1,721,132 research outputs found
Exhumation history of the Red River shear zone in northern Vietnam: New insights from zircon and apatite fission-track analysis
No full textA new set of zircon and apatite fission-track ages from the Ailao Shan and Day Nui Con Voi (DNCV) metamorphic massifs of the Red River shear zone (RRSZ) and neighboring rocks in northern Vietnam is presented. A complex, along-strike diachronous, denudation history is revealed. The southern sector of the DNCV cooled to about 100 °C by the Late Oligocene, whereas its central compartment was affected by the later thermotectonic evolution of the Song Chay dome to the E of the RRSZ, whose final exhumation occurred during the Early Miocene. The northern sector of the RRSZ is characterized by the 35 Ma Phan Si Pang pre- to synkinematic intrusion. Fission-track ages from a vertical section within the Phan Si Pang granite indicate rapid exhumation and cooling. The Paleozoic tectonic block to the west of the RRSZ (fission-track ages between 40 and 30 Ma) was exhumed and cooled earlier than the fault mylonite belt (fission track ages of 30 Ma and younger) and also than the eastern block. Its structural level is consistent with field observations that suggest the RRSZ in northern Vietnam to be a transtensional system, with a regional NE-SW oriented extension component. © 2007 Elsevier Ltd. All rights reserved
Revised structure and stratigraphy of the northwestern Repparfjord Tectonic Window, northern Norway
No full textThe Repparfjord Tectonic Window (RTW) consists of a ~8 km-thick low-grade, weakly deformed Early Palaeoproterozoic volcano-sedimentary
succession that hosts abundant Cu mineralisations. Two major, sediment-hosted, Cu-deposits occur in the Saltvatnet Group, a <6 km-thick
sequence of mainly coarse-grained clastic rocks that forms the core subject of the present study. Results from new mapping, structural analysis
and whole-rock geochemistry from the northwestern part of the RTW are presented and used in association with high-resolution geophysical
data to propose a revised stratigraphy of the Repparfjord volcano-sedimentary succession and its tectonic and geodynamic settings. The Saltvatnet
Group is better characterised as containing four formations, now including also the uppermost volcaniclastic siltstones and dolostone members of
the herein newly defined Gorahatjohka Formation. Field oberservations and structural analysis show that the current geometry of the Saltvatnet
Group is that of a km-scale, upright, NE–SW-trending anticline, the Ulveryggen anticline, cored by the Saltvatnet Group, which thus represents
the lowermost exposed rocks of the RTW. New and existing geochemical data are used to demonstrate that the two volcanic-dominated Nussir
and Holmvatnet groups, which crop out on either side of the Ulveryggen anticline, have similar geochemical character and can be interpreted
as temporal and lateral correlatives. The basaltic Ulveryggen intrusions within the Saltvatnet Group are geochemically different from any other
igneous rocks in the RTW, suggesting that these intruded prior to the volcanic-dominated groups in the RTW. The revised stratigraphy of the
RTW is interpreted as reflecting deposition and volcanism in an evolving continental back-arc, which contrasts with the purely intraplate rift
setting inferred for most parts of the Fennoscandian Shield in the Early Palaeoproterozoic
Inclined transpression at the toe of an arcuate thrust: an example from the Precambrian ‘Mylonite Zone’ of the Sveconorwegian orogen
No full textThe ‘Mylonite Zone’ (MZ) forms a major, arcuate terrane boundary in the Precambrian
Sveconorwegian orogen of SW Scandinavia. SE-directed thrusting along this curvilinear shear
zone emplaced the higher-grade Idefjorden Terrane to the west onto the lower-grade Eastern
Segment terrane to the east. Detailed structural characterization of the MZ mylonites in two different
localities (Va ̈rmlandsna ̈s and Bua peninsulas) reveals a complex three-dimensional strain
pattern. Inclined transpression is inferred on the basis of coexisting (and broadly coeval) foliation-
parallel oblique shearing (resolvable in a strike-slip and dip-slip component) and acrossfoliation
shortening. The former accommodated the transpressive component of the MZ, and its
kinematics is either sinistral or dextral depending on the local strike of the MZ with respect to
the regional thrust shortening vector. The latter led to pure-shear shortening perpendicular to the
thrust sheet and subsequent lateral extrusion parallel to the mylonitic foliation via the development
of antithetic displacements. No significant strain partitioning is observed at the meso-scale and
strain is thus truly triclinic. The example described is an outstanding case of triclinic deformation,
confirms theoretical analyses of complex strain models and adds valuable natural field constraints
to our knowledge of deformation in the crust
New constraints on 1.7Gyr of brittle tectonic evolution in southwestern Finland derived from a structural study at the site of a potential nuclear waste repository (Olkiluoto Island)
No full textThe brittle deformation history of southwestern Finland was reconstructed through paleostress inversion of fault-slip data from outcrops and drill cores at the site of a planned nuclear waste repository. Seven distinct paleostress states have been resolved spanning 1.7Ga of geological history. (1) NW-SE to NNW-SSE transpression acted soon after 1.75Ga, when conditions leading to brittle deformation were first attained. (2) N-S to NE-SW transpression caused partial reactivation of (1). (3) NW-SE extension followed during the Gothian event and at the time of rapakivi magmatism. (4) Renewed NE-SW transtension occurred between 1.60 and 1.30Ga, forming the NW-SE-elongated Satakunta graben. (5) A significant and so far unrecognised phase of NE-SW compression has been found to postdate both rapakivi granites and 1.27Ga olivine diabase sills. (6) E-W transpression followed during the early stages of the Mesoproterozoic Sveconorwegian orogeny and predated (7) almost coaxial E-W extension of the late Sveconorwegian orogenic collapse. Our results prove that the shield's 'crust fragmentation' (i.e. generation of new fractures) was short lived in comparison to its 'jostling phase' (i.e. reactivation of old fractures). The shield thus reached structural maturity with respect to its saturation with brittle structures already in the late Mesoproterozoic. We predict that that future stress changes will most probably be accommodated by reactivation of the existing and inherited brittle structures rather than by formation of new deformation zones. © 2014 Elsevier Ltd
Structural and temporal evolution of a reactivated brittle-ductile fault - Part I: Fault architecture, strain localization mechanisms and deformation history
No full textFaults are by nature dynamic, as their architecture and composition evolve progressively in space and through time steered by the interplay between strain weakening and hardening mechanisms. This study combines structural analysis, geochemistry and chlorite geothermometry to investigate deformation and strain localization mechanisms of the Kvenklubben fault, a Paleozoic brittle-ductile thrust in northern Norway, with the goal to constrain their temporal variations and the consequences thereof on fault architecture development and rheological behavior. The fault evolved from an initially discrete brittle feature slipping mainly by seismogenic ruptures to a wide brittle-ductile phyllonite deforming by aseismic creep. The formation of mechanically weak phyllosilicates by decarbonation of footwall dolostones and carbonation of hanging wall metabasalts was the main weakening mechanism, whereas partitioning of fluid flow and fracture sealing following transient high pore pressure-driven embrittlement caused episodic and localized strain hardening. The interplay between strain weakening and hardening mechanisms caused the fault core to widen. We suggest that the ability for carbonate-hosted faults to slip by seismogenic rupture is also a function of the faults' structural-evolutionary stage, and that it decreases progressively with fault maturity. This study demonstrates the importance of calibrating the present-day fault anatomy against the dynamic character of faults, which evolve geometrically, compositionally and mechanically in space and through time
Multiple reactivation and strain localization along a Proterozoic orogen-scale deformation zone: The Kongsberg-Telemark boundary in southern Norway revisited
No full textStructural analysis defines a multiphase Sveconorwegian tectonic evolution for the boundary zone between the Kongsberg and Telemark lithotectonic units in S Norway, referred to as the Kongsberg-Telemark Boundary Zone (KTBZ). This large-scale weakness zone developed predominantly within and at the margin of a c. 110. km long granitic belt, the intrusion of which is dated between 1170. ±. 11 and 1146. ±. 5. Ma by U-Pb SIMS zircon geochronology. The oldest KTBZ ductile fabric formed during the Sveconorwegian orogenic cycle (c. 1140-900. Ma) as a penetrative top-to-the-W shear fabric, which was subsequently reactivated selectively by sinistral transpression that formed characteristic mylonitic shear zones within the granitic belt. Later folding affected the area at the northern end of the Kongsberg lithotectonic unit. Analysis of the subregional foliation trajectories unravels the occurrence of a large-scale fold structure, the "Norefjell-Hønefoss Fold". All these structures are in turn cut by late-Sveconorwegian, E-dipping shear zones and normal faults, which accommodated a distinct phase of exhumation of the Telemark lithotectonic unit in the footwall of the KTBZ. This extensional detachment widens toward the north, where it might have controlled the emplacement of the late-orogenic Flå granite. Since late Sveconorwegian times, the KTBZ was repeatedly reactivated in a brittle fashion forming complex fault patterns, extensive quartz vein networks and leading to the generation of the so-called "Store Rivningsbreksje", a 100. km long brittle fault zone that follows the trend of the KTBZ and that locally juxtaposes blocks with different ductile precursor histories. The newly established deformation history helps to refine existing models for the orogenic evolution of the central Sveconorwegian orogen. The characterization of the Norefjell-Hønefoss fold structure provides a new perspective on Sveconorwegian geometries and fabrics in the area. The reactivation history established for the KTBZ helps to better understand the dynamics of long-lived weakness zones of Precambrian origin in general
From XY tracking to buckling: Axial plane cleavage fanning and folding during progressive deformation
No full textFolding of axial plane cleavage can occur during progressive deformation without a change in the overall background flow. Two field examples of upright (Lachlan Fold Belt, SE Australia) and recumbent (Naukluft Nappe Complex, central Namibia) folds are presented, in which strongly refracted pressure solution cleavage in competent layers on the fold limbs is buckled as a result of ongoing fold amplification. Finite element modelling confirms that cleavage refraction on limbs can be sufficient for cleavage planes to be subsequently shortened and therefore folded. Cleavage refraction is unequally developed on opposite limbs of asymmetric folds formed by oblique shortening of a layer in coaxial flow or by folding in a more general shear environment. The differences in finite strain on opposite limbs can be quite marked even when the fold shapes themselves are not obviously asymmetric. For folding in simple shear flow, as specifically modelled here, refraction is only strong on the fold limb that rotates against the imposed sense of shear. In known shear environments, this provides a potential kinematic indicator in folded units at relatively low strain (e.g. in simple shear, γ of around one), where other higher-strain indicators, typical of mylonites, are not yet sufficiently developed or are equivocal. © 2005 Elsevier Ltd. All rights reserved
Passive kimberlite intrusion into actively dilating dyke-fracture arrays: Evidence from fibrous calcite veins and extensional fracture cleavage
No full textCalcite veins are invariably associated with en-echelon kimberlite dyke-fracture arrays. A detailed microstructural study of veining indicates four vein types. Type I stretched or ataxial veins are defined by high aspect ratio calcite fibers that are crystallographically continuous with calcite of the kimberlite matrix wall rock, by elongated phenocrystic phlogopite with sharp crystal terminations centered on contacts between adjacent calcite fibers and by phenocrystic phlogopite that grows or extends across these veins. Type I vein mineralogy indicates syn-dilational crystallization of vein minerals in local tensional areas within the kimberlite. Vein Types II (stretched to syntaxial elongate-blocky) and III (antitaxial) indicate late crystallization vein mineral growth during subsequent or repeated dilation. Calcite fibers in Type I to Type III veins are orthogonal to the contacts of their host dykes regardless of the orientation of vein margins. Type IV calcite veins, with blocky or mosaic/polycrystalline textures, are attributed to minor post-intrusion extension, which was potentially accompanied by repeated kimberlite intrusion within a given dyke array. Syn-crystallization/syn-intrusion Type I veins and an ubiquitous dyke-parallel fracture cleavage, in a zone up to 4 m on either side of dyke contacts, suggest that en-echelon kimberlite dyke-fracture arrays occupied the approximate center of zones of active dilation within the brittle carapace of the upper crust. Type II and III veins indicate that extension or dilation continued, independently of an occupying kimberlite fluid phase, after initial intrusion. Arrested mobile hydrofracturing, under low differential stress within the upper brittle or seismic carapace of the continental crust, followed by repeated dilation of the dyke-fracture system, is proposed as a mechanism for producing the features observed in this study. The conditions constrained in this study indicate passive dyke intrusion into dilating fracture arrays during crustal extension. © 2004 Elsevier B.V. All rights reserved
Geological record of Paleoarchean oceanic flake tectonics preserved in the c. 3.3 Ga Kromberg volcanic type-section, Barberton greenstone belt, South Africa
No full textThe geology and existing geodynamic models for the c. 3.3 Kromberg mafic–ultramafic sequence of rocks in the Barberton greenstone belt (South Africa) have been
the subject of intense debate as, in addition to their regional significance, they have bearing also upon possible Paleoarchean tectonic processes on the early Earth.
Models for the origin of the Kromberg sequence of rocks range from formation in an autochthonous, plume-type setting to that of an allochthonous, ophiolite thrust
sheet emplaced in a modern-style plate-tectonic regime. This study reports new high-resolution field mapping results, drill core observations and secondary ion
microprobe U-Pb detrital zircon age constraints across the Kromberg type-section on both the eastern and western sides of the Komati River. The new observations
indicate that the Kromberg type-section represents a ca. 1.65 km thick dismembered and variably silicified mélange of oceanic supracrustal rocks with minor
metadunites, which are structurally bound by chlorite-carbonate-quartz shear zones. Field observations and microstructural analysis allow for the rheological
behaviour of the Paleoarchean oceanic lithosphere to be explored. It is proposed herein that the observed structural and volcanic geology is compatible with a
Paleoarchean oceanic flake tectonic process, involving detachment along a brittle-ductile transition zone and horizontal accretion of thin, rigid, silicified slivers of
upper Archean oceanic crust. These accreted allochtonous oceanic fragments would have delaminated from thick, weaker, lower oceanic crust that was recycled into
the hot Paleoarchean mantle, and thus removed from the geological record. The model invoked here suggests that, although the well-preserved mafic–ultramafic
Kromberg volcanic sequence may share some significant similarities with modern Phanerozoic ophiolites in the field, it most likely did not form by modern-style
plate-tectonic processes. The new field observations herein support highly elevated oceanic geothermal gradients at the time due to a hotter, more vigorously
convecting, Paleoarchean mantle and crustal geodynamic processes unique to the Paleoarchean era of Earth history in the absence of a global-scale, fully developed
plate-tectonic regime
Dating syn-orogenic exhumation of subducted continental crust: The case of the Northern Apennines
Full text linkDating deformation is key to unravel the evolution of orogens. Unfortunately, this is commonly challenged by the occurrence in the rock record of multiple overprinting deformation stages that lead to repeated blastesis and form multiple fabrics during both prograde and retrograde evolutions. Additionally, recrystallization is not always complete, which causes a mixture of inherited and neoblastic mineral phases. Therefore, an approach integrating age dating with microstructural and petrographic analyses is necessary to extract structurally constrained ages from the rock record. We used this approach to contribute to the long-lasting debate about exhumation of continental metamorphic units subducted to high-pressure conditions. We studied continental metasedimentary sequences of the Tuscan Metamorphic Units of the Northern Apennines (Italy) from two localities: the Island of Giglio to the west and the Monticiano-Roccastrada Unit to the east. We obtained two analytically distinct 40Ar/39Ar age ranges: an older 21–16 Ma age cluster is related to syn-orogenic top-to-the E contractional deformation occurring at ca. 1 GPa and 350 °C. A younger 15–11 Ma age group is related, instead, to exhumation to shallower structural levels and retrogression of these units to greenschist facies conditions. The interpretation of the results is based on white mica chemical compositions and rock fabrics. By integrating our results with published literature, we conclude that the investigated high-pressure continental metasedimentary sequences experienced early syn-orogenic exhumation from blueschist to greenschist facies conditions predominantly by a combination of extrusion and out- and in-sequence thrusting, with only minor contributions by extensional shearing during regional crustal thinning. Therefore, this study highlights the role of contractional fabrics in accommodating significant exhumation amounts of deeply subducted continental rocks
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