Steep Belt Research Articles

A near-shore clastic-carbonate mixing mode in a continental rift basin (early Oligocene, eastern Shijiutuo Uplift, Bohai Bay Basin, China): sedimentology, reservoir characteristics and exploration practice

A comprehensive sedimentary and reservoir analysis was conducted based on seismic, well logging, core and relative test data, taking Members 1 and 2 of Shahejie Formation of the early Oligocene in the steep slope belt, eastern Shijiutuo Uplift (STU), Bohai Bay Basin (BBB) as a case. The study indicates that a near-shore mixed fan deposit formed in the study area and developed characteristics and pattern of a high-quality reservoir. The mixed clastic-carbonate rocks constitute Members 1 and 2 of Shahejie Formation which developed along the steep slope belt and is named as a near-shore mixed fan. The mixed fan of the study area is mainly composed of microfacies of proximal channel, mixed deposited channel, mixed clastic beach, mixed bioclastic (grain) beach, with vertical multi-stage superimposition feature, and basically a similar shape as modern near-shore fans. It constitutes a new depositional type developing in the steep slope belt of a characteristic and complex lacustrine rift basin in the study area. This mixed fan in the steep slope of eastern STU is controlled by comprehensive factors including tectonics, clastic material supply, climate, palaeogeomorphology and hydrodynamic conditions. The reservoir quality of Members 1 and 2 of Shahejie Formation of eastern STU is, however, actually controlled by the sedimentary environment and diagenesis processes. Coarse-grained mixed rocks of near-shore fans, rich in bioclastics, can form excellent reservoirs, characterized by resistance to compaction, easy to dissolution, little influenced by burial depth and high production of oil and gas, which enable them become key exploration targets of medium-deep strata of BBB. Analyses of high-quality reservoir, its controlling factors and the oil and gas exploration implications of the near-shore mixed fan developing in the study area give a deeper insight into discussions of the same type of mixed rocks of other lacustrine rift basins worldwide.

Pre-Alpine contrasting tectono-metamorphic evolutions within the Southern Steep Belt, Central Alps

In the Southern Steep Belt, Italian Central Alps, relicts of the pre-Alpine continental crust are preserved. Between Valtellina and Val Camonica, a poly-metamorphic rock association occurs, which belongs to the Austroalpine units and includes two classically subdivided units: the Languard-Campo nappe (LCN) and the Tonale Series (TS). The outcropping rocks are low to medium grade muscovite, biotite and minor staurolite-bearing gneisses and micaschists, which include interlayered garnet- and biotite-bearing amphibolites, marbles, quartzites and pegmatites, as well as sillimanite-bearing gneisses and micaschists. Permian intrusives (granitoids, diorites and minor gabbros) emplaced in the metamorphic rocks. We performed a detailed structural, petrological and geochronological analysis focusing on the two main lithotypes, namely, staurolite-bearing micaschists and sillimanite-bearing paragneisses, to reconstruct the Variscan and Permian-Triassic history of this crustal section. The reconstruction of the tectono-metamorphic evolution allows for the distinction between two different tectono-metamorphic units during the early pre-Alpine evolution (D1) and predates the Permian intrusives, which comprise rocks from both TS and LCN. In the staurolite-bearing micaschists, D1 developed under amphibolite facies conditions (P = 0.7–1.1 GPa, T = 580–660 °C), while in the sillimanite-bearing paragneisses formed under granulite facies conditions (P = 0.6–1.0 GPa, T> 780 °C). The two tectono-metamorphic units coupled together during the second pre-Alpine stage (D2) under granulite-amphibolite facies conditions at a lower pressure (P = 0.4–0.6 GPa, T = 620–750 °C) forming a single tectono-metamorphic unit (Languard-Tonale Tectono-Metamorphic Unit), which comprised the previously distinguished LCN and TS. Geochronological analyses on zircon rims indicate ages ranging between 250 and 275 Ma for D2, contemporaneous with the emplacement of Permian intrusives. This event developed under a high thermal state, which is compatible with an extensional tectonic setting that occurred during the exhumation of the Languard-Tonale Tectono-Metamorphic Unit. The extensional regime is interpreted as being responsible for the thinning of the Adriatic continental lithosphere during the Permian, which may be related to an early rifting phase of Pangea.

Clastic sedimentation in the Late Oligocene Southalpine Foredeep: from tectonically controlled melting to tectonically driven erosion

The Villa Olmo Conglomerate (lower member of the Como Conglomerate Formation, Gonfolite Lombarda Group, Southern Alps, Italy) represents the first coarse clastic inputs into the Oligocene Southalpine Foredeep. A number of techniques including sedimentary lithofacies analyses, clast counts on turbidite conglomerate bodies, sandstone petrography through Gazzi–Dickinson point‐count method and XRF analyses, and optical and minero‐chemical analyses on single clasts have been performed, in order to better define the sediment source area and geodynamic conditions which promoted sedimentation in the Southalpine Foredeep at the end of the Oligocene. The Villa Olmo Conglomerate interdigitates with the upper part of the Chiasso Formation, and gradually passes upward into the overlying Como Conglomerate Formation. Provenance analyses (conglomerate clast counts and sandstone petrography) reveal a strong metamorphic provenance signal, likely sourced from eroded Southalpine basement. An increase in igneous plutonic clasts reflects sediment supply from the Southern Steep Belt and a decrease of volcano‐sedimentary Mesozoic cover sequences. Optical and minero‐chemical analyses on volcanic detritus detect the presence of sub‐intrusive to effusive, andesite to rhyolite products, ascribable to the Varese‐Lugano Permian volcanoclastic suite, as well as Oligocene andesite products. Plutonic clasts document the presence of tonalites, granites, and brittle deformed granodiorites (with two micas), being likely sourced from the tonalite tail of the Bergell Pluton and the plutonic units of the Bellinzona‐Dascio Zone. The identification of this provenance suite implies palaeo‐drainage from the region between Varese (Southern Alps) and the Bellinzona‐Dascio Zone (Central Alps). The Villa Olmo Conglomerate is the first depositional record of the onset of tectonically driven erosion in the Alpine belt. We infer that the previous low sediment budget regime (Eocene–Middle Oligocene) was a consequence of a tectonically controlled melting phase, during which tectonic events promoted magmatic production in the middle crust of the Central Alps at rates higher than those of crustal deformation, so inhibiting sediment production. We conclude that changes in the deep structures of the Alpine Orogenic chain have controlled the main geodynamic processes during Oligocene–Neogene times, and have controlled sediment composition and supply into the Southalpine Foredeep. Copyright © 2015 John Wiley & Sons, Ltd.

Tectonic Characteristics and its Effects on the Control of the Oil and Gas Accumulation in Bayanhushu Depression

In this paper, it makes full use of seismic and Wells in the single well data on the basis of prede- cessors' research. On the basis of structural geology theory as the instruction, the detailed characteristics of structure and fracture were studied in this paper. Based on the fracture characteristics, the authors studied the basement depth and the development of hydrocarbon source rocks characteristics, and the restoration method of sag tectonic evolution characteristics are analyzed as well. The authors also discussed the construction control of the oil and gas distribution rules. Through this research, the authors found out the regional tectonic framework, the fault distribution and the tectonic evolution stage, illustrated the structure of oil and gas accumulation condi- tions, the accumulation mode, and the distribution rule of control law. Through prototype basin restoration, the authors given concerns about the early depression and tectonic background of stuck faults control source, the distribution of sand body, the western steep slope fan delta and gentle slope belt of eastern braided river delta front sand body is favorable reservoir. Depression after Nantun group and the end of the Yimin group at the end of the reversal of the two big changes, ring between the concave structure development, favorable traps formed early reverse changes, late reverse change on the early formation of trap destruction; reservoir by the late Nantun group is formed by the reverse change control in the construction of the steep slope belt wing structure oil and gas enrichment.

TERTIARY PEGMATITE DIKES OF THE CENTRAL ALPS

The largest field of Alpine Oligocene pegmatite dikes is in the Central Alps within the Southern Steep Belt (SSB) of the Alpine nappes; it extends for about 100 km in an E–W direction and 15 km in a N–S direction north of the Periadriatic Fault, from the Bergell pluton (to the east) to the Ossola valley (to the west). The pegmatite field geographically overlaps (1) the highest temperature domain of the Lepontine Barrovian metamorphic dome and (2) the zone of Alpine migmatization. We have studied pegmatites in two areas: (1) the Codera area on the western border of the Bergell pluton and (2) the Bodengo area between the Mera and the Mesolcina valleys. Most pegmatites show a simple mineral assemblage consisting of K-feldspar, quartz, and muscovite ± biotite, and only a minor percentage of the dikes (< 5%) contains Sn-Nb-Ta-Y-REE-U oxide, Y-REE phosphate, Mn-Fe-phosphate, Ti-Zr-silicate, Be-Y-REE-U-silicate and oxide minerals (beryl, chrysoberyl, bertrandite, bavenite, and milarite), garnet (almandine-spessartine), tourmaline (schorl to rare elbaite), bismuthinite, magnetite, and rarely dumortierite and helvite. The mineral assemblages, geological context, and chemical compositions allow the distinction between LCT (lithium, cesium, tantalum) and mixed LCT-NYF (niobium, yttrium, fluorine) pegmatites (with only one exception of an NYF dike in the Bodengo area). The LCT pegmatites of the Central Alps did not reach a high degree of geochemical evolution. The most fractionated pegmatites are found in the Codera area and contain Mn-rich elbaite, triplite, pink-beryl, and Cs-Rb-rich feldspar. In the Bodengo area pegmatites locally contain miarolitic cavities and the most evolved pegmatites correspond to the beryl-columbite-phosphate type. From a structural point of view two main types of pegmatites can be distinguished: (1) pegmatites that were involved in ductile deformation and (2) pegmatites that postdated the main ductile deformation of the SSB. Many pegmatites of the Codera valley belong to the first structural type: they were emplaced at relatively high ambient temperature (ca. 500 °C) and locally show a pervasive recrystallization of quartz and a mylonitic structure. The Codera dikes trend about 70° and are steeply dipping. In the Bodengo area the main set of pegmatites (trending approximately N–S to NNE–SSW) crosscuts the ductile deformation structures of the SSB, but the area also includes an earlier generation of boudinaged and folded pegmatite dikes. The undeformed pegmatites from this area may contain miarolitic pockets. There is no systematic difference in the mineral assemblage between the two structural types of pegmatites. However, the chemistry of pegmatite minerals, especially of garnet, in addition to field data suggests that the dikes of the Codera and Bodengo areas represent two distinct generations of pegmatites. Structural data and the few existing radiometric ages suggest that pegmatites were emplaced over a time span between 29 and 25 Ma (and possibly as young as 20 Ma), with the youngest dikes postdating the ductile deformations of the Alpine nappes. The present work presents a first comprehensive field description and geochemical – mineralogical characterization of the Alpine pegmatite field of the Central Alps.

Magnetic study of a late Alpine dike crosscutting the regional foliation

Dikes crosscutting existing folds or foliation are one of the classic time markers used in establishing relative geological chronology. We examine the paleomagnetic directions and magnetic fabric in a deformed gneiss from the Southern Steep Belt, which lies immediately north of the Insubric fault, and in an aplite dike that transects this foliation. No clear foliation or other evidence of internal deformation is discernible in the aplite dike in the field and it was originally interpreted as post-tectonic, with its intrusion age placing a young limit on the timing of deformation in the adjacent gneiss. The aplite dike does not carry a stable remanent magnetization and cannot be used to resolve the age of intrusion. The characteristic paleomagnetic remanence in the gneiss is compatible with Eurasian paleomagnetic directions during the last 20Ma. These results suggest that the timing of magnetization postdates dextral motion along the Insubric and Rhine–Rhone faults. The low-field anisotropy of magnetic susceptibility (AMS) in the gneiss is controlled by the regional schistosity. High-field AMS suggests that both paramagnetic and ferromagnetic minerals contribute to the magnetic fabric, whereby the ferromagnetic minerals show a stronger lineation. This is further confirmed by the anisotropy of anhysteretic remanent magnetization (AARM). Rock magnetic experiments show that multi-domain magnetite is the main ferromagnetic mineral in the gneiss. Unexpectedly, the AMS results from the aplite dike show that it also has a magnetic fabric, which is shown by high-field AMS, AARM and rock magnetic experiments to be carried by superparamagnetic magnetite. The presence of magnetic fabric in the dike demonstrates that regional deformation was still on-going in the Southern Steep Belt of the Central Alps at the time of its intrusion.

Tectonics of the Lepontine Alps: ductile thrusting and folding in the deepest tectonic levels of the Central Alps

The Lepontine dome represents a unique region in the arc of the Central and Western Alps, where complex fold structures of upper amphibolite facies grade of the deepest stage of the orogenic belt are exposed in a tectonic half-window. The NW-verging Mont Blanc, Aar und Gotthard basement folds and the Lower Penninic gneiss nappes of the Central Alps were formed by ductile detachment of the upper European crust during its Late Eocene–Early Oligocene SE-directed underthrust below the upper Penninic and Austroalpine thrusts and the Adriatic plate. Four underthrust zones are distinguished in the NW-verging stack of Alpine fold nappes and thrusts: the Canavese, Piemont, Valais and Adula zones. Up to three schistosities S1–S3, folds F1–F3 and a stretching lineation XI with top-to-NW shear indicators were developed in the F1–F3 fold nappes. Spectacular F4 transverse folds, the SW-verging Verzasca, Maggia, Ziccher, Alpe Bosa and Wandfluhhorn anticlines and synclines overprint the Alpine nappe stack. Their formation under amphibolite facies grade was related to late ductile folding of the southern nappe roots during dextral displacement of the Adriatic indenter. The transverse folding F4 was followed since 30 Ma by the pull-apart exhumation and erosion of the Lepontine dome. This occurred coevally with the formation of the dextral ductile Simplon shear zone, the S-verging backfolding F5 and the formation of the southern steep belt. Exhumation continued after 18 Ma with movement on the brittle Rhone-Simplon detachment, accompanied by the N-, NW- and W-directed Helvetic and Dauphiné thrusts. The dextral shear is dated by the 29–25 Ma crustal-derived aplite and pegmatite intrusions in the southern steep belt. The cooling by uplift and erosion of the Tertiary migmatites of the Bellinzona region occurred between 22 and 18 Ma followed by the exhumation of the Toce dome on the brittle Rhone–Simplon fault since 18 Ma.

Metamorphic grade of source rocks revealed by chemical fingerprints of detrital amphibole and garnet

Abstract Amphibole and garnet are among the most widespread heavy minerals in orogenic sediments. Their chemical composition and optical properties vary markedly and systematically with temperature and pressure conditions during growth, and thus provide important information on the metamorphic evolution of source areas that is crucial in palaeotectonic and palaeogeodynamic reconstructions. This study investigates the chemical composition of detrital amphiboles and garnets derived from parent rocks of progressively increasing metamorphic grade through a well-studied composite section across the Central and Southern Alps, including the granulite-facies core of the Late Palaeozoic orogen exposed in the Ivrea–Verbano Zone and the amphibolite-facies core of the Cenozoic orogen exposed in the Lepontine Dome. We specifically focus on metamorphic grade because it represents the best proxy for tectono-stratigraphic crustal level, and hence degree of unroofing of source areas. In river sands collected between metamorphic isograds corresponding to crystallization temperatures ranging from c. 500 °C to c. 850 °C, TiO 2 gradually increases in detrital amphibole while its colour progressively changes from blue-green in the lower amphibolite-facies where actinolite, hornblende and tschermakite are most abundant, to brown in the granulite facies where pargasite is dominant. Detrital garnets display moderate gradual changes across the amphibolite-facies Lepontine Dome, where low-Mg ‘type B’ garnets predominate. Almandine-spessartine is spatially associated with abundance of pegmatites while entering the zone of anatexis (Southern Steep Belt), where grossular or grossular-andradite-spessartine are occasionally found. A sharp change occurs while reaching granulite-facies in the Ivrea–Verbano Zone, where high-Mn garnets disappear and ‘type A’ almandine-pyrope (from ‘stronalite’ metasediments) and ‘type C’ almandine-pyrope-grossular (from metagabbros of the Mafic Complex) predominate. Also redefined in this article are a series of numerical indices based on amphibole colour and relative abundances of diverse key minerals (chloritoid, staurolite, andalusite, kyanite, fibrolitic and prismatic sillimanite), useful to accurately assess the average metamorphic grade of meta-igneous and metasedimentary source rocks. Supplementary material: Chemical composition of detrital amphiboles and garnets and full information on the location and mineralogical composition of studied samples is available at http://www.geolsoc.org.uk/SUP18618 .

Fluid Flow of Daxing Conglomerate in Langgu Depression and its Controlling Effects on Reservoir

The Daxing conglomerate in Langgu depression is typical glutenite fan sediment in steep slope belt of faulted basin. By analyzing the data of core, petrographic thin section, scanning electron microscope and “X” ray diffraction, we’ve found that fluid flow controls the reservoir development to a certain degree. The results show that there are four types of fluid affecting the reservoir of Daxing conglomerate-meteoric fresh water, organic acid, alkaline fluid and abnormal high pressure fluid. We’ve studied fluid flows in different diagenetic stages combining the observation of clay mineral and carbonate cement and we find that two acid fluid events and abnormal high pressure fluid have determined the development of three pore belts in the longitudinal section.

The Dalradian rocks of the north-east Grampian Highlands of Scotland

Abstract The north-east Grampian Highlands, as described here, are bounded to the north-west by the Grampian Group outcrop of the northern Grampian Highlands and to the south by the Southern Highland Group outcrop in the Highland Border region. The Dalradian succession therefore encompasses the whole of the Appin and Argyll groups, but also includes an extensive outlier of Southern Highland Group strata in the north of the region. The succession includes shallow-marine sequences, glacigenic deposits at two stratigraphical levels, the earliest evidence for volcanism in the Dalradian, a later major development of basaltic and picritic sub-marine lavas, and thick turbiditic sequences. In the south, the Grampian–Appin group boundary is a high-strain zone, with no obvious dislocation or stratigraphical excision, which was formerly termed the Boundary Slide. Shear-zones at higher structural levels are associated with pre-tectonic granites, such as the Ben Vuirich Granite, which have been dated at c. 600 Ma and hence place limits on the timing of sedimentation, deformation and metamorphism. The region is divided from north to south by a major zone of shearing and dislocation with associated igneous intrusions, termed the Portsoy Lineament. To the west of the lineament, the stratigraphy is more-or-less continuous along strike with that of the central Grampian Highlands. D1, D2 and D3 structures extend from the Tummel Steep Belt north-eastwards throughout this area. The stratigraphical succession is broadly continuous across the Portsoy Lineament but to the east, in the Buchan Block, correlations are more tenuous and do not extend below subgroup level. High-grade migmatitic paragneisses were once interpreted as pre-Dalradian basement but they are now assigned to the Crinan Subgroup, within the Dalradian succession. Within the Buchan Block the outcrop pattern is controlled by two broad, open, post-metamorphic folds, the Turriff Syncline and the Buchan Anticline. The Buchan Block is the international type area for the high-temperature/low-pressure Buchan-type regional metamorphism. To the south and west, this passes into higher pressure Barrovian-type metamorphism. South of Deeside, metamorphic conditions reached 820 °C and over 8 kbar, well into granulite facies and the highest recorded in the Grampian Terrane. The detailed relationship between the high heat-flow and the emplacement of large bodies of basic and silicic magma is a matter of ongoing research. Plutons of the north-east Grampian Basic Suite, emplaced at c. 474–470 Ma, during or shortly after the peak of metamorphism and the D3 deformation, provide key evidence for the timing of the Grampian orogenic event.

Provenance System Analysis of the Nantun Formation of the Huhehu Sag in Hailar Basin

Abstract:Huhehu Sag is a new battlefield for the future oil and gas exploration in the Hailar Basin. At present, exploration of this area stays at a low level and the understanding of the provenance system of the Nantun Formation is comparatively weak. So, to carry out the detailed depiction of the provenance system in the study area is significant for the sedimentary system reconstruction, for the prediction of reservoir distribution and favorable areas. Based on comprehensive analysis of paleogeomorphy, light‐heavy mineral and seismic reflection features, sandstone thickness and sandstone percentage, this paper analyzes in detail the provenance system of the Nantun Formation in Huhehu Sag from various angles and levels. It shows that provenances of the Huhehu Sag mainly originated from the eastern Xilinbeier Salient and western Bayan Mountain Uplift. The provenances surrounding the lake basin converged to the center of the basin, among which, the steep slope belt and the gentle slope belt are the main provenances, and the southern uplift is a secondary one. The distribution framework of the provenance fundamentally determines the distribution rules of the depositional system and its sand body developing during the Nantun period.

Palaeoproterozoic metamorphism and cooling of the northern Nagssugtoqidian orogen, West Greenland

Lu–Hf and Sm–Nd garnet geochronology combined with geothermobarometric calculations allowed us to gain new insight into the metamorphic evolution of the Northern Nagssugtoqidian orogen (NNO), in West Greenland. Three samples were collected from the southern (SM 310 and SM 316) and northern (SM 339) parts of the NNO. They revealed amphibolite-grade Palaeoproterozoic metamorphism under peak temperature conditions decreasing from c. 810 to 840°C at 7.5kbar (SM 310, 316) to c. 690°C at 10kbar (SM 339) in the southern and northern part of the NNO, respectively. Our garnet dating defines two episodes: (1) at c. 1780Ma, determined by Lu–Hf in sample SM 316 and Sm–Nd in sample SM 339, most likely approximating the time of garnet growth, and (2) at c. 1750Ma defined by the remaining Sm–Nd dates, reflecting time of cooling below isotopic closure. Original garnet growth patterns preserved by HREE (SM 316) and clearly seen zonation of Sm and Nd (SM 339) suggest that the garnets may not have been completely reset and thus, the obtained age of 1780Ma could be fairly close to the time of garnet crystallisation.The growth of peak metamorphic mineral assemblages in the southern NNO took place after the end of deformation, as demonstrated by sample SM 316 that was collected from an undeformed mafic dyke cutting across the regional fabric. In contrast, sample SM 339 revealed synkinematic growth of peak metamorphic paragenesis that crystallised in a low angle top-to-SW shear zone overprinted on the pre-existing fabric. Our results are consistent with the interpretation of the NNO as an Archaean crustal block, the Aasiaat domain, that was affected by Palaeoproterozoic metamorphism and localised deformation at 1780Ma, i.e., 70Ma after the Nagssugtoqidian collision. This event can be compared to the formation of steep belts (major left-lateral strike-slip shear zones) in the central part of the Nagssugtoqidian orogen. The Aasiaat domain is considered to be an independent microplate separating the Nagssugtoqidian and Rinkian orogens which define respectively its southern and northern margins. The interpretation presented implies the existence of two discrete tectonic sutures to the north and south of the Aasiaat domain.

Late Cretaceous extension overprinting a steep belt in the Northern Calcareous Alps (Schesaplana, Rätikon, Switzerland and Austria)

The Triassic to Cretaceous sediment succession of the Lechtal Nappe in the western part of the Northern Calcareous Alps (NCA) has been deformed into large-scale folds and crosscut by thrust and extensional faults during Late Cretaceous (Eoalpine) and Tertiary orogenic processes. The following sequence of deformation is developed from overprinting relations in the field: (D1) NW-vergent folds related to thrusting; (D2) N–S shortening leading to east–west-trending folds and to the formation of a steep belt (Arlberg Steep Zone) along the southern border of the NCA; (D3) E–W to NE–SW extension and vertical shortening, leading to low-angle normal faulting and recumbent “collapse folds” like the Wildberg Syncline. D1 and D2 are Cretaceous in age and predate the Eocene emplacement of the Austroalpine on the Penninic Nappes along the Austroalpine basal thrust; the same is probably true for D3. Finally, the basal thrust was deformed by folds related to out-of-sequence thrusting. These results suggest that the NCA were at least partly in a state of extension during the sedimentation of the Gosau Group in the Late Cretaceous.

Stability and isotopic dating of monazite and allanite in partially molten rocks: examples from the Central Alps

We investigated the stability of monazite and allanite as a function of bulk rock composition within several types of Tertiary Alpine anatexites, characterized by different compositions and melting reactions, but similar P-T conditions of melting. The investigated rocks consist of: (1) orthogneisses in which the melting reaction was triggered by water infiltration from the Bergell pluton; (2) anatectic tonalites, which were affected by water-assisted melting; and (3) metapelitic migmatites, which underwent muscovite dehydration melting. The studied anatexites cover a large range of Ca contents and water activities during partial melting, and allow an assessment of how much these parameters affect the stability of accessory phases. The different melting reactions that affected these rocks generated different water activities during the melt-present stage; they were highest in the water-saturated, contact metamorphic anatexites, and lowest in the metapelitic anatexites that underwent dehydration melting. These differences go together with different accessory phases within the migmatites. Whereas metapelitic anatexites only contain monazite, anatexites derived from tonalitic and granodioritic protoliths mainly contain allanite. This is consistent with observations made on Tertiary Alpine anatexites, suggesting that the growth of specific accessory phases is determined by the water activity and Ca content during melting. We measured single-grain monazite U/Pb isotope ages. One grain has relics of old cores, which have also been detected in Y-zonation patterns of the monazite. The data of unzoned monazites indicate partial melting in the Southern Steep Belt between 30.78 ± 0.14 and 28.10 ± 0.28 Ma.

Tectonics of the Simplon massif and Lepontine gneiss dome: deformation structures due to collision between the underthrusting European plate and the Adriatic indenter

This study presents a review of published geological data, combined with original observations on the tectonics of the Simplon massif and the Lepontine gneiss dome in the Western Alps. New observations concern the geometry of the Oligocene Vanzone back fold, formed under amphibolite facies conditions, and of its root between Domodossola and Locarno, which is cut at an acute angle by the Miocene, epi- to anchizonal, dextral Centovalli strike-slip fault. The structures of the Simplon massif result from collision over 50 Ma between two plate boundaries with a different geometry: the underthrusted European plate and the Adriatic indenter. Detailed mapping and analysis of a complex structural interference pattern, combined with observations on the metamorphic grade of the superimposed structures and radiometric data, allow a kinematic model to be developed for this zone of oblique continental collision. The following main Alpine tectonic phases and structures may be distinguished: 1. NW-directed nappe emplacement, starting in the Early Eocene (~50 Ma); 2. W, SW and S-verging transverse folds; 3. transpressional movements on the dextral Simplon ductile shear zone since ~32 Ma; 4. formation of the Bergell – Vanzone backfolds and of the southern steep belt during the Oligocene, emplacement of the mantle derived 31–29 Ma Bergell and Biella granodiorites and porphyritic andesites as well as intrusions of 29–25 Ma crustal aplites and pegmatites; 5. formation of the dextral discrete Rhone-Simplon line and the Centovalli line during the Miocene, accompanied by the pull-apart development of the Lepontine gneiss dome – Dent Blanche (Valpelline) depression. It is suggested that movements of shortening in fan shaped NW, W and SW directions accompanied the more regular NW- to WNW-directed displacement of the Adriatic indenter during continental collision.

Tertiary subduction, collision and exhumation recorded in the Adula nappe, central Alps

Abstract The Adula nappe in the Central Alps represents a lithospheric mélange assembled in a south-dipping subduction zone during the Tertiary orogenic cycle. It consists of several heterogeneous lobes which are stacked in a forward-dipping duplex geometry. Eclogites, garnet peridotites and garnet-white-mica schists record southward-increasing peak pressure conditions which culminate at 12–17 kbar/500–600 °C in the north and 30 kbar/800–850 °C in the south. Some studies infer even higher peak pressures for the garnet peridotite body of Alpe Arami. The present-day metamorphic field gradient for peak pressures exceeds the lithostatic pressure gradient. So far, only eclogites and garnet peridotites from the Cima Lunga complex in the south and the adjacent Southern Steep Belt have yielded Tertiary metamorphic ages for the peak-pressure stage. Some recent studies propose that the Adula nappe got assembled after the formation of high-pressure assemblages in eclogites and garnet peridotites and reject regional high-pressure conditions in Tertiary times. This scenario, however, is in conflict with the observed continuity of metamorphic field gradients and post-peak-pressure structures. Amphibolite facies conditions post-date formation of the Central Alpine nappe stack. In this paper, the associated field gradient is explained through southward-increasing temperatures during near-isothermal decompression. The main mylonitic foliation in the Adula nappe post-dates peak-pressure conditions. It is associated with top-to-the-north shearing and southward-increasing amounts of decompression from eclogite facies to amphibolite facies conditions. Also, the present-day supra-lithostatic field gradient for peak pressures probably results from this deformation phase and is here related to substantial vertical flattening during northward shearing. All subsequent structures affect established nappe boundaries. Pervasive Oligocene deformation events in the Adula nappe are coeval with intense shearing along the so-called Insubric mylonites and occur during ongoing isothermal decompression to around 5 kbar. They are associated with orogen-oblique to orogen-parallel stretching of unspecified amount which may considerably contribute to the exhumation of the Lepontine dome already before the onset of the well-known Miocene extension.

On the role and importance of orogen-parallel and -perpendicular extension, transcurrent shearing, and backthrusting in the Monte Rosa nappe and the Southern Steep Belt of the Alps (Penninic zone, Switzerland and Italy)

Abstract During Europe–Adria collision in Tertiary times, the Monte Rosa nappe was penetratively deformed in several stages after an eclogite-facies pressure peak: (1) top-to-the-NW thrust shearing (Mattmark phase, after 40 Ma); (2) orogen-parallel, top-to-the-SW extensional shearing and folding (Malfatta phase); (3) orogen-perpendicular, top-to-the-SE extensional shearing and folding (Mischabel phase, before 30 Ma); and (4) large-scale, upright, SE-vergent folding (Vanzone phase, c . 29–28 Ma). Structural analysis and neutron texture goniometry of quartz mylonites show that the Stellihorn shear zone in the Monte Rosa nappe accommodated a complex and multidirectional sequence of shearing movements during the Mattmark, Malfatta and Mischabel phases, and was folded in the Vanzone phase. In the tail-shaped eastward prolongation of the Monte Rosa nappe in the Southern Steep Belt of the Alps, both dextral and sinistral mylonites (Olino phase) were formed during and after the formation of the Vanzone fold, reflecting renewed orogen-parallel (SW–NE) extension contemporaneous with NW–SE shortening from c . 29 Ma onward. A similar sequence of deformation stages was identified in the Adula nappe at the eastern border of the Lepontine metamorphic dome. Important consequences arise for the Insubric fault at the southern border of the Lepontine dome: (1) the NW- to N-dipping orientation of the Insubric fault is not a primary feature but resulted from rotation of an originally SE-dipping shear zone after c . 30 Ma; and (2), the strong contrast in metamorphic grade across this fault (upper amphibolite facies to the north versus anchizone to the south) results from north-side-up faulting coupled with orogen-parallel extension of the northern block (Lepontine dome), while no such extension occurred in the southern block (Southern Alps). Extension in the northern block started in the Malfatta phase and continued in the Mischabel phase when the foliation in the area which later became the Southern Steep Belt still dipped towards south. During Vanzone/Olino deformation, further unroofing and uplift of the Lepontine dome relative to the South Alpine block took place while the Southern Steep Belt was progressively rotated into its present, overturned position, changing its character from a normal fault into a backthrust. Complex deformation paths in the Southern Steep Belt resulted from the combination of extension of the northern block with strike-slip motion along the Insubric fault.

Tectonically controlled fluid flow and water-assisted melting in the middle crust: An example from the Central Alps

Melting triggered by influx of a free aqueous fluid in the continental crust has commonly been inferred, but the source of water in such contexts remains a matter of debate. We focus on the Tertiary migmatites in the Southern Steep Belt of the Central Alps (Switzerland) to discuss the petrology, structures and geodynamic setting of water-assisted melting. These migmatites comprise various structural types (e.g. metatexites, diatexites, melt in shear zones), which reflect variable leucosome fractions. The melting event itself as well as the variable melt fractions are related to the amount of aqueous fluids. At a given P and T, melt-fractions in rocks of minimum melt composition correlate with the amount of infiltrated aqueous fluids. In more granodioritic systems the water distributes between melt and newly crystallizing hydrous phases such as amphibole, such that the melt fraction correlates with the contents of H 2O, Al, and Ca in the system. Phase-equilibrium modelling indicates that the stabilization of amphibole leads to slightly lower melt fractions than in a granitic system at the same P, T and bulk water content. Phase-equilibrium models further indicate that in the Alpine migmatite belt: (1) several wt.% water (fluid:rock ratio of ∼ 1:30) are necessary to produce the inferred melt fraction; (2) the activity of H 2O in the fluid is high; and (3) spatially associated metapelites are unlikely as a source for the required aqueous fluids. We present a tectonic scenario for the southern margin of the Central Alps, to which these migmatites are confined, and we propose that water was produced from dehydration reactions in metapelites in the Southern Alps. We model fluid production rates at the time of melting and demonstrate that the resulting fluid flow pattern is mainly controlled by the differences in permeability between the fluid source region and melting region. The proposed model requires strong gradients in temperature and permeability for the two tectonic blocks. This is consistent with the scenario involving indenter tectonics at the boundary between the Central and the Southern Alps in Oligocene times.

Explanatory notes to the Geological Map of Greenland, 1:100 000, Ussuit 67 V.2 Nord

The Ussuit map area is situated around the inner Nassuttooq (Nordre Strømfjord) in central West Greenland, in the core of the Palaeoproterozoic Nagssugtoqidian orogen. The orogen largely consists of reworked Archaean gneisses, as well as Palaeoproterozoic ortho- and paragneisses in its central part. Easy access through the branched fjord system and good exposures along its coastlines, combined with less intense reworking compared to neighbouring areas to the west and south, have made the Ussuit map area the most intensely studied and best known part of the orogen. The most recent research and mapping projects were organised by the Danish Lithosphere Centre (1994–1999) and the Geological Survey of Denmark and Greenland (2000–2001).&#x0D; The predominant rocks are late Archaean tonalitic and granodioritic orthogneisses, intruded by postkinematic granites. Archaean supracrustal rocks are predominantly of mafic composition, but only few have been recognised. Palaeoproterozoic rocks occur as tectonic sheets (the Ussuit unit) which are always in tectonic contact with the Archaean orthogneisses. The most abundant Palaeoproterozoic rock types are biotite schist and biotite-bearing paragneiss, besides orthogneiss of the Arfersiorfik intrusive suite. The latter rocks are mainly deformed quartz diorites intrusive into the metasedimentary rocks and interpreted as remnants of a magmatic arc above a subduction zone. The Ussuit unit also includes amphibolite, marble and calc-silicate rocks, and lenses of ultrabasic rocks. Small bodies of younger syn- and post-tectonic granites occur throughout the map area. The interleaved Palaeoproterozoic and Archaean rocks form a major anticlinal fold structure between two crustal-scale shear zones, the Nordre Strømfjord shear zone in the north and the Nordre Isortoq steep belt in the south. These shear zones formed during the latest ductiledeformation event (D4), following ductile thrusting (D1) and kilometre-scale folding (D2 and D3). The deformation and high grade metamorphism are the result of collision of two Archaean blocks at c. 1850 Ma, with a presumed strongly deformed suture rooted in the southern Ussuit area.&#x0D; No economically feasible mineral occurrences have been discovered to date within the Ussuit map area. Minor sulphide mineralisation related to hydrothermal activity occurs in faults and shear zones, and minor stratabound iron formations have been observed. The most promising industrial mineral deposits are minor diopside occurrences and potential dimension stone in migmatised orthogneiss.

Explanatory notes to the Geological Map of Greenland, 1:100 000, Ussuit 67 V.2 Nord

The Ussuit map area is situated around the inner Nassuttooq (Nordre Strømfjord) in central West Greenland, in the core of the Palaeoproterozoic Nagssugtoqidian orogen. The orogen largely consists of reworked Archaean gneisses, as well as Palaeoproterozoic ortho- and paragneisses in its central part. Easy access through the branched fjord system and good exposures along its coastlines, combined with less intense reworking compared to neighbouring areas to the west and south, have made the Ussuit map area the most intensely studied and best known part of the orogen. The most recent research and mapping projects were organised by the Danish Lithosphere Centre (1994–1999) and the Geological Survey of Denmark and Greenland (2000–2001).&#x0D; The predominant rocks are late Archaean tonalitic and granodioritic orthogneisses, intruded by postkinematic granites. Archaean supracrustal rocks are predominantly of mafic composition, but only few have been recognised. Palaeoproterozoic rocks occur as tectonic sheets (the Ussuit unit) which are always in tectonic contact with the Archaean orthogneisses. The most abundant Palaeoproterozoic rock types are biotite schist and biotite-bearing paragneiss, besides orthogneiss of the Arfersiorfik intrusive suite. The latter rocks are mainly deformed quartz diorites intrusive into the metasedimentary rocks and interpreted as remnants of a magmatic arc above a subduction zone. The Ussuit unit also includes amphibolite, marble and calc-silicate rocks, and lenses of ultrabasic rocks. Small bodies of younger syn- and post-tectonic granites occur throughout the map area. The interleaved Palaeoproterozoic and Archaean rocks form a major anticlinal fold structure between two crustal-scale shear zones, the Nordre Strømfjord shear zone in the north and the Nordre Isortoq steep belt in the south. These shear zones formed during the latest ductiledeformation event (D4), following ductile thrusting (D1) and kilometre-scale folding (D2 and D3). The deformation and high grade metamorphism are the result of collision of two Archaean blocks at c. 1850 Ma, with a presumed strongly deformed suture rooted in the southern Ussuit area.&#x0D; No economically feasible mineral occurrences have been discovered to date within the Ussuit map area. Minor sulphide mineralisation related to hydrothermal activity occurs in faults and shear zones, and minor stratabound iron formations have been observed. The most promising industrial mineral deposits are minor diopside occurrences and potential dimension stone in migmatised orthogneiss.