,800 km 2 Jurassic intrusive body in southeastern British Columbia surrounded by a contact aureole, 0.7–1.8 km wide, developed in graphitic argillaceous rocks that show only minor variations in bulk composition. Contrasting prograde sequences of mineral assemblages are developed around the aureole in a regular pattern, reflecting different pressures of contact metamorphism. The following assemblages are seen going from lower to higher pressure (all assemblages contain muscovite + biotite + quartz ± Mn-rich garnet): (1) cordierite-only assemblages, (2) mix of cordierite-only and cordierite + andalusite assemblages, locally with cordierite + K-feldspar and andalusite + K-feldspar assemblages at higher grade, (3) andalusite-only assemblages, with sillimanite + andalusite assemblages and locally sillimanite + K-feldspar assemblages at higher grade, (4) staurolite-only assemblages, (5) staurolite ± andalusite assemblages, with sillimanite-bearing and locally sillimanite + K-feldspar assemblages at higher grade. The higher-pressure sequences with staurolite ± andalusite are restricted to the aureole surrounding the east half of the batholith, whereas the lower-pressure cordierite ± andalusite are restricted to the aureole surrounding the west half of the batholith and its northern and southern tips. The sequences of mineral assemblages correspond closely to the facies series of Pattison & Tracy (1991) and are interpreted to represent a series of approximately isobaric metamorphic field-gradients below the Al2SiO5 triple point, providing an excellent opportunity to evaluate thermodynamically calculated low-pressure phase equilibria in the metapelitic system. The total difference in pressure represented by the contrasting assemblages is about 1.0 kbar, showing that they are a sensitive measure of small differences in pressure within the stability field of andalusite. Thermobarometry results from the aureole are moderately consistent with the mineral assemblage constraints, but carry pressure uncertainties larger than the total range of pressure represented by the aureole's assemblages. Pressures of the intrusive rocks derived from hornblende barometry are scattered, and many do not agree with the pressure constraints from the aureole. The mineral-assemblage constraints indicate down-to-the-west post-metamorphic tilting of the batholith and aureole of about 10°, interpreted to be due to a combination of eastward thrusting of the Nelson Batholith over crustal-scale ramps during Cretaceous–Paleocene shortening and Eocene east-side-down normal motion on the Slocan Lake – Champion Lakes fault system that forms the western boundary of the batholith.

Crustal architecture above the high-pressure belt of the Grenville Province in the Manicouagan area: new structural, petrologic and U–Pb age constraints

Recent U–Pb age determinations and P–T estimates allow us to characterize the different levels of a formerly thickened crust, and provide further constraints on the make up and tectono-thermal evolution of the Grenville Province in the Manicouagan area. An important tectonic element, the Manicouagan Imbricate zone (MIZ), consists of mainly 1.65, 1.48 and 1.17 Ga igneous rocks metamorphosed under 1400–1800 MPa and 800–900 °C at ∼1.05–1.03 Ga, during the Ottawan episode of the Grenvillian orogenic cycle, coevally with intrusion of gabbro dykes in shear zones. The MIZ has been interpreted as representing thermally weakened deep levels of thickened crust extruded towards the NW over a parautochthonous crustal-scale ramp. Mantle-derived melts are considered as in part responsible for the high metamorphic temperatures that were registered. New data show that mid-crustal levels structurally above the MIZ are represented by the Gabriel Complex of the Berthé terrane, that consists of migmatite with boudins of 1136±15 Ma gabbro and rafts of anatectic metapelite with an inherited monazite age at 1478±30 Ma. These rocks were metamorphosed at about the same time as the MIZ (metamorphic zircon in gabbro: 1046±2 Ma; single grains of monazite in anatectic metapelite: 1053±2 Ma) and under the same T range (800–900 °C) but at lower P conditions (∼1000–1100 MPa). They are mainly exposed in an antiformal culmination above a high-strain zone, which has tectonic lenses of high P–T rocks from the MIZ and is intruded by synmetamorphic gabbroic rocks. This zone is interpreted as part of the hangingwall of the MIZ during extrusion. A gap of 400 MPa in metamorphic pressures between the tectonic lenses and the country rocks, together with the broad similarity in metamorphic ages, are consistent with rapid tectonic transport of the high P–T rocks over a ramp prior to the incorporation of the mafic lenses in the hangingwall. Between the antiformal culmination of the Gabriel Complex and the MIZ 1.48 Ga old granulites of the Hart Jaune terrane are exposed. They are intruded by unmetamorphosed 1228±3 Ma gabbro sills and 1166±1 Ma anorthosite. Hart Jaune Terrane represents relatively high crustal levels that truncate the MIZ-Gabriel Complex contact and are preserved in a synformal structure. Farther south, the Gabriel Complex is overlain by the Banded Complex, a composite unit including 1403+32/−25 Ma granodiorite and 1238+16/−13−1202+40/−25 Ma granite. This unit has been metamorphosed under relatively low-P (∼800 MPa) granulite-facies conditions. Metamorphic U–Pb data, limited to zircon lower intercept ages (971±38 Ma and 996±27 Ma) and a titanite (990±5 Ma) age, are interpreted to postdate the metamorphic peak. The general configuration of units along the section is consistent with extrusion of the MIZ during shortening and, finally, normal displacement along discrete shear zones.

Applications of seismic pattern recognition and gravity inversion techniques to obtain enhanced subsurface images of the Earth's crust under the Central Metasedimentary Belt, Grenville Province, Ontario

SUMMARY Project Lithoprobe's Abitibi–Grenville transect seismic reflection lines 32 and 33 traverse the exposed Central Metasedimentary Belt (CMB) located in the Grenville province of the Precambrian Shield of Canada in southern Ontario. These seismic lines image a zone with a protracted deformational history spanning more than 300 Myr. Detailed examination of the commercially processed stacked sections reveals a number of significant deficiencies in some important areas. The image quality in these zones of reduced coherency needs to be enhanced to examine specific features and their relation to the surface geology. Examination of near-vertical seismic data from Lines 32 and 33 revealed that the signal-to-noise ratio was not improved by stacking, due to misalignment of signals even after static, normal moveout corrections and residual static corrections. The presumed reason is that reflected seismic energy following long ray paths in heterogeneous media suffers from relative advances and delays in its propagation, and hence arrives at slightly different times at the receivers, tending to be poorly aligned relative to its theoretical traveltime curves. A pattern recognition (PR) method for signal enhancement followed by energy stacking in moving time windows was used in this study to improve the images in spite of misalignments. Reprocessing has refined the geometry of the reflection profiles. The objective of this paper is to use enhanced images of the seismic reflection data obtained by using a PR approach together with gravity data, using 2.5-D forward and 3-D inversion routines, to give an improved model of subsurface structure in the vicinity of lines 32 and 33. Line 32 is dominated by southeast-dipping reflectors soling into the lower crust. The listric geometry of the strong reflection packages of the CMB boundary thrust zone is interpreted to represent a crustal-scale ramp–flat geometry that accommodated northwest-directed tectonic transport of the CMB. This interpretation is also supported by the gravity data.

Crustal geometry and tectonic evolution of the Archean crystalline basement beneath the southern Alberta Plains, from new seismic reflection and potential-field studies

Deep seismic reflection profiling of the southern Alberta crystalline basement, namely the Archean Medicine Hat block (MHB), has revealed that the crust is characterized by west- and southwest-dipping reflection fabrics, a layered lower crust, and geometric cutoffs that are interpreted to be remnants of a complex tectonic history. Upper to middle crustal dipping reflections are interpreted to be associated with Archean rocks drilled below the Western Canada Sedimentary Basin and their geometry is consistent with east- and northeast-verging imbrication. Layered lower crustal reflections of the MHB are interpreted as Early Proterozoic delaminated Loverna block crust and injected mafic material, based on geometric relationships, xenolith dating, and gravity modelling. The reflection Moho is diffuse below the MHB at ~15 s two-way time (~47 km). This is similar to estimates based on adjacent seismic refraction data and is up to ~8 km deeper than regions north of the MHB. The MHB crust is interpreted to have undergone two episodes of crustal deformation in the Precambrian: (1) Late Archean formation of the MHB crust by continental collision of two Archean domains along a crustal-scale ramp; and (2) Early Proterozoic crustal shortening associated with the formation of the Vulcan structure, the delamination of the Loverna block crust, and postcollisional injection of mafic melts beneath the MHB crust. Radiometric dating of lower crustal xenoliths suggests a significant Early Proterozoic thermal event in the deep crust beneath the MHB that is coeval with orogenic activity in the Trans-Hudson Orogen and the East Alberta Orogen.

Crustal evolution along a seismic section across the Grenville Province (western Quebec)

Results of deep seismic reflection survey along a 375 km long transect of the Grenville Province in western Quebec are combined with a review of geological observations and published isotopic ages. The seismic profile offers a remarkably clear image of the crust-mantle boundary and a good definition of the various crustal blocks. Crust about 44 km thick beneath the Grenville Front zone thins abruptly to ca. 36 km southeastward, perhaps the result of extension on southeast-dipping surfaces extending to the Moho. Other zones of relatively thin crust, although less pronounced, occur where Proterozoic crust overlies Archean crust, and beneath the Morin anorthosite complex. The thickest crust is found at the extreme southeast of the transect, east of the Morin anorthosite. From northwest to southeast, three main crustal subdivisions are (1) deformed Archean rocks with southeast-dipping reflectors in the Grenville Front zone, (2) an Archean parautochthon with northwest-dipping reflectors extending to the lower crust, and (3) an overlying three-layer crust interpreted as accreted Proterozoic terranes. The boundary between (2) and (3) is a major, southeast-dipping, crustal-scale ramp (Baskatong ramp) interpreted to have accommodated strain during and after accretion. U-Pb and Pb-Pb ages on detrital zircons show that metasedimentary rocks of the allochthons (Mont-Laurier, Réservoir Cabonga, and Lac Dumoine terranes) range from Archean to as young as 1.21 Ga. A single zone with 1.4 Ga old Sm-Nd model ages appears to lack Archean components and may be considered as a fragment of juvenile Mesoproterozoic crust pinched in a shear zone (Renzy shear zone) that could be raised to the status of terrane (Renzy terrane). In the allochthons, U-Pb ages of metamorphic zircon and monazite cluster around 1.17 Ga (Mont-Laurier and Réservoir Cabonga terrane) and 1.07 Ga (Renzy and Lac Dumoine terrane) and are interpreted to record late and post-accretion crustal reworking, a common feature of the Grenville orogen. A final high-grade metamorphic event (ca. 1.0 Ga) documented only in the parautochthon and the Grenville Front zone records large-scale, piggyback-style thrusting of allochthonous slabs onto the parautochthon. The age of transcurrent displacement following peak metamorphism affecting both the allochthons and the parautochthon decreases northwestward from 1.07 to 1.00 Ga. Dating thus shows that Grenvillian deformation in western Quebec occurred in pulses over an interval of 180 million years, with a tendency to propagate from the inner part of the orogen toward the Grenville Front. Reworked migmatites from the parautochthon cooled from the ca. 1.0 Ga peak of metamorphism through about 450°C (Ar closure in hornblende) at ca. 0.96 Ga with calculated cooling rates of about 6°C per million years, and unroofing rates of 0.33 km per million years. The cooling-unroofing history of the allochthons is not so straightforward, probably due to tectonic disturbances related to allochthon emplacement. Cooling through 450°C occurred between 1.04 and 1.01 Ga, at least 50 million years earlier than cooling in the parautochthon; this contrast agrees with the northwestward propagation of the orogen.

Tectono-thermal evolution of deep crust in a Mesoproterozoic continental collision setting: the Manicouagan example

High-pressure metamorphic rocks in continental collision belts provide important information on the orogenic evolution of deep crust. In the Grenville Province, a prime example is the Manicouagan Imbricate Zone (MIZ), a Mesoproterozoic crustal section that was metamorphosed up to 1800 MPa and 850-950°C at ~1050 Ma. High temperatures and the presence of synmetamorphic within-plate tholeiitic gabbro intrusion in the MIZ attest to thermal perturbation during burial and emplacement of mantle-derived melts, consistent with thinning of lithospheric mantle. Subsequently, the MIZ was extruded by a combination of northwest-directed thrusting over a crustal-scale ramp and extension at higher crustal levels. In the middle MIZ, extension was coeval with thrusting, but perpendicular to the thrust direction, and was apparently controlled by ductility contrasts between slices. In contrast, the highest levels show evidence of southeast-directed extension, i.e., of opposite sense to the thrust direction, that postdated thrusting within the MIZ. This episode was followed by renewed northwest-directed thrusting of medium-pressure Mesoproterozoic units over the MIZ, and then by final southeast-directed extension. Overall configuration of the MIZ was achieved between ~1040-990 Ma and is consistent with outward propagation of the orogen. Alternating periods of thrusting and extension displacement are a likely result of adjustments permitting maintenance of dynamic equilibrium within the advancing stack. The tectonothermal characteristics of the MIZ allow comparison between the Grenville Province and younger, thermally perturbed orogens (i.e., Variscan Belt) and may be the final product of a Tibetan Plateau style evolution.

Water as a Petrologic Catalyst Driving 18O/16O Homogenization and Anatexis of the Middle Crust in the Metamorphic Core Complexes of British Columbia

In the Thor-Odin and Valhalla metamorphic core complexes, we have documented a remarkable uniformity of mineral δ18O values in the middle continental crust beneath the detachment faults. For example, in the Thor-Odin Complex, throughout an 8 km thick section of metasedimentary rocks and early Tertiary leucogranites in the hanging wall of the Monashee decollement (MD), quartz δ18O = 12.3 ± 0.5% (lσ S.D.) for metapelite (N = 11), 12.0 ± 0.1% for quartzite (N = 2), 12.6 ± 0.6% (N = 4) for < 1 m thick amphibolite layers, and 12.1 ± 0.4% (N = 24) for the concordant leucogranites. No exceptions have been found to this remarkable 18O/16O homogeneity except locally in a couple of thick amphibolites and within a ductile, relatively impermeable, marble-rich section. Similar zones of 18O/16O homogeneity associated with leucogranite genesis are observed throughout the mid-crustal section of the Valhalla Complex and just beneath the MD in the Monashee Complex, the only difference being that those rocks are overall 0.5 to 1.5% lower in δ18O than in the middle crust at Thor-Odin. These zones of pervasive homogenization in 18O/l6O must be a result of exchange with magmatic or metamorphic H2O, and these same volatiles appear to have been responsible for the leucogranite anatexis. A wide range in quartz δ18O from +8 to +16 within and below the MD suggests that this major thrust fault was impermeable to aqueous fluid flow during the partial melting stage; at that time, the basement appears to have been isolated from the mid-crustal metamorphichydrothermal system. LITHOPROBE crustal seismic profiles establish the MD as a W-dipping, crustal-scale ramp with 20 km of vertical relief, and Carr (1992) proposed an anatectic origin for the leucogranites during decompression melting associated with tectonic shortening as the mid-crustal section moved up this thrust ramp. Partial melting of metapelites and feldspathic grits from the Late Precambrian Windermere Supergroup began in response to influx of metamorphic H2O, aided by internal muscovite dehydration at ≈8 kbar and ≈750°C at the base of the Monashee ramp. Metapelites are volatile rich, but feldspar poor, whereas the opposite is true for the grit lithologies. Thus, at the base of the Monashee ramp large-scale (≈30°) partial melting of the metapelites produced magmas near H2O saturation (10 tol4 wt°), whereas the intercalated arkosic grit-derived magmas were undersaturated (5 to 6 wt°). As these H2O-rich, pelite-derived leucogranite melts moved upward to shallower depths, they cooled adiabatically and underwent decompressive exsolution of H2O. The released H2O was then able to exchange oxygen with lithologies infertile to melting as it concurrently migrated through the section toward the feldspathic grit layers, where it could act as a catalyst and be re-used, promoting further hydrothermal melting of the arkosic grits. Continued decompression melting and exsolution occurred simultaneously in different parts of the section during uplift, tectonic shortening, and buoyant uprise of the magma bodies, until final crystallization of all of the leucogranites took place much higher in the crust, where almost all of the H2O was released and again re-used for a final episode of 18O/l6O exchange with the unmelted metamorphic lithologies. In addition to the direct l8O/16O exchange that takes place between the metamorphic rocks and the migrating leucogranite magmas, this use and re-use of the same H2O during repeated episodes of partial melting and exsolution in different parts of the section seems adequate to explain the pervasive oxygen isotopic homogenization of these metasedimentary rocks. It is estimated that 25 to 30° partial melting of a typical section of the Windermere Supergroup occurred as a result of these cumulative processes, and this probably played a pivotal role in determining the susceptibility of this orogen to subsequent extensional collapse along the detachment faults.

Constraints on Himalayan deformation inferred from vertical velocity fields in Nepal and Tibet

Spirit leveling data from the Nepal Himalaya between 1977 and 1990 indicate localized uplift at 2–3 mm/yr in the Lesser Himalaya with spatial wavelengths of 25–35 km and at 4–6 mm/yr in the Greater Himalaya with a wavelength of ≈40 km. Leveling data with significantly sparser spatial sampling in southern Tibet between 1959 and 1981 suggest that the Himalayan divide may be rising at a rate of 7.5±5.6 mm/yr relative to central Tibet. We use two‐dimensional dislocation modeling methods to examine a number of structural models that yield vertical velocity fields similar to those observed. Although these models are structurally nonunique, dislocation models that satisfy the data require aseismic slip rates of 2–7 mm/yr on shallow dipping faults beneath the Lesser Himalaya and rates of 9–18 mm/yr on deep thrust faults dipping at ≈25°N near the Greater Himalaya. Unfortunately, the leveling data cannot constrain long‐wavelength uplift (&gt;100 km) across the Himalaya, and unequivocal estimates of aseismic slip in central Nepal are therefore not possible. In turn, the poor spatial density of leveling data in southern Tibet may inadequately sample the processes responsible for the uplift of the Greater Himalaya. Despite these shortcomings in the leveling data, the pattern of uplift is consistent with a crustal scale ramp near the Greater Himalaya linking shallow northward dipping thrust planes (3–6°) beneath the Lesser Himalaya and southern Tibet. Aseismic slip on the potential rupture surface of future great earthquakes beneath the Nepal Himalaya south of this ramp appears not to exceed 30% of the total convergence rate between India and southern Tibet resulting in an accumulating slip deficit of 13±8 mm/yr.

Crustal structure of the external variscides in northwest spain from deep seismic reflection profiling

Recent deep-crustal seismic reflection profiling escicantabrica-1 provides a detailed image of the external parts of the Variscan Belt in northwest Spain. These seismic data together with surface geological information provide constraints on the crustal structure of the foreland thrust and fold belt (Cantabrian Zone) and the transition to the hinterland areas (Narcea Antiform area). The image in the Cantabrian Zone includes strong inclined reflections in the upper 5 s that correspond to mappable Paleozoic rocks at the surface which have been deformed by thrusting and folding during the Carboniferous. Some of the major thrust faults imaged show a ramp-flat geometry at depth and merge into a highly reflective band between 5 and 6 s that corresponds to the Cantabrian Zone detachment. The Precambrian basement is almost transparent underneath the Cantabrian Zone detachment. There are a few subhorizontal discontinuous reflections at 11 s that may correspond to the reflection Moho. At the transition to the hinterland, the seismic profile includes short inclined reflections grouped in several reflective bands in the upper 6 s. These correspond to the Narcea Antiform, which comprises at the surface a stack of thrust sheets containing Precambrian rocks. The Narcea Antiform is placed above a crustal-scale ramp imaged by strong W-dipping reflections between 6 and 9 s. These reflections merge upwards into the Cantabrian Zone detachment at 6 s, and downwards into a reflective lower-crustal zone located between 9 and 12 s. These midcrustal reflectors are interpreted as a major Variscan shear zone that marks the transition between the thin-skinned tectonics of the external areas and the thick-skinned tectonics of the hinterland areas. The lower crust is highly reflective below the Narcea Antiform area and reflectivity fades out to the east beneath the foreland thrust and fold belt. The different seismic responses at middle- and lower-crustal levels in the foreland and hinterland areas suggest that Variscan tectonics strongly affected deep-crustal levels in the hinterland areas, whereas they remained undeformed beneath the foreland.

Crustal architecture across Phanerozoic Australia along the Eromanga-Brisbane Geoscience Transect: evolution and analogues

The crustal architecture across Phanerozoic Australia has been established from seismic data along an 1100-km-long east-west transect, the Eromanga-Brisbane Geoscience Transect. This has enabled a better understanding of deep structures and processes that have controlled the development of major sedimentary basin systems. It has shown that crustal dynamics throughout geological history have played an important role in the development of these sedimentary basins and that structures developed during the early Palaeozoic have influenced, and continue to influence, basin systems. The transect crosses three major basement provinces of the Tasman Orogenic System in eastern Australia - the Thomson, northern Lachlan, and New England Orogens. The basement geology in the transect region has, until now, been only poorly understood because it is largely obscured by the Mesozoic cover rocks of the Eromanga, Surat and Clarence-Moreton Basins. The transect interpretation has firmly identified crustal-scale ramp structures, multiple intra-crustal detachment surfaces, strike-slip fault architecture, lower-crustal magmatism/underplating, Mono remobilisation, and intra-crustal terranes in the geological reconstructions of southern Queensland. The boundaries between the orogens, the Foyleview and Burunga—Mooki Geosutures, have been identified as lithospheric-scale structures that have influenced the evolution of the Tasman Orogenic System as a whole.

Contrasting tectonic styles in the northern Grenville province: Implications for the dynamics of orogenic fronts

Research Article| December 01, 1993 Contrasting tectonic styles in the northern Grenville province: Implications for the dynamics of orogenic fronts T. Rivers; T. Rivers 1Department of Earth Sciences, Memorial University, St. John's, Newfoundland A1B 3X5, Canada Search for other works by this author on: GSW Google Scholar J.A.M. van Gool; J.A.M. van Gool 1Department of Earth Sciences, Memorial University, St. John's, Newfoundland A1B 3X5, Canada Search for other works by this author on: GSW Google Scholar J. N. Connelly J. N. Connelly 1Department of Earth Sciences, Memorial University, St. John's, Newfoundland A1B 3X5, Canada Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (12): 1127–1130. https://doi.org/10.1130/0091-7613(1993)021<1127:CTSITN>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation T. Rivers, J.A.M. van Gool, J. N. Connelly; Contrasting tectonic styles in the northern Grenville province: Implications for the dynamics of orogenic fronts. Geology 1993;; 21 (12): 1127–1130. doi: https://doi.org/10.1130/0091-7613(1993)021<1127:CTSITN>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In western Labrador, tectonic elements of the parautochthonous belt of the northern Grenville orogen include a regional, mid-crustal, thin-skinned, metamorphic fold-thrust belt structurally overlain by a thick-skinned thrust sheet with a high-pressure metamorphic signature. In central Ontario, the parautochthonous belt is composed of a major reverse-sense shear zone, along which deep crustal rocks have been uplifted and exhumed. Tectonic models for both areas involve crustal shortening by delamination in the lower crust and tectonically driven exhumation up a crustal-scale ramp. The fold-thrust belt northwest of the ramp in Labrador is a result of the development of critical topography in the thrust wedge at the top of the ramp and the accretion of thrust slices to the base of the wedge during its propagation onto the foreland. The fold-thrust belt is absent in Ontario because syntectonic erosion of the thrust wedge prevented the development of critical topography. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Crustal-scale ramp in a Middle Proterozoic orogen, Northwest Territories, Canada

Deep crustal seismic data from the Fort Goodhope area, Northwest Territories, Canada, image crustal structures associated with Middle Proterozoic compressional deformation. These include 10–20 km wide antiforms and thrust faults that lie above a west-dipping crustal-scale ramp with at least 10 km of vertical relief. The deformation is interpreted as being associated with structures observed in the subsurface to the east and may be partly coeval with deformation originally detected in outcrop in the Rackla Range of the Wernecke Mountains. These new deep crustal profiles, coupled with data to the east that delineate structures to 15 km depth, reveal large-scale similarity between this Middle Proterozoic orogen and many Phanerozoic compressional orogens.

Rift basin evolution in Africa: the influence of reactivated steep basement shear zones

Summary Phanerozoic rift basin evolution in Africa has been strongly influenced by the pre-existing basement structure inherited from Precambrian tectonics. Two modes of basement reactivation may be identified; the largely dip-slip reactivation of gently dipping shear zone and thrust structures and the largely strike-slip reactivation of steep shear zones and faults. In particular, crustal scale steep basement shear zones have exercised a profound control on rift basin formation on a regional scale. Such shear zone structures are seen to govern the location, trend and structural style of rifts and appear to constrain the orientation of displacements generating the rifts. The shear zones are relatively narrow, high strain zones formed during ductile deformation. They occur as steep, crustal scale ramps that define a pronounced mechanical anisotropy within the African continental crust. These structures are difficult to reactivate by dip-slip movement and thus tend to act as major strike-slip transfer zones during later extensional events. This paper discusses examples of the steep shear zone-rift relationship from three of the rift systems of Africa: the Upper Palaeozoic (Karoo) rifts of Central Africa, the Mesozoic rifts of the Sudan and the Cenozoic rifts of East Africa. These discussions indicate that steep shear zones are sites of rift nucleation and play a fundamental role in the mechanisms of rift propagation and subsequent continental breakup.