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>

Metamorphism and the P-T History of Alpine Schist from the Newton Range, Southern Alps, New Zealand

<p>Metamorphic rocks have the potential to record in their mineral assemblages, mineral compositional zoning, and textures, information about geological changes and processes that occur during tectonic events. Interpretations of metamorphic pressure-temperature (P-T) records have traditionally relied on results of geothermobarometry studies, but that approach is not suitable in every case. Metamorphosed greywacke, which makes up ~95% of the New Zealand Southern Alps, has long proven problematic for traditional geothermobarometry because it develops intractable mineral compositions and/or assemblages, especially at relatively low temperature (greenschist facies) conditions. An alternative forward modelling approach using the computer program THERMOCALC was recently used to extract the first detailed P-T history (P-T path) from such previously intractably difficult "greyschist" rocks from a single site in the New Zealand Southern Alps. The present study is the first attempt to apply those new methods to rocks from another study area, and is the first detailed geological study of the Newton Range in the New Zealand Southern Alps. The Newton Range is a ~15 km-long, east-west trending range located ~30 km southeast of the town of Hokitika, ~110 km northeast of the Franz Josef-Fox Glacier region, and immediately to the east of the Alpine Fault in the Southern Alps, South Island, New Zealand. The rocks in the Newton Range are mainly derived from Torlesse Terrane accretionary prism greywacke and argillite (Alpine Schist, greyschist), together with a large pods of ultramafic rock (part of the Pounamu Ultramafic Belt (PUB)) and minor associated metabasic layers (greenschist), all metamorphosed to greenschist facies conditions. The dominant mineral assemblage in the greyschist (Qtz + Ms+ Bt ± Chl ± Ep ± Pl ± Ilm ± Ttn ± Grt ± Zrn ± Tur ± Ap ± Cal), much like that found elsewhere in the Southern Alps. As elsewhere in the Southern Alps, the dominant high-grade metamorphic mineral assemblages in the Alpine Schist in the Newton Range are inherited. The mineral assemblages, compositions, and some textures thus record evidence of processes that took place during tectonic events, presumably mainly in Cretaceous time, prior to the formation of the modern Southern Alps, which are forming today by the ongoing oblique continent-continent collision of the Pacific Plate against the Australian Plate at the Alpine Fault. Compositional zoning in garnet from the greyschist is an important record of the metamorphic P-T path traversed by the host rock as the garnet grew. Occasionally, garnet from the study area contains an inmost core (stage 0) of unusual (anomalously high- or low-MnO) composition. The cores with extremely low MnO are possibly detrital in origin, and those with extremely high MnO may perhaps have grown in the early tectonic episode that formed the Otago Schist. Typically, garnet shows the following core- to rim zoning sequence. Stages 1 & 2 show a progressive decrease in MnO and increase in FeO from core to rim, with higher MnO cores present in rocks with higher whole-rock MnO compositions. Stage 3 is characterised by a gradual decrease in CaO and signifies the growth of Ca-bearing oligoclase late in the garnet growth history. Stage 4 is a discontinuous overgrowth characterised by an abrupt increase in CaO. Such overgrowths have in the past been attributed to garnet growth accompanying the development of the Alpine Fault mylonite zone in the late Cenozoic. In the Newton Range they were only observed on garnet adjacent to the main outcrop of the PUB at ~4.5km from the Alpine Fault, far from the mylonite zone, so local element availability during decompression (and possibly fluid flow and/or metasomatism) may have played a part in the growth of these rims. A P-T path for Alpine Schist from the Newton Range has been estimated using detailed garnet composition data measured along core-to-rim transects across individual garnets, together with predicted garnet compositions and P-T pseudosection results calculated using THERMOCALC. The P-T path starts at ~3.5kbar/400°C, where both garnet and albite coexist, and increases in pressure and temperature to ~6.5bar/500°C where garnet coexists with both albite and oligoclase. The estimated peak metamorphic conditions probably correspond to peak metamorphic pressures, unlike in the Franz Josef-Fox Glacier region where peak conditions (~9.2kbar and 620°C) probably coincided with peak metamorphic temperatures.</p>

Metamorphism and the P-T History of Alpine Schist from the Newton Range, Southern Alps, New Zealand

<p>Metamorphic rocks have the potential to record in their mineral assemblages, mineral compositional zoning, and textures, information about geological changes and processes that occur during tectonic events. Interpretations of metamorphic pressure-temperature (P-T) records have traditionally relied on results of geothermobarometry studies, but that approach is not suitable in every case. Metamorphosed greywacke, which makes up ~95% of the New Zealand Southern Alps, has long proven problematic for traditional geothermobarometry because it develops intractable mineral compositions and/or assemblages, especially at relatively low temperature (greenschist facies) conditions. An alternative forward modelling approach using the computer program THERMOCALC was recently used to extract the first detailed P-T history (P-T path) from such previously intractably difficult "greyschist" rocks from a single site in the New Zealand Southern Alps. The present study is the first attempt to apply those new methods to rocks from another study area, and is the first detailed geological study of the Newton Range in the New Zealand Southern Alps. The Newton Range is a ~15 km-long, east-west trending range located ~30 km southeast of the town of Hokitika, ~110 km northeast of the Franz Josef-Fox Glacier region, and immediately to the east of the Alpine Fault in the Southern Alps, South Island, New Zealand. The rocks in the Newton Range are mainly derived from Torlesse Terrane accretionary prism greywacke and argillite (Alpine Schist, greyschist), together with a large pods of ultramafic rock (part of the Pounamu Ultramafic Belt (PUB)) and minor associated metabasic layers (greenschist), all metamorphosed to greenschist facies conditions. The dominant mineral assemblage in the greyschist (Qtz + Ms+ Bt ± Chl ± Ep ± Pl ± Ilm ± Ttn ± Grt ± Zrn ± Tur ± Ap ± Cal), much like that found elsewhere in the Southern Alps. As elsewhere in the Southern Alps, the dominant high-grade metamorphic mineral assemblages in the Alpine Schist in the Newton Range are inherited. The mineral assemblages, compositions, and some textures thus record evidence of processes that took place during tectonic events, presumably mainly in Cretaceous time, prior to the formation of the modern Southern Alps, which are forming today by the ongoing oblique continent-continent collision of the Pacific Plate against the Australian Plate at the Alpine Fault. Compositional zoning in garnet from the greyschist is an important record of the metamorphic P-T path traversed by the host rock as the garnet grew. Occasionally, garnet from the study area contains an inmost core (stage 0) of unusual (anomalously high- or low-MnO) composition. The cores with extremely low MnO are possibly detrital in origin, and those with extremely high MnO may perhaps have grown in the early tectonic episode that formed the Otago Schist. Typically, garnet shows the following core- to rim zoning sequence. Stages 1 & 2 show a progressive decrease in MnO and increase in FeO from core to rim, with higher MnO cores present in rocks with higher whole-rock MnO compositions. Stage 3 is characterised by a gradual decrease in CaO and signifies the growth of Ca-bearing oligoclase late in the garnet growth history. Stage 4 is a discontinuous overgrowth characterised by an abrupt increase in CaO. Such overgrowths have in the past been attributed to garnet growth accompanying the development of the Alpine Fault mylonite zone in the late Cenozoic. In the Newton Range they were only observed on garnet adjacent to the main outcrop of the PUB at ~4.5km from the Alpine Fault, far from the mylonite zone, so local element availability during decompression (and possibly fluid flow and/or metasomatism) may have played a part in the growth of these rims. A P-T path for Alpine Schist from the Newton Range has been estimated using detailed garnet composition data measured along core-to-rim transects across individual garnets, together with predicted garnet compositions and P-T pseudosection results calculated using THERMOCALC. The P-T path starts at ~3.5kbar/400°C, where both garnet and albite coexist, and increases in pressure and temperature to ~6.5bar/500°C where garnet coexists with both albite and oligoclase. The estimated peak metamorphic conditions probably correspond to peak metamorphic pressures, unlike in the Franz Josef-Fox Glacier region where peak conditions (~9.2kbar and 620°C) probably coincided with peak metamorphic temperatures.</p>

Sources and Effects of Fluids in Continental Retrograde Shear Zones: Insights from the Kuckaus Mylonite Zone, Namibia

Midcrustal rocks in retrograde metamorphic settings are typically H2O-undersaturated and fluid-absent and have low permeability. Exhumed continental retrograde faults, nonetheless, show evidence for the operation of fluid-mediated weakening mechanisms during deformation at midcrustal conditions. To explore the origin and effects of fluids in retrograde faults, we study the Kuckaus Mylonite Zone (KMZ), an exhumed crustal-scale, strike-slip shear zone in the southern Namibian Namaqua Metamorphic Complex. The KMZ deformed quartzofeldspathic migmatised gneisses at midcrustal retrograde conditions (450-480°C, 270-420 MPa) in the Mesoproterozoic, 40 Ma after granulite facies peak metamorphism at 825°C and 550 MPa. The mylonites contain fully hydrated retrograde mineral assemblages, predominantly adjacent to anastomosing high-strain zones, providing evidence of local H2O saturation and fluid presence during deformation. Whole rock and quartz vein δ18O values suggest that at least some of the fluids were meteoric in origin. The rocks across the shear zone retain the effect of different protoliths, implying little effect of fluid-rock interaction on whole rock major element chemistry. Together with a general scarcity of quartz veins, this suggests that fluid/rock ratios remained low in the KMZ. However, even small amounts of H2O allowed reaction weakening and diffusion-precipitation, followed by growth and alignment of phyllosilicates. In the ultramylonites, a fine grain size in the presence of fluids allowed for grain size sensitive creep. We conclude that the influx of even small volumes of fluids into retrograde shear zones can induce drastic weakening by facilitating grain size sensitive creep and retrograde reactions. In retrograde settings, these reactions consume fluids, and therefore elevated fluid pressures will only be possible after considerable weakening has already occurred. Our findings imply that the range of seismic styles recently documented at active retrograde transform faults may not require high fluid pressures but could also arise from other local weakening mechanisms.

Processes, properties, and microstructures in faults active at retrograde conditions

&lt;p&gt;Faults that are active at retrograde conditions tend to contain metastable fault rock assemblages that are prone to undergo fluid-consuming reactions. These reactions typically lead to growth of minerals that are viscously and frictionally weaker than the reactants. This is illustrated in the well-studied Outer Hebrides Fault Zone (OHFZ) of Scotland, and we add observations from the Kuckaus Mylonite Zone (KMZ), Namibia. In both locations, deformation is localised in anastomosing networks of phyllosilicates that developed during deformation of amphibolite and/or granulite assemblages at greenschist facies conditions. Microstructures of these phyllonites show generally well aligned phyllosilicates wrapping around fractured feldspars and quartz with features indicating dislocation creep.&lt;/p&gt;&lt;p&gt;In the KMZ, further localization occurred in ultramylonites within the mylonite zone. These are characterised by a similar phyllosilicate proportion to surrounding mylonites, but lack interconnected phyllosilicate networks. Instead, they contain a very fine-grained assemblage of quartz, feldspar, and phyllosilicate, where both quartz and feldspar lack a CPO. We interpret this assemblage as having deformed through grain-size sensitive creep, at lower shear stress than the surrounding mylonite. It is possible that the ultramylonites developed by dismembering an earlier interconnected weak phase microstructure with increasing finite strain, as has been suggested experimentally by Cross and Skemer (2017).&lt;/p&gt;&lt;p&gt;Whereas these exhumed fault zones deformed at greenschist facies conditions, continued activity would exhume similar fault rocks to shallower depth. We explored frictional properties and microstructure of greenschist facies fault rock at low temperature conditions by deforming chlorite-amphibole-epidote assemblages in single-direct shear at room temperature and 10 MPa normal stress under fluid saturated conditions. As inferred at greater depth, presence of chlorite weakens and promotes aseismic creep along these experimental faults. Presence of chlorite also correlates with the development of striations on fault surfaces. Lack of chlorite, on the other hand, leads to velocity-weakening behaviour and, in epidotite, a fault surface containing very fine grains that do not develop when &amp;#8805; 50 % chlorite is present. We suggest that chlorite supresses wear at contact asperities between stronger minerals, and therefore also supresses velocity-weakening behaviour.&lt;/p&gt;&lt;p&gt;Overall, we see that growth of retrograde phyllosilicates lead to profound weakening, strain localisation, and frictional stabilisation of major shear zones, from greenschist facies to near-surface conditions. These processes and properties are, however, reliant on external fluids to allow hydration reactions in otherwise relatively dry host rocks. From scattered syn-deformational quartz veins, in the KMZ, such fluids appear to be of surface origin, whereas in the OHFZ, fluids were likely of a deeper, metamorphic or magmatic origin. Ready incorporation of such fluids into retrograde minerals would prevent substantial or widespread fluid overpressures from developing. These fluid sources are similar to present-day inferred fluid regimes in the Alpine and San Andreas Faults, respectively. We speculate that the variable slip behaviour seen on active retrograde faults relate to their degree of retrogression, and the development of time and strain-dependent microstructures with specific strengths and behaviours.&lt;/p&gt;

Left-lateral shearing before coeval exhumation between the high pressure Yuli belt and the underlying Mesozoic Tailuko belt? Insights from the Shoufeng fault in eastern Taiwan

&lt;p&gt;Based on field investigations, microscopic observations, and available geophysical, geochemical and geochronological data, this study intends to better understand the structural characteristics of the Eurasian continental margin (e.g., the eastern Central Range in Taiwan) during subduction and exhumation while the Philippine Sea plate has been approaching in the vicinity of Taiwan since the Miocene time.&amp;#160; The eastern Central Range is composed of two major geological units: 1) the Tailuko belt, the Mesozoic metamorphic subduction complex, retro-metamorphosed in green schist facies and exhumed since late Miocene, and 2) the Yuli belt, continental margin rocks that contain high-pressure minerals (omphacite, glaucophane, garnet) with Miocene-Pliocene ages suggesting rapid exhumation from mantle depths of 40-50 km.&lt;/p&gt;&lt;p&gt;We conducted detailed field surveys around the Shoufeng fault which represents the boundary between the Tailuko belt and the Yuli belt. We found a mylonite zone of several kilometers wide in the boundary of these two belts. Based on the meso- and microscopic scale observations we define the boundary as ultra-mylonite, mylonite, and proto-mylonite zones. Within the ultra-mylonite and mylonite zones, rocks from two belts are intercalated each other in varied widths. The main dominant schistosity/cleavage in the mylonite zones (Sm/S3) remains the same orientation of striking in NE/NNE and dipping to the west. Also, the main composition layers, which we tentatively called S2 for the sake of field investigations, were more intensively deformed (i.e., crenulated, folded, etc.) from outside toward the core of the mylonite zones. As a result, the Sm/S3 becomes less persistent outside of the mylonite zones in the Yuli belt.&lt;/p&gt;&lt;p&gt;The mylonite zones exhibit left-lateral Sm/S3-related shearing without significant down-dip component. We also observed a general S2/S3-related top-to-west sense of shear across the two belts. As a consequence, we tend to interpret that the Yuli belt and the Tailuko belt have been mylonitically sheared (Sm/S3) in a left-lateral movement at the depth and that they exhumed coevally up to the surface level. The schistosity of the main composition layers S2 probably occurred before the mylonization during the transition from subduction to exhumation. The shallow dipping and less dominant S3 outside the mylonite zone might imply an upward unroofing process during the rapid exhumation of the eastern Central Range of Taiwan.&lt;/p&gt;

Late Ottawan orogenic collapse of the Adirondacks in the Grenville province of New York State (USA): Integrated petrologic, geochronologic, and structural analysis of the Diana Complex in the southern Carthage-Colton mylonite zone

Crustal-scale shear zones can be highly important but complicated orogenic structures, therefore they must be studied in detail along their entire length. The Carthage-Colton mylonite zone (CCMZ) is one such shear zone in the northwestern Adirondacks of northern New York State (USA), part of the Mesoproterozoic Grenville province. The southern CCMZ is contained within the Diana Complex, and geochemistry and U-Pb zircon geochronology demonstrate that the Diana Complex is expansive and collectively crystallized at 1164.3 ± 6.2 Ma. Major ductile structures within the CCMZ and Diana Complex include a northwest-dipping penetrative regional mylonitic foliation with north-trending lineation that bisects a conjugate set of mesoscale ductile shear zones. These ductile structures formed from the same 1060–1050 Ma pure shear transitioning to a top-to-the-SSE shearing event at ∼700 °C. Other important structures include a ductile fault and breccia zones. The ductile fault formed immediately following the major ductile structures, while the breccia zones may have formed at ca. 945 Ma in greenschist facies conditions. Two models can explain the studied structures and other regional observations. Model 1 postulates that the CCMZ is an Ottawan orogeny (1090–1035 Ma) thrust, which was later reactivated locally as a tectonic collapse structure. Model 2, the preferred model, postulates that the CCMZ initially formed as a subhorizontal mid-crustal mylonite zone during collapse of the Ottawan orogen. With continued collapse, a metamorphic core complex formed and the CCMZ was rotated into is current orientation and overprinted with other structures.