scholarly journals Temporal and spatial variations in magmatism and transpression in a Cretaceous arc, Median Batholith, Fiordland, New Zealand

Lithosphere ◽  
2019 ◽  
Vol 11 (5) ◽  
pp. 652-682 ◽  
Author(s):  
Luisa F. Buriticá ◽  
Joshua J. Schwartz ◽  
Keith A. Klepeis ◽  
Elena A. Miranda ◽  
Andy J. Tulloch ◽  
...  
Keyword(s):  
Mass Spectrometry ◽  
New Zealand ◽  
Shear Zone ◽  
High Strain ◽  
Shear Zones ◽  
Separation Point ◽  
Ca 125 ◽  
Arc Magmatism ◽  
Spatial And Temporal Patterns ◽  
Middle Crust

Abstract We investigated the interplay between deformation and pluton emplacement with the goal of providing insights into the role of transpression and arc magmatism in forming and modifying continental arc crust. We present 39 new laser-ablation–split-stream–inductively coupled plasma–mass spectrometry (LASS-ICP-MS) and secondary ion mass spectrometry (SIMS) 206Pb/238U zircon and titanite dates, together with titanite geochemistry and temperatures from the lower and middle crust of the Mesozoic Median Batholith, New Zealand, to (1) constrain the timing of Cretaceous arc magmatism in the Separation Point Suite, (2) document the timing of titanite growth in low- and high-strain deformational fabrics, and (3) link spatial and temporal patterns of lithospheric-scale transpressional shear zone development to the Cretaceous arc flare-up event. Our zircon results reveal that Separation Point Suite plutonism lasted from ca. 129 Ma to ca. 110 Ma in the middle crust of eastern and central Fiordland. Deformation during this time was focused into a 20-km-wide, arc-parallel zone of deformation that includes previously unreported segments of a complex shear zone that we term the Grebe shear zone. Early deformation in the Grebe shear zone involved development of low-strain fabrics with shallowly plunging mineral stretching lineations from ca. 129 to 125 Ma. Titanites in these rocks are euhedral, are generally aligned with weak subsolidus fabrics, and give rock-average temperatures ranging from 675 °C to 700 °C. We interpret them as relict magmatic titanites that grew prior to low-strain fabric development. In contrast, deformation from ca. 125 to 116 Ma involved movement along subvertical, mylonitic shear zones with moderately to steeply plunging mineral stretching lineations. Titanites in these shear zones are anhedral grains/aggregates that are aligned within mylonitic fabrics and have rock-average temperatures ranging from ∼610 °C to 700 °C. These titanites are most consistent with (re)crystallization in response to deformation and/or metamorphic reactions during amphibolite-facies metamorphism. At the orogen scale, spatial and temporal patterns indicate that the Separation Point Suite flare-up commenced during low-strain deformation in the middle crust (ca. 129–125 Ma) and peaked during high-strain, transpressional deformation (ca. 125–116 Ma), during which time the magmatic arc axis widened to 70 km or more. We suggest that transpressional deformation during the arc flare-up event was an important process in linking melt storage regions and controlling the distribution and geometry of plutons at mid-crustal levels.

scholarly journals The internal structure and composition of a plate boundary-scale serpentinite shear zone: The Livingstone Fault, New Zealand

2019 ◽  
Author(s):  
Matthew S. Tarling ◽  
Steven A. F. Smith ◽  
James M. Scott ◽  
Jeremy S. Rooney ◽  
Cecilia Viti ◽  
...  
Keyword(s):  
New Zealand ◽  
Internal Structure ◽  
Shear Zone ◽  
Plate Boundary ◽  
Shear Zones ◽  
East Side ◽  
Local Dispersion ◽  
Fine Grained ◽  
Shear Sense ◽  
Tectonic Settings

Abstract. Deciphering the internal structural and composition of large serpentinite-dominated shear zones will lead to an improved understanding of the rheology of the lithosphere in a range of tectonic settings. The Livingstone Fault in New Zealand is a > 1000 km long terrane-bounding structure that separates the basal portions (peridotite; serpentinised peridotite; metagabbros) of the Dun Mountain Ophiolite Belt from quartzofeldspathic schists of the Caples or Aspiring Terranes. Field and microstructural observations from eleven localities along a strike length of c. 140 km show that the Livingstone Fault is a steeply-dipping, serpentinite-dominated shear zone tens to several hundreds of metres wide. The bulk shear zone has a pervasive scaly fabric that wraps around fractured and faulted pods of massive serpentinite, rodingite and partially metasomatised quartzofeldspathic schist up to a few tens of metres long. S-C fabrics and lineations in the shear zone consistently indicate a steep Caples-side-up (i.e. east-side-up) shear sense, with significant local dispersion in kinematics where the shear zone fabrics wrap around pods. The scaly fabric is dominated (> 98 vol %) by fine-grained (≪ 10 μm) fibrous chrysotile and lizardite/polygonal serpentine, but infrequent (

scholarly journals Interplay of Cretaceous transpressional deformation and continental arc magmatism in a long-lived crustal boundary, central Fiordland, New Zealand

Geosphere ◽  
2020 ◽  
Vol 16 (5) ◽  
pp. 1225-1248
Author(s):  
Hannah J. Blatchford ◽  
Keith A. Klepeis ◽  
Joshua J. Schwartz ◽  
Richard Jongens ◽  
Rose E. Turnbull ◽  
...  
Keyword(s):  
New Zealand ◽  
Thermal Structure ◽  
Shear Zones ◽  
Arc Magmatism ◽  
Magmatic Arc ◽  
Continental Arcs ◽  
Metasedimentary Rocks ◽  
Spatio Temporal ◽  
Continental Magmatic Arc ◽  
Lower Crustal

Abstract Recovering the time-evolving relationship between arc magmatism and deformation, and the influence of anisotropies (inherited foliations, crustal-scale features, and thermal gradients), is critical for interpreting the location, timing, and geometry of transpressional structures in continental arcs. We investigated these themes of magma-deformation interactions and preexisting anisotropies within a middle- and lower-crustal section of Cretaceous arc crust coinciding with a Paleozoic boundary in central Fiordland, New Zealand. We present new structural mapping and results of Zr-in-titanite thermometry and U-Pb zircon and titanite geochronology from an Early Cretaceous batholith and its host rock. The data reveal how the expression of transpression in the middle and lower crust of a continental magmatic arc evolved during emplacement and crystallization of the ∼2300 km2 lower-crustal Western Fiordland Orthogneiss (WFO) batholith. Two structures within Fiordland’s architecture of transpressional shear zones are identified. The gently dipping Misty shear zone records syn-magmatic oblique-sinistral thrust motion between ca. 123 and ca. 118 Ma, along the lower-crustal WFO Misty Pluton margin. The subhorizontal South Adams Burn thrust records mid-crustal arc-normal shortening between ca. 114 and ca. 111 Ma. Both structures are localized within and reactivate a recently described >10 km-wide Paleozoic crustal boundary, and show that deformation migrated upwards between ca. 118 and ca. 114 Ma. WFO emplacement and crystallization (mainly 118–115 Ma) coincided with elevated (>750 °C) middle- and lower-crustal Zr-in-titanite temperatures and the onset of mid-crustal cooling at 5.9 ± 2.0 °C Ma−1 between ca. 118 and ca. 95 Ma. We suggest that reduced strength contrasts across lower-crustal pluton margins during crystallization caused deformation to migrate upwards into thermally weakened rocks of the mid-crust. The migration was accompanied by partitioning of deformation into domains of arc-normal shortening in Paleozoic metasedimentary rocks and domains that combined shortening and strike-slip deformation in crustal-scale subvertical, transpressional shear zones previously documented in Fiordland. U-Pb titanite dates indicate Carboniferous–Cretaceous (re)crystallization, consistent with reactivation of the inherited boundary. Our results show that spatio-temporal patterns of transpression are influenced by magma emplacement and crystallization and by the thermal structure of a reactivated boundary.

Rheological properties of the shallow subduction interface: insights from the Chugach Complex, Alaska

2020 ◽  
Author(s):  
Zoe Braden ◽  
Whitney Behr
Keyword(s):  
Fluid Flow ◽  
Rheological Properties ◽  
Shear Zone ◽  
Strain Localization ◽  
High Strain ◽  
Shear Zones ◽  
Fluid Injection ◽  
Structural Mapping ◽  
Wide Range ◽  
Shallow Subduction

<p>The plate interface in subduction zones accommodates a wide range of seismic styles over different depths as a function of pressure-temperature conditions, compositional and fluid-pressure heterogeneities, deformation mechanisms, and degrees of strain localization. The shallow subduction interface (i.e. ~2-10 km subduction depths), in particular, can exhibit either slow slip events (e.g. Hikurangi) or megathrust earthquakes (e.g. Tohoku). To evaluate the factors governing these different slip behaviors, we need better constraints on the rheological properties of the shallow interface. Here we focus on exhumed rocks within the Chugach Complex of southern Alaska, which represents the Jurassic to Cretaceous shallow subduction interface of the Kula and North American plates. The Chugach is ideal because it exhibits progressive variations in subducted rock types through time, minimal post-subduction overprinting, and extensive along-strike exposure (~250 km). Our aims are to use field structural mapping, geochronology, and microstructural analysis to examine a) how strain is localized in different subducted protoliths, and b) the deformation processes, role of fluids, and strain localization mechanisms within each high strain zone. We interpret these data in the context of the relative ‘strengths’ of different materials on the shallow interface and possible styles of seismicity.  </p><p>Thus far we have characterized deformation features along a 1.25-km-thick melange belt within the Turnagain Arm region southeast of Anchorage.  The westernmost melange unit is sediment poor and consists of deep marine rocks with more chert, shale and mafic rocks than units to the east. The melange fabric is variably developed (weakly to strongly) throughout the unit and is steeply (sub-vertical) west-dipping with down-dip lineations. Quartz-calcite-filled dilational cracks are oriented perpendicular to the main melange fabric.</p><p>Drone imaging and structural mapping reveals 3 major discrete shear zones and 6-7 minor shear zones within the melange belt, all of which exhibit thrust kinematics. Major shear zones show a significant and observable strain gradient into a wide (~1 m) region of high strain and deform large blocks while minor shear zones are generally developed in narrow zones (~10-15 cm) of high strain between larger blocks. One major shear zone is developed in basalt and has closely-spaced, polished slip surfaces that define a facoidal texture; the basalt shear zone is ~1 m thick. Preserved pillows are observable in lower strain areas on either side of the shear zone but are deformed and indistinguishable within the high strain zone. The other two major shear zones are developed in shale and are matrix-supported with wispy, closely-spaced foliation and rotated porphyroclasts of chert and basalt; the shale shear zones are ~0.5-2 m thick.  </p><p>Abundant quartz-calcite veins parallel to the melange fabric and within shale shear zones record multiple generations of fluid-flow; early veins appear to be more silicic and later fluid flow involved only calcite precipitation. At the west, trench-proximal end of the mélange unit there is a 5-10 m thick silicified zone of fluid injection that is bound on one side by the basalt shear zone. Fluid injection appears to pre-date or be synchronous with shearing.</p>

Strain variation in shear belts

1970 ◽  
Vol 7 (3) ◽  
pp. 786-813 ◽  
Author(s):  
J. G. Ramsay ◽  
R. H. Graham
Keyword(s):  
Shear Zone ◽  
Strain State ◽  
Finite Strain ◽  
High Strain ◽  
Shear Zones ◽  
Finite Strains ◽  
Strain Variation ◽  
Strain Ellipsoid ◽  
Intensity Of Development ◽  
Variable Displacement

In rocks deformed by natural orogenic processes it is usual to find that the finite strain state varies from locality to locality. In some deformed rocks high strain states are localized within approximately planar zones commonly known as "shear belts".The general relationships that exist between variable displacement and variable strain state are established, and these general equations are solved for particular types of strain within shear zones. Only a limited number of types of solution are possible. Using these solutions the geometric forms of the structures found in shear zones in several regions are analyzed. Methods for computing the finite strain through these zones are described, and these finite strains are integrated to determine the total displacements across these zones. Schistosity is developed in some of the shear zones described. It is not parallel to the walls of the shear zone and is therefore not parallel to the dominant displacement (shear) directions. The schistosity appears to be formed perpendicular to the principal finite shortening (i.e. perpendicular to the shortest axis of the finite strain ellipsoid). Variations of the schistosity planes represent variations in the finite strain trajectories of XY planes in the strain states ([Formula: see text] ellipsoid axes). The intensity of development of the schistosity is correlated with the values of the principal finite strains.

Crustal melt granites and migmatites along the Himalaya: melt source, segregation, transport and granite emplacement mechanisms

2009 ◽  
Vol 100 (1-2) ◽  
pp. 219-233 ◽  
Author(s):  
M. P. Searle ◽  
J. M. Cottle ◽  
M. J. Streule ◽  
D. J. Waters
Keyword(s):  
Partial Melting ◽  
Shear Zone ◽  
Internal Heat ◽  
Shear Zones ◽  
Source Rocks ◽  
Crustal Thickening ◽  
Main Central Thrust ◽  
Crustal Layer ◽  
Middle Crust ◽  
The Himalaya

ABSTRACTIndia–Asia collision resulted in crustal thickening and shortening, metamorphism and partial melting along the 2200 km-long Himalayan range. In the core of the Greater Himalaya, widespread in situ partial melting in sillimanite+K-feldspar gneisses resulted in formation of migmatites and Ms+Bt+Grt+Tur±Crd±Sil leucogranites, mainly by muscovite dehydration melting. Melting occurred at shallow depths (4–6 kbar; 15–20 km depth) in the middle crust, but not in the lower crust. 87Sr/86Sr ratios of leucogranites are very high (0·74–0·79) and heterogeneous, indicating a 100 crustal protolith. Melts were sourced from fertile muscovite-bearing pelites and quartzo-feldspathic gneisses of the Neo-Proterozoic Haimanta–Cheka Formations. Melting was induced through a combination of thermal relaxation due to crustal thickening and from high internal heat production rates within the Proterozoic source rocks in the middle crust. Himalayan granites have highly radiogenic Pb isotopes and extremely high uranium concentrations. Little or no heat was derived either from the mantle or from shear heating along thrust faults. Mid-crustal melting triggered southward ductile extrusion (channel flow) of a mid-crustal layer bounded by a crustal-scale thrust fault and shear zone (Main Central Thrust; MCT) along the base, and a low-angle ductile shear zone and normal fault (South Tibetan Detachment; STD) along the top. Multi-system thermochronology (U–Pb, Sm–Nd, 40Ar–39Ar and fission track dating) show that partial melting spanned ̃24–15 Ma and triggered mid-crustal flow between the simultaneously active shear zones of the MCT and STD. Granite melting was restricted in both time (Early Miocene) and space (middle crust) along the entire length of the Himalaya. Melts were channelled up via hydraulic fracturing into sheeted sill complexes from the underthrust Indian plate source beneath southern Tibet, and intruded for up to 100 km parallel to the foliation in the host sillimanite gneisses. Crystallisation of the leucogranites was immediately followed by rapid exhumation, cooling and enhanced erosion during the Early–Middle Miocene.

scholarly journals Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

2016 ◽  
Author(s):  
Frances J. Cooper ◽  
John P. Platt ◽  
Whitney M. Behr
Keyword(s):  
High Strain ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Lower Zone ◽  
Middle Crust ◽  
Metamorphic Core Complexes ◽  
Brittle Ductile Transition ◽  
Ductile Shear ◽  
Metamorphic Core ◽  
Whipple Mountains

Abstract. High strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada) suggest the presence of two distinct rheological transitions in the middle crust. (1) The brittle-ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear. (2) The localized-distributed transition or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths. In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. In some examples (e.g. the Whipple Mountains) the lower zone is spatially distinct from the detachment, although high-strain rocks from the lower zone were subsequently exhumed along the detachment. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization.

scholarly journals Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Solid Earth ◽  
2017 ◽  
Vol 8 (1) ◽  
pp. 199-215 ◽  
Author(s):  
Frances J. Cooper ◽  
John P. Platt ◽  
Whitney M. Behr
Keyword(s):  
High Strain ◽  
High Stress ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Middle Crust ◽  
Metamorphic Core Complexes ◽  
Brittle Ductile Transition ◽  
Ductile Shear ◽  
Metamorphic Core ◽  
Whipple Mountains

Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

scholarly journals Whole-lithosphere shear during oblique rifting

Geology ◽  
2021 ◽  
Author(s):  
Brandon M. Lutz ◽  
Gary J. Axen ◽  
Jolante W. van Wijk ◽  
Fred M. Phillips
Keyword(s):  
Shear Zone ◽  
Fault Zone ◽  
Shear Zones ◽  
Death Valley ◽  
Necessary Condition ◽  
Middle Crust ◽  
Metamorphic Core Complexes ◽  
Dextral Shear ◽  
Depth Gradients ◽  
Metamorphic Core

Processes controlling the formation of continental whole-lithosphere shear zones are debated, but their existence requires that the lithosphere is mechanically coupled from base to top. We document the formation of a dextral, whole-lithosphere shear zone in the Death Valley region (DVR), southwest United States. Dextral deflections of depth gradients in the lithosphere-asthenosphere boundary and Moho are stacked vertically, defining a 20–50-km-wide, lower lithospheric shear zone with ~60 km of shear. These deflections underlie an upper-crustal fault zone that accrued ~60 km of dextral slip since ca. 8–7 Ma, when we infer that whole-lithosphere shear began. This dextral offset is less than net dextral offset on the upper-crustal fault zone (~90 km, ca. 13–0 Ma) and total upper-crustal extension (~250 km, ca. 16–0 Ma). We show that, before ca. 8–7 Ma, weak middle crust decoupled upper-crustal deformation from deformation in the lower crust and mantle lithosphere. Between 16 and 7 Ma, detachment slip thinned, uplifted, cooled, and thus strengthened the middle crust, which is exposed in metamorphic core complexes collocated with the whole-lithosphere shear zone. Midcrustal strengthening coupled the layered lithosphere vertically and therefore enabled whole-lithosphere dextral shear. Where thick crust exists (as in pre–16 Ma DVR), midcrustal strengthening is probably a necessary condition for whole-lithosphere shear.

scholarly journals New Insight into the Genetic Mechanism of Shear Zone Type Gold Deposits from Muping-Rushan Metallogenic Belt (Jiaodong Peninsula of Eastern China)

Minerals ◽  
2019 ◽  
Vol 9 (12) ◽  
pp. 775
Author(s):  
Nannan Cheng ◽  
Quanlin Hou ◽  
Mengyan Shi ◽  
Miao He ◽  
Qing Liu ◽  
...  
Keyword(s):  
Shear Zone ◽  
Gold Mineralization ◽  
Electron Backscatter Diffraction ◽  
Eastern China ◽  
Fluid Pressure ◽  
Shear Zones ◽  
Gold Deposits ◽  
Genetic Mechanism ◽  
Forming Process ◽  
Middle Crust

Most gold deposits are genetically controlled by shear zones, which are called shear zone type gold deposits (SZTGD). A better understanding of kinematics of shear zones and its constraint on the ore-forming process is critical to reveal the genetic mechanism of the SZTGD and favorable to mineral exploration. By conducting detailed structural analysis including field and microscopic observations and electron backscatter diffraction (EBSD) and fractal dimension analysis in the Muping-Rushan shear zone (MR) as well as several gold deposits, the kinematic characteristics of the MR are well recognized and the metallogenic process of the SZTGD are discussed. The main conclusions are as follows: (1) petrology, geometry, kinematics, macro- and micro-structures imply that the MR has experienced a progressive shearing history exhumed via middle crust to subsurface level under the NW-SE extensional regime from late Jurassic to early Cretaceous; (2) in the MR, gold may precipitate both in the brittle fractures at middle crust level and brittle deformation part at shallow crust level during the stress-chemical process and (3) comparison of gold deposits between the MR and other areas show that the SZTGD has a uniform metallogenic mechanism, which is from (multi-stage) pluton emplacement, hydrothermal fluid action, shearing action, brittle fracturing, sudden reduction of fluid pressure, flash vaporization to (gold) mineralization.

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