Structures within greenschist facies Alpine Schist, central Southern Alps, New Zealand
39
Citation
24
Reference
10
Related Paper
Citation Trend
Abstract:
Abstract Ductile structures in the greenschist facies rocks of the Alpine Schist that lie in the headwaters of the Whataroa, Callery, and Balfour valleys formed during Mesozoic deformation, similar to those in the Otago Schist to the south. A general westward increase in metamorphic grade is locally disrupted and repeated by several late metamorphic faults. The first appearance of biotite occurs at tectonic boundaries in the Callery and Balfour sections. Metagreywackes are juxtaposed against a greenschist‐metapelite‐metachert sequence by a synmetamorphic ductile high strain zone in the lower Balfour valley. A prominent quartz rodding lineation in the high strain zone defines a stretching direction that plunges about 30° southwest. Structures in the high strain zone are locally cut by quartz (±biotite) veins, which have been weakly deformed. Fluid inclusions in quartz veins indicate that the veins formed about 8–10 km deep and that uplift was initially nearly isothermal. Pervasive foliation that formed during Mesozoic deformation was deformed into upright, open, kilometre‐scale folds, with some foliation‐parallel boudinage, associated with the Miocene inception of the Alpine Fault. The resultant north to northeast trending folds plunge gently southwards. Deformation associated with Alpine Fault movement was only minor in the Alpine Schist greenschist facies and resulted in passive differential uplift of deeper parts of the schist pile in the west.Keywords:
Greenschist
Lineation
Abstract Shape, size and orientation measurements of quartz grains sampled along two transects that cross zones of increasing metamorphic grade in the Otago Schist, New Zealand, reveal the role of quartz in the progressive development of metamorphic foliation. Sedimentary compaction and diagenesis contributed little to the formation of a shape‐preferred orientation (SPO) within the analysed samples. Metamorphic foliation was initiated at sub‐greenschist facies conditions as part of a composite S1‐bedding structure parallel to the axial planes of tight to isoclinal F1 folds. An important component of this foliation is a pronounced quartz SPO that formed dominantly by the effect of dissolution–precipitation creep on detrital grains in association with F1 strain. With increasing grade, the following trends are evident from the SPO data: (i) a progressive increase in the aspect ratio of grains in sections parallel to lineation, and the development of blade‐shaped grains; (ii) the early development of a strong shape preferred orientation so that blade lengths define the linear aspect of the foliation (lineation) and the intermediate axes of the blades define a partial girdle about the lineation; (iii) a slight thinning and reduction in volume of grains in the one transect; and (iv) an actual increase in thickness and volume in the survivor grains of the second transect. The highest‐grade samples, within the chlorite zone of the greenschist facies, record segregation into quartz‐ and mica‐rich layers. This segregation resulted largely from F2 crenulation and marks a key change in the distribution, deformation and SPO of the quartz grains. The contribution of quartz SPO to defining the foliation lessens as the previously discrete and aligned detrital quartz grains are replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and reduced alignment. Pressure solution now affects the margins of quartz‐rich layers rather than individual grains. In higher‐grade samples, therefore, the rock structure is characterized increasingly by segregation layering parallel to a foliation defined predominantly by mica SPO.
Lineation
Greenschist
Foliation (geology)
Phyllite
Cite
Citations (10)
Continental crustal rocks, now structurally beneath the allochthonous Samail ophiolite, underwent blueschist to eclogite facies metamorphism prior to the emplacement of the Oman ophiolite onto the Arabian margin. The recognition of a major low angle fault within this polydeformed and polymeta‐morphosed sequence of metacarbonates, metabasites, quartzites and quartz mica schists greatly simplifies the interpretation of the structure and metamorphic zonation within the Saih Hatat window, NE Oman. Carpholite‐bearing upper plate rocks consist of pre‐Permian to Jurassic rocks that have been folded into large recumbent nappe structures which exhibit a marked increase in deformation intensity toward the boundary with the lower plate. The lower plate rocks have higher peak pressure and temperature assemblages; they are exposed in two windows separated by Jabal Abu Daud. Although more intensely deformed, the lower plate rocks are recognizable as metamorphosed continental platform sediments. Fold axes are parallel to the regional NNE‐SSW lineation. Sense of shear indicators yield a transport direction of south over north in the lower plate, opposite to the sense of motion inferred for the emplacement of the ophiolite. Lower plate eclogite‐facies metabasalts are only preserved in kilometer‐scale megaboudins found in the easternmost window at As Sifah. These metamorphic assemblages along with their remnant east‐west fabrics define the existence of a short‐lived, Arabian platform‐directed, nascent subduction zone. The stretching lineation elsewhere within the upper and lower plates, the Hatat Schist (the basement), and the metamorphic sole of the ophiolite is consistently NNE‐SSW, suggesting that the exhumation of high‐pressure metamorphic rocks of Saih Hatat is related to the ophiolite obduction. Exhumation of the high pressure rocks was accompanied by intense deformation involving regional‐scale fold nappes in a convergent margin setting. The geotherm remained suppressed for a period (>30 Myr) greater than the thermal relaxation time of the crust. The geometric and thermal constraints from Oman may be applicable to the general problem of the formation and preservation of high‐pressure, low‐temperature rocks.
Lineation
continental collision
Greenschist
Blueschist
Continental Margin
Dalradian
Cite
Citations (79)
The median antiformal axis of the Otago Schist, New Zealand, is marked by a zone of relatively high‐grade (up to garnet‐biotite‐albite) greenschist facies rocks. 40Ar/39Ar geochronology has been carried out in conjunction with structural analysis in regions distant from the effects of the Alpine Fault to determine the origin of this metamorphic welt. We have determined that the metamorphic welt is bounded on its northern and southern sides by multistage ductile shear zone(s) marked by intensely developed fabrics, and/or low‐angle normal faults. These structures extend over a strike length of >∼200 km and on the southern side of the metamorphic welt they mark the boundary between the Caples and the Torlesse terranes. The oldest such shear zone formed between 122 and 118 Ma. The metamorphic welt was exhumed beneath low‐angle normal faults and ductile shear zones that formed from 112 to 109 Ma. The shear zones form the carapace to elongate domal culminations in the central Otago Schist. These geomorphological features are Cretaceous metamorphic core complexes dissected by younger Quaternary faults. Exhumation of the shear zones occurred shortly before volcanogenic sediments began to deposit on a Cretaceous unconformity. We propose that both the Cretaceous unconformity and the underlying ductile shear zones and low‐angle faults are a direct result of extensional tectonism. The ductile shear zones display both 'cross‐belt' and 'belt‐parallel' stretching lineations. Cross‐belt stretching may have been caused by rollback of the subducting Pacific slab. Belt parallel extension is interpreted to have taken place during extension associated with rifting between Australia and Antarctica, prior to breakup.
Metamorphic core complex
Greenschist
Lineation
Detachment fault
Mylonite
Cite
Citations (61)
Abstract In this contribution, we present new structural, microstructural, fabrics, and geochronological data from the southern Chong Shan complex, one of the metamorphic complexes in the southeastern Tibetan Plateau that were sheared and exhumed during the India-Eurasia convergence. The NW-SE–striking complex is comprised of a central high-grade metamorphic zone (Unit I) flanked by two low-grade metamorphic zones (units II and III) on the northeastern and southwestern sides, respectively. High-grade metamorphic rocks (e.g., amphibolites, sillimanite-mica schists) of up to amphibolite facies, of the Proterozoic Chong Shan group and granitic intrusions of Permo-Triassic to Cenozoic in age in Unit I are characterized by high-temperature deformation. Units II (i.e., the Wuliangshan group) and III (i.e., the Lancang group) on both sides of the high-grade Unit I consist of metamorphic rocks of low greenschist facies (e.g., phyllites) with low-temperature deformation. The high- and low-grade units possess consistent kinematics, i.e., northwestward motion of the core rocks relative to the two limbs, and they are separated by large scale shear discontinuities. Thereby, the high- and low-grade units are kinematically linked but mechanically decoupled. Zircon laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of syn-shearing granitic dikes reveals that ductile shearing occurred from 29 to 19 Ma. Structural analysis reveals that these units constitute an A-type dome that has long axis parallel to the stretching lineations and fold axes of outcrop-scale A-type folds. It is shown that three stages of deformation contributed to the formation of the southern Chong Shan dome, during which subhorizontal shearing were in connection with regional doming. The events occurred as the consequence of middle to lower crustal flow that led to lateral flow and vertical exhumation of crustal masses. Therefore, the lateral crustal flow was not only limited along the boundary high strain zones of the Sundaland block, but distributed within the southeastern Tibetan Plateau. We would argue that the tectonic extrusion of the Sundaland block occurred through ductile crustal flow of a viscous middle and lower crust in the plate interior combined with concurring channel flow along the block margins.
Lineation
Greenschist
Metamorphic core complex
Crenulation
Mylonite
Tectonite
continental collision
Cite
Citations (3)
Southwest of Kimberley, southeastern British Columbia, the Matthew Creek metamorphic zone occupies the core of a structural dome in Mesoproterozoic rocks of the Lower Aldridge formation (lower Purcell Supergroup). It comprises (1) a core zone of ductilely deformed sillimanite-grade metapelites, thin foliated mafic sills, and sheared quartz-plagioclase-tourmaline pegmatites; and (2) a thin transition zone of ductilely deformed metasediments which marks a textural and metamorphic transition between the core zone and overlying regionally extensive, brittlely deformed, biotite-grade semipelitic Lower Aldridge formation metasediments and thick Moyie sills. The core zone and transition zone in combination cover an area of 30 km 2 . The deepest exposed rocks in the core zone have a strong foliation and lineation (D 1 deformation) formed during late M 1 metamorphism at conditions of 580650°C and 3.5 ± 0.5 kbar. The timing of this metamorphic-structural episode is constrained to the interval 13521341 Ma based on near-concordant UPb ages from monazite in pelitic schist near the mouth of Matthew Creek. Later, weaker metamorphic and deformation episodes variably overprinted the rocks of the Matthew Creek metamorphic zone. The juxtaposition of low-grade, weakly deformed rocks above high-grade, strongly deformed rocks across a zone of ductile deformation is interpreted to be due to a subhorizontal shear zone.
Lineation
Metamorphic core complex
Sill
Sillimanite
Isograd
Cite
Citations (17)
Mylonite
Cataclastic rock
Crenulation
Lineation
Greenschist
Cite
Citations (175)
Mylonite
Metamorphic core complex
Detachment fault
Lineation
Greenschist
Tectonite
Cite
Citations (54)
Massif
Forsterite
Cite
Citations (1)
Greenschist
Ultramafic rock
Amphibole
Cite
Citations (14)
The Tatla Lake Metamorphic Complex (TLMC), which lies on the southwest side of the Intermontane Belt (IMB) in British Columbia, has characteristics typical of a metamorphic core complex: anticlinorial amphibolite‐grade gneissic and migmatitic core underlying a 1− to 2.5+‐km‐thick zone of mylonite and ductilely sheared metamorphic rocks which is in fault contact beneath an upper plate of low‐metamorphic‐grade cover rocks of the 1MB. Ductile shearing in the TLMC involved tonalitic to granodioritic orthogneiss and structurally overlying amphibolite‐grade metasedimentary rocks and greenschist‐grade chlorite‐actinolite‐albite schist. Structures observed throughout the ductilely sheared rocks include a gently dipping mylonitic foliation (Ss), containing a mineral lineation (Ls) which trends toward 280° (100°) ± 20°. Minor folds of variable trend (Fs), almost exclusively confined to metasedimentary rocks, are interpreted as synductile shear. Vergence of these folds defines a movement sense and direction of top toward 290° ± 20°. Kinematic indicators from rocks not deformed by synductile shear folds indicate a tops‐to‐the‐west sense of shear, while those within metasedimentary rocks (deformed by Fs folds) yield conflicting results, with a tops‐to‐the‐west sense predominating. Calculated a directions from Fs folds which deform Ls lineations indicate nearly horizontal Ds movement, subparallel to 290°–110°. The entire metamorphic core of the TLMC has been deformed by upright, west to west‐northwest trending, shallowly plunging map‐scale folds (F3). The steeply dipping, northwest trending Yalakom fault cuts all units and forms the southwestern margin of the TLMC. U‐Pb zircon geochronology has documented the existence of Cretaceous (107–79 Ma, in the core) and Eocene 55–47 Ma, (in the mylonitic zone) deformation and metamorphism in the TLMC. K‐Ar dates for biotite and hornblende of 53.4–45.6 Ma record the uplift and cooling of the TLMC. During early and middle Eocene time (55–47 Ma) metamorphic rocks of the TLMC were carried to higher crustal levels along the footwall of the TLMC normal ductile shear zone. Final uplift and development of F3 folds (post‐47 Ma) are possibly related to dextral motion along the Yalakom fault The TLMC has structural style and timing of deformation similar to other metamorphic core complexes in southeastern British Columbia. Local and regional evidence is consistent with the formation of the TLMC in a regional extensional setting within a vigorous magmatic arc. Similar interlayered gneisses with near‐horizontal layering and foliation may underlie the entire southern IMB.
Mylonite
Lineation
Metamorphic core complex
Greenschist
Crenulation
Isograd
Foliation (geology)
Shearing (physics)
Cite
Citations (40)