Kinematics of the Central Alpine Fault Mylonite Zone, Tatare Stream, South Island, New Zealand

<p>The hanging wall of the Alpine Fault (AF) near Franz Josef Glacier has been exhumed during the past ~3 m. y. providing a sample of the ductilely deformed middle crust via obliquereverse slip on the AF. The former middle crust of the Pacific Plate occurs as an eastward-tilted slab that has been upramped from depths of ~25–35 km. A mylonitic high strain zone abuts the eastern edge of the AF in Tatare Stream. This ductile shear zone is locally ~2 km thick. The Tatare Stream locality is remarkable along the AF in the Central Southern Alps for the apparent lack of near surface segmentation of the fault there; instead its mylonitic shear zone appears uniformly inclined by ~63° to the SE. I infer this foliation is parallel to the shear zone boundary (SZB). In the distal part of the mylonite zone in extensional C' shear bands cross-cut the older non-mylonitic Alpine foliation (S3), and deflect that pre-existing fabric in a dextral-reverse sense. Based on the attitude of these shears the ductile shearing direction in the Alpine mylonite zone (AMZ) during extensional shear band activity is inferred to have trended 090 ± 6° (2σ), which is ~20° clockwise of sea floor spreading based estimates for the azimuth of the Pacific Plate motion. This indicates that slip on this central part of the AF is not fully “unpartitioned”. Measurements of the mean spacing, per-shear offset, C’ orientation, and per-shear thickness on >1000 extensional C’ shears provides perhaps the largest field-based data set of extensional shear band geometrical parameters so far compiled for a natural shear zone. The mean spacing between C’ shears decreases towards the AF from ~6 cm to ~0.2 cm. The per-shear offsets (8.2 ± 5 mm 1σ) and thickness (128 ± 20 1σ) of the extensional shears remains consistent despite a finite shear strain gradient. Using shear offset data I calculate a bulk finite shear strain accommodated by slip on C’ shears of 0.4 ± 0.3 (1σ), and a mean intra-shear band (C’ local) finite shear strain of 12.6 ± 5.4 (1σ). Consistency in the intra-shear band finite shear strain throughout the mylonite zone, together with increased C’ density implies that the quartzose rocks have behaved with a strain hardening rheology as the shears evolved. The dominant C’ (synthetic) extensional shears are disposed at a mean dihedral angle of 30° ± 2.2 (2σ), whereas the C’’ (antithetic) shears are 135 ± 3° (2σ) to the foliation (SZB). The C’ and C’’ shears appear to lie approximately parallel to planes of maximum instantaneous shear strain rate from which I obtain an estimate for Wk of 0.5 for the AMZ. I have measured the geometrical orientation of Mesozoic Alpine Schist garnet inclusion trails and tracked these pre-mylonitic age porphyroblastic garnets through the distal and main mylonite zones to determine their rotational response to late Cenozoic shearing. Electron microprobe analysis indicates that all the garnets examined in Tatare Stream are prograde from the regional (M2) Barrovian metamorphism. The mean inclusion trail orientations in the distal mylonite zone have been forward rotated by 35° relative to their equivalent orientation in the adjacent, less deformed non-mylonitic Alpine Schist. This rotation is synthetic to the dextralreverse shear of the AF zone. The rotation of approximately spherical shaped garnet porphyroblasts in the distal mylonite implies a finite shear strain of 1.2 in that zone. In the main part of the mylonite zone an additional forward rotation of 46° implies a finite shear strain there of 2.8. The inclusion trail rotational axis measured trends approximately perpendicular to the shear direction and parallel to the inferred late Cenozoic vorticity vector of ductile shearing. Using GhoshFlow, a program for simulating rotation of rigid passive objects in plane strain general shear a new kinematic vorticity number (Wn) estimate of 0.5 – 0.7 is established for the AMZ. The transition zone between the distal mylonite and the main mylonite zone, though little described in the literature, is well exposed in Tatare Stream. A distinct quartz rodding lineation, inherited from the non-mylonitic schist as an object into the mylonite zone, is distorted in the plane of the foliation across the transition from SW plunges to NE plunges. Because the foliation plane is here parallel to the SZB and by special reference to strongly curved lineation traces I have been able to isolate the pure shear component of deformation considering a simple 2D deformation on that slip plane; by modeling the distortional reorientation of inherited lineations in that plane. The direction of maximum finite elongation that I calculate in this plane trends 89 ± 3.8° (2σ). I believe this records the finite strain related to the co-axial component only. The parallelism of the previously calculated mylonitic ductile shearing direction to this stretching direction (also trending 090) indicates that the late Cenozoic ductile flow path in the central AMZ has been approximately monoclinic. I estimate a Wn of 0.8 ± 0.06 (2σ) based on the observed finite shearing in the mylonite zone (garnet rotation) and on the co-axial strain observed deforming the inherited lineations.</p>

Kinematics of the Central Alpine Fault Mylonite Zone, Tatare Stream, South Island, New Zealand

<p>The hanging wall of the Alpine Fault (AF) near Franz Josef Glacier has been exhumed during the past ~3 m. y. providing a sample of the ductilely deformed middle crust via obliquereverse slip on the AF. The former middle crust of the Pacific Plate occurs as an eastward-tilted slab that has been upramped from depths of ~25–35 km. A mylonitic high strain zone abuts the eastern edge of the AF in Tatare Stream. This ductile shear zone is locally ~2 km thick. The Tatare Stream locality is remarkable along the AF in the Central Southern Alps for the apparent lack of near surface segmentation of the fault there; instead its mylonitic shear zone appears uniformly inclined by ~63° to the SE. I infer this foliation is parallel to the shear zone boundary (SZB). In the distal part of the mylonite zone in extensional C' shear bands cross-cut the older non-mylonitic Alpine foliation (S3), and deflect that pre-existing fabric in a dextral-reverse sense. Based on the attitude of these shears the ductile shearing direction in the Alpine mylonite zone (AMZ) during extensional shear band activity is inferred to have trended 090 ± 6° (2σ), which is ~20° clockwise of sea floor spreading based estimates for the azimuth of the Pacific Plate motion. This indicates that slip on this central part of the AF is not fully “unpartitioned”. Measurements of the mean spacing, per-shear offset, C’ orientation, and per-shear thickness on >1000 extensional C’ shears provides perhaps the largest field-based data set of extensional shear band geometrical parameters so far compiled for a natural shear zone. The mean spacing between C’ shears decreases towards the AF from ~6 cm to ~0.2 cm. The per-shear offsets (8.2 ± 5 mm 1σ) and thickness (128 ± 20 1σ) of the extensional shears remains consistent despite a finite shear strain gradient. Using shear offset data I calculate a bulk finite shear strain accommodated by slip on C’ shears of 0.4 ± 0.3 (1σ), and a mean intra-shear band (C’ local) finite shear strain of 12.6 ± 5.4 (1σ). Consistency in the intra-shear band finite shear strain throughout the mylonite zone, together with increased C’ density implies that the quartzose rocks have behaved with a strain hardening rheology as the shears evolved. The dominant C’ (synthetic) extensional shears are disposed at a mean dihedral angle of 30° ± 2.2 (2σ), whereas the C’’ (antithetic) shears are 135 ± 3° (2σ) to the foliation (SZB). The C’ and C’’ shears appear to lie approximately parallel to planes of maximum instantaneous shear strain rate from which I obtain an estimate for Wk of 0.5 for the AMZ. I have measured the geometrical orientation of Mesozoic Alpine Schist garnet inclusion trails and tracked these pre-mylonitic age porphyroblastic garnets through the distal and main mylonite zones to determine their rotational response to late Cenozoic shearing. Electron microprobe analysis indicates that all the garnets examined in Tatare Stream are prograde from the regional (M2) Barrovian metamorphism. The mean inclusion trail orientations in the distal mylonite zone have been forward rotated by 35° relative to their equivalent orientation in the adjacent, less deformed non-mylonitic Alpine Schist. This rotation is synthetic to the dextralreverse shear of the AF zone. The rotation of approximately spherical shaped garnet porphyroblasts in the distal mylonite implies a finite shear strain of 1.2 in that zone. In the main part of the mylonite zone an additional forward rotation of 46° implies a finite shear strain there of 2.8. The inclusion trail rotational axis measured trends approximately perpendicular to the shear direction and parallel to the inferred late Cenozoic vorticity vector of ductile shearing. Using GhoshFlow, a program for simulating rotation of rigid passive objects in plane strain general shear a new kinematic vorticity number (Wn) estimate of 0.5 – 0.7 is established for the AMZ. The transition zone between the distal mylonite and the main mylonite zone, though little described in the literature, is well exposed in Tatare Stream. A distinct quartz rodding lineation, inherited from the non-mylonitic schist as an object into the mylonite zone, is distorted in the plane of the foliation across the transition from SW plunges to NE plunges. Because the foliation plane is here parallel to the SZB and by special reference to strongly curved lineation traces I have been able to isolate the pure shear component of deformation considering a simple 2D deformation on that slip plane; by modeling the distortional reorientation of inherited lineations in that plane. The direction of maximum finite elongation that I calculate in this plane trends 89 ± 3.8° (2σ). I believe this records the finite strain related to the co-axial component only. The parallelism of the previously calculated mylonitic ductile shearing direction to this stretching direction (also trending 090) indicates that the late Cenozoic ductile flow path in the central AMZ has been approximately monoclinic. I estimate a Wn of 0.8 ± 0.06 (2σ) based on the observed finite shearing in the mylonite zone (garnet rotation) and on the co-axial strain observed deforming the inherited lineations.</p>

Evolution of Quartz and Calcite Microstructures Exhumed From Deep Brittle-Ductile Shear Zones in the Southern Alps of New Zealand

<p><b>Arrays of brittle-ductile shears exposed in the Southern Alps of New Zealand, haveprovided a superb natural laboratory for insight into the microstructural evolution of lowercrustal shear zones during exhumation. Shears are exposed in the central section of theSouthern Alps at Sam Peak, Chancellor Ridge, and Baumann Glacier in a zone ~2 kmwide that is located 6–8 km structurally above the Alpine Fault. An array ofsystematically spaced shear zones that formed by embrittlement and faulting ofquartzofeldspathic schist took place at the same time as ductile shearing of quartzcarbonateveins embedded within the schist. This study has used field-based structuralmapping along with optical microscopy and universal stage measurements ofcrystallographic preferred orientations (CPO) to resolve the shear zone kinematics andrheology. On the basis of these data, the strain path can be reconstructed for the shearedveins during their progressive deformation. This began with their incidence as backshearsat the base of the Alpine Fault ramp and ended with their subsequent recrystallisation,uplift, and exhumation.</b></p> <p>The near-vertical shear planes have mean orientation of 221@89 NW ± 1o (n =780). They are inferred to have formed as backshears accommodating uplift of the PacificPlate as it was translated onto the oblique footwall ramp of the Alpine Fault during lateCenozoic oblique convergence. Detailed fault offset transect surveys across the shears atChancellor Ridge and Baumann Glacier reveal a mean spacing between the shear zones of25 ± 5 cm (n = 410). Quartz-carbonate marker veins are displaced in a dextral west-sideupshear sense. Fault offset geometry and a consistent arrangement of mineral fibrelineations that decorate fault surfaces, indicate that the mean displacement vector pitches35o SW in the shear plane (trend and plunge of: 262, 35 ± 7o). Ductilely deformed markerveins have been subject to a mean displacement of 9.9 ± 1.4 cm (n = 344) and a meanfinite ductile shear strain of 4.8 ± 0.3 (n = 219). A strain-rate for the ductile deformationof the veins is estimated at 3 x 10-11 sec-1 based on the observed finite ductile shear strain,an escalator kinematic model, and assumptions about the width of the deforming zone.</p> <p>Five deformation phases have affected the sheared veins during their transport upthe fault ramp: 1) initial brittle faulting and ductile shearing; 2) grain boundary sliding ofmylonitic quartz in response to a post-ramping differential stress drop; 3) recrystallisationand grain growth; 4) renewed late-stage dislocation creep; and 5) semibrittle deformationand exhumation. In the schist, the shears initiated as planar brittle faults at lower crustal depths of~21 km at a temperature of 450 ± 50oC. They developed in a zone of transiently highshear strain-rates near the base of the Alpine Fault ramp. Dislocation creep caused a CPOof quartz and calcite to develop in sheared veins. Using the flow law of Hirth et al. (2001)and the estimated strain-rate, a differential stress of ~165 MPa is inferred for ductiledeformation of the veins. Near-lithostatic (λ = 0.85) fluid pressures would have causedthe rocks to undergo brittle failure, a situation that is confirmed by a late component ofbrittle deformation that over prints the ductilely sheared veins. Syntectonic quartz-calciteveins infill the shear fractures, and these themselves have been sheared. The deformationof the veins was not a simple shear process but one with triclinic flow symmetry. This isinferred from discordance between the shear direction and the near-vertical principleextension direction that is revealed by the pattern and symmetry of quartz and calcite CPOfabrics.</p> <p>After the shears move away from the ramp-step, grain boundary sliding (GBS)accommodated by solid-state diffusion creep is inferred to have affected quartz veins.</p> <p>This deformation mechanism takes place because of 1) the small 8 μm grain size inheritedfrom Phase 1; 2) the presence of fluid in the shear zone; and 3) a stress drop to ~22 MPathat followed the initial up-ramping. Quartz CPO fabrics in the sheared veins areremarkably weak considering their large shear strains. GBS is inferred to have been achief deformation mechanism that caused the weakening of quartz CPO fabrics in thehighly sheared sections of deformed veins. Calcite has also affected the quartz fabricstrength as those veins containing >5% calcite have very weak quartz CPO fabrics. Incontrast to quartz, the CPO fabrics for the co-existing calcite remained strong andcontinued to develop by dislocation creep.</p> <p>The third phase of deformation, a process that may have contributed to subsequentweakening of quartz CPO fabrics, was recrystallisation and grain growth to 126 μm and anequigranular-polygonal grain shape fabric. This fabric was overprinted by late-stagedislocation creep microstructures in the fourth deformation phase in response increaseddifferential stress encountered by the rocks at lower temperatures in the upper crust. Thefinal phase of deformation to affect the sheared veins was semibrittle deformation atdifferential stresses of <189 MPa and temperatures of 200–280oC as the rocks passedthrough the steady-state brittle-ductile transition zone at depths of 8–10 km before beingexhumed at the surface.</p>

Evolution of Quartz and Calcite Microstructures Exhumed From Deep Brittle-Ductile Shear Zones in the Southern Alps of New Zealand

<p><b>Arrays of brittle-ductile shears exposed in the Southern Alps of New Zealand, haveprovided a superb natural laboratory for insight into the microstructural evolution of lowercrustal shear zones during exhumation. Shears are exposed in the central section of theSouthern Alps at Sam Peak, Chancellor Ridge, and Baumann Glacier in a zone ~2 kmwide that is located 6–8 km structurally above the Alpine Fault. An array ofsystematically spaced shear zones that formed by embrittlement and faulting ofquartzofeldspathic schist took place at the same time as ductile shearing of quartzcarbonateveins embedded within the schist. This study has used field-based structuralmapping along with optical microscopy and universal stage measurements ofcrystallographic preferred orientations (CPO) to resolve the shear zone kinematics andrheology. On the basis of these data, the strain path can be reconstructed for the shearedveins during their progressive deformation. This began with their incidence as backshearsat the base of the Alpine Fault ramp and ended with their subsequent recrystallisation,uplift, and exhumation.</b></p> <p>The near-vertical shear planes have mean orientation of 221@89 NW ± 1o (n =780). They are inferred to have formed as backshears accommodating uplift of the PacificPlate as it was translated onto the oblique footwall ramp of the Alpine Fault during lateCenozoic oblique convergence. Detailed fault offset transect surveys across the shears atChancellor Ridge and Baumann Glacier reveal a mean spacing between the shear zones of25 ± 5 cm (n = 410). Quartz-carbonate marker veins are displaced in a dextral west-sideupshear sense. Fault offset geometry and a consistent arrangement of mineral fibrelineations that decorate fault surfaces, indicate that the mean displacement vector pitches35o SW in the shear plane (trend and plunge of: 262, 35 ± 7o). Ductilely deformed markerveins have been subject to a mean displacement of 9.9 ± 1.4 cm (n = 344) and a meanfinite ductile shear strain of 4.8 ± 0.3 (n = 219). A strain-rate for the ductile deformationof the veins is estimated at 3 x 10-11 sec-1 based on the observed finite ductile shear strain,an escalator kinematic model, and assumptions about the width of the deforming zone.</p> <p>Five deformation phases have affected the sheared veins during their transport upthe fault ramp: 1) initial brittle faulting and ductile shearing; 2) grain boundary sliding ofmylonitic quartz in response to a post-ramping differential stress drop; 3) recrystallisationand grain growth; 4) renewed late-stage dislocation creep; and 5) semibrittle deformationand exhumation. In the schist, the shears initiated as planar brittle faults at lower crustal depths of~21 km at a temperature of 450 ± 50oC. They developed in a zone of transiently highshear strain-rates near the base of the Alpine Fault ramp. Dislocation creep caused a CPOof quartz and calcite to develop in sheared veins. Using the flow law of Hirth et al. (2001)and the estimated strain-rate, a differential stress of ~165 MPa is inferred for ductiledeformation of the veins. Near-lithostatic (λ = 0.85) fluid pressures would have causedthe rocks to undergo brittle failure, a situation that is confirmed by a late component ofbrittle deformation that over prints the ductilely sheared veins. Syntectonic quartz-calciteveins infill the shear fractures, and these themselves have been sheared. The deformationof the veins was not a simple shear process but one with triclinic flow symmetry. This isinferred from discordance between the shear direction and the near-vertical principleextension direction that is revealed by the pattern and symmetry of quartz and calcite CPOfabrics.</p> <p>After the shears move away from the ramp-step, grain boundary sliding (GBS)accommodated by solid-state diffusion creep is inferred to have affected quartz veins.</p> <p>This deformation mechanism takes place because of 1) the small 8 μm grain size inheritedfrom Phase 1; 2) the presence of fluid in the shear zone; and 3) a stress drop to ~22 MPathat followed the initial up-ramping. Quartz CPO fabrics in the sheared veins areremarkably weak considering their large shear strains. GBS is inferred to have been achief deformation mechanism that caused the weakening of quartz CPO fabrics in thehighly sheared sections of deformed veins. Calcite has also affected the quartz fabricstrength as those veins containing >5% calcite have very weak quartz CPO fabrics. Incontrast to quartz, the CPO fabrics for the co-existing calcite remained strong andcontinued to develop by dislocation creep.</p> <p>The third phase of deformation, a process that may have contributed to subsequentweakening of quartz CPO fabrics, was recrystallisation and grain growth to 126 μm and anequigranular-polygonal grain shape fabric. This fabric was overprinted by late-stagedislocation creep microstructures in the fourth deformation phase in response increaseddifferential stress encountered by the rocks at lower temperatures in the upper crust. Thefinal phase of deformation to affect the sheared veins was semibrittle deformation atdifferential stresses of <189 MPa and temperatures of 200–280oC as the rocks passedthrough the steady-state brittle-ductile transition zone at depths of 8–10 km before beingexhumed at the surface.</p>

Crustal structure of the East African Limpopo margin, a strike-slip rifted corridor along the continental Mozambique Coastal Plain and North Natal Valley

Coincident wide-angle and multi-channel seismic data acquired within the scope of the PAMELA Moz3-5 project allow us to reconsider the formation mechanism of East African margins offshore of southern Mozambique. This study specifically focuses on the sedimentary and deep-crustal architecture of the Limpopo margin (LM) that fringes the eastern edge of the Mozambique’s Coastal Plain (MCP) and its offshore southern prolongation the North Natal Valley (NNV). It relies primarily on the MZ3 profile that runs obliquely from the northeastern NNV towards the Mozambique basin (MB) with additional inputs from a tectonostratigraphy analysis of industrial onshore–offshore seismic lines and nearby or crossing velocity models from companion studies. Over its entire N–S extension the LM appears segmented into (1) a western domain that shows the progressive eastward crustal thinning and termination of the MCP/NNV continental crust and its overlying pre-Neocomian volcano-sedimentary basement and (2) a central corridor of anomalous crust bounded to the east by the Mozambique fracture zone (MFZ) and the oceanic crust of the MB. A prominent basement high marks the boundary between these two domains. Its development was most probably controlled by a steep and deeply rooted fault, i.e., the Limpopo fault. We infer that strike-slip or slightly transtensional rifting occurred along the LM and was accommodated along this Limpopo fault. At depth we propose that ductile shearing was responsible for the thinning of the continental crust and an oceanward flow of lower crustal material. This process was accompanied by intense magmatism that extruded to form the volcanic basement and gave the corridor its peculiar structure and mixed nature. The whole region remained at a relative high level during the rifting period and a shallow marine environment dominated the pre-Neocomian period during the early phase of continent–ocean interaction. It is only some time after break-up in the MB and the initiation of the MFZ that decoupling occurred between the MCP/NNV and the corridor, allowing for the latter to subside and become covered by deep marine sediments. A scenario for the early evolution and formation of the LM is proposed taking into account both recent kinematic and geological constraints. It implies that no or little change in extensional direction occurred between the intra-continental rifting and subsequent phase of continent–ocean interaction.

Structural, petrological, and tectonic constraints on the Loch Borralan and Loch Ailsh alkaline intrusions, Moine thrust zone, northwestern Scotland

During the Caledonian orogeny, the Moine thrust zone in northwestern Scotland (UK) emplaced Neoproterozoic Moine Supergroup rocks, metamorphosed during the Ordovician (Grampian) and Silurian (Scandian) orogenic periods, westward over the Laurentian passive margin in the northern highlands of Scotland. The Laurentian margin comprises Archean–Paleoproterozoic granulite and amphibolite facies basement (Scourian and Laxfordian complexes, Lewisian gneiss), Proterozoic sedimentary rocks (Stoer and Torridon Groups), and Cambrian–Ordovician passive-margin sediments. Four major thrusts, the Moine, Ben More, Glencoul, and Sole thrusts, are well exposed in the Assynt window. Two highly alkaline syenite intrusions crop out within the Moine thrust zone in the southern Assynt window. The Loch Ailsh and Loch Borralan intrusions range from ultramafic melanite-biotite pyroxenite and pseudoleucite-bearing biotite nepheline syenite (borolanite) to alkali-feldspar–bearing and quartz-bearing syenites. Within the thrust zone, syenites intrude up to the Ordovician Durness Group limestones and dolomites, forming a high-temperature contact metamorphic aureole with diopside-forsterite-phlogopite-brucite marbles exposed at Ledbeg quarry. Controversy remains as to whether the Loch Ailsh and Loch Borralan syenites were intruded prior to thrusting or intruded syn- or post-thrusting. Borolanites contain large white leucite crystals pseudomorphed by alkali feldspar, muscovite, and nepheline (pseudoleucite) that have been flattened and elongated during ductile shearing. The minerals pseudomorphing leucites show signs of ductile deformation indicating that high-temperature (~500 °C) deformation acted upon pseudomorphed leucite crystals that had previously undergone subsolidus breakdown. New detailed field mapping and structural and petrological observations are used to constrain the geological evolution of both the Loch Ailsh and the Loch Borralan intrusions and the chronology of the Moine thrust zone. The data supports the interpretation that both syenite bodies were intruded immediately prior to thrusting along the Moine, Ben More, and Borralan thrusts.

40Ar/39Ar Dating of Paleoproterozoic Shear Zones in the Ellesmere-Devon Crystalline Terrane, Nunavut, Canadian Arctic

Paleoproterozoic gneisses of the Ellesmere–Devon crystalline terrane on southeast Ellesmere Island are deformed by m-scale, E-striking mylonite zones. The shear zones commonly offset pegmatitic dikes and represent the last episode of ductile deformation. Samples were dated by the ⁴⁰Ar/³⁹Ar step-heating method to put an upper limit on the time of deformation. Biotite from one tonalitic protolith and five shear zones give geologically meaningful results. Clusters of unoriented biotite grains pseudomorph granulite-facies orthopyroxene in some of the weakly deformed gneisses, whereas the shape preferred orientation of biotite defines the mylonitic fabric. The intrusive age of the tonalitic protolith is 1958 ± 12 Ma, based on previous U-Pb dating of zircon. 40Ar/39Ar analysis of biotite from the same sample gave a plateau age of 1929 ± 23 Ma, which is interpreted as cooling from regional granulite facies metamorphism. Three nearby samples of mylonitic tonalite have ⁴⁰Ar/³⁹Ar ages that range from ≈1870–1840 Ma. Biotite from two granitic mylonites over 80 km away return high-resolution Ar spectra in the same range, implying that widespread ductile shearing occurred between ≈1870–1840 Ma, or ≈90 m.y. after cooling from regional metamorphism. Although the 2.0–1.9 Ga gneisses of southeast Ellesmere Island correlate with the Inglefield Mobile Belt in North-West Greenland and the Thelon Tectonic Zone, the late shear zones are superimposed on that juvenile arc long after the 1.97 Ga Thelon orogeny.

SHEAR PERFORMANCE OF SPECIAL DRY JOINTS FOR PRECAST CONCRETE SEGMENTS

The special dry joints for precast prestressed concrete segments are invented in this study toovercome the limitation of conventional dry joints. Eight specimens of special dry joints were madeand subjected to direct shear test. Test parameters comprise concrete compressive strength (normaland high strength concrete) and steel fiber volume added in the special dry joint (0%, 0.5%, and1.0%). Test results revealed that the inclusion of steel fibers remarkably enhanced the shear capacityand ductility index. Failure mode of specimens was changed from shearing off to concrete crackingaround shear key corners, defined as ductile shearing-off failure. Furthermore, the existing equationsfor predicting shear capacity of keyed joints were validated by the experimental results. Amongavailable equations from literatures, the Turmo’s equation yields better prediction of the shearcapacity for the special dry joint made with normal strength concrete.

Intense deformation of the caprock on salt extrusions in the Iranian Zagros Mountains – Insights from geological mapping and analogue modeling

&lt;p&gt;The Zagros fold-and-thrust belt in Southern Iran is famous for its spectacular outcrops of salt diapirs. Most of these diapirs already existed prior to the onset of the Zagros orogeny, but tectonic shortening caused their reactivation and extrusion of the salt. Thus, the diapir exposures often provide access to intense internal deformation of the Hormuz salt series and its entrained interlayers. However, highly soluble evaporites (mainly halite) were already dissolved in many of the exposures leaving behind degraded &amp;#8216;caprock&amp;#8217;, which is built of a multi-compositional residuum of less soluble minerals and rocks. Based on geological field studies on two iconic salt diapirs in Southern Iran, the Karmostaj (Gach) and the Siah Taq diapir, we ascertained that the caprock is also intensively deformed. The accessible part of the caprock is roughly 200 m thick and consists of a fine-grained, laminated gypsum containing fragments of brecciated carbonates and siliciclastics.&amp;#160; Especially in the down- and mid-slope regions of the salt exposure, this mixture is sheared and folded, but also dissected by thrust faults. Since such deformation processes in the caprock were not described before, there is a lack in explanations for the timing, the depth of formation and the structural evolution of these structures. For instance, it is unclear if the ductile shearing of the relatively competent gypsum matrix and the brecciation of the clasts took place near the surface or in larger depths (a few hundreds of meters), where confining pressure is higher.&lt;/p&gt;&lt;p&gt;In this study, we want to classify the observed structures in the caprock, characterize deformation mechanisms and differentiate typical deformation domains. Based on that, we speculate about the timing and structural evolution of the caprock deformation and suggest that three scenarios can be imagined: (1) Pre-extrusion deformation: The caprock exposed today was buried by a thicker caprock package and, therefore, is compacted and mechanically strong. &amp;#160;With the onset of the Zagros orogeny, tectonic shortening of the buried diapir caused lateral deformation before the salt extrusion. (2) Syn-extrusion deformation: The caprock is relatively young and was mechanically weak after its formation. Thus, it was deformed during diapir extrusion and, then, solidified during degradation of the salt. (3) Post-extrusion deformation: The caprock was mainly formed after salt extrusion, but it remained relatively immobile. The caprock matrix is occasionally weakened by the infiltration of meteoric water, and continued to be deformed due to gravitational gliding even after the dissolution of the rock salt. &amp;#160;In order to test these hypotheses, we intend to carry out analogue experiments in which we try to model a squeezed diapir. In a parameter study, the thickness and the material of the covering layer simulating the caprock will be varied to assess possible differences in the deformation patterns.&lt;/p&gt;