In the Karnali valley of west Nepal, detailed mapping, thermobarometry, quartz-petrofabrics, vorticity analysis, and thermochronology delineate three tectonometamorphic domains separated by structural and metamorphic discontinuities. The lowest domain, the Lesser Himalayan sequence, is weakly metamorphosed and preserves evidence of primary sedimentary features and a polydeformational history. The Greater Himalayan sequence (GHS) is pervasively sheared and metamorphosed and overlies the Lesser Himalayan sequence along the Main Central thrust. The Greater Himalayan sequence is sub-divided into two tectonometamorphic domains that display contrasting metamorphic histories. The lower portion of the Greater Himalayan sequence contains garnet- to kyanite-grade rocks whose peak metamorphic assemblages developed during top-to-the-south directed shear and a metamorphic pressure gradient that increases up structural section. The upper portion of the Greater Himalayan sequence contains kyanite and sillimanite-grade migmatites that preserve polymetamorphic assemblages and a metamorphic pressure gradient that decreases up structural section. The upper and lower portions of the Greater Himalayan sequence are separated by a metamorphic discontinuity that roughly coincides with the bottom of the lowest migmatite unit. Vorticity estimates indicate roughly equal contributions of pure and simple shear during deformation of the upper and lower portions of the GHS. Quartz petrofabrics suggest deformation temperatures are equivalent to peak metamorphic temperatures in the lower Greater Himalayan sequence. These observations are consistent with channel flow tectonic models whereby the upper portion of the Greater Himalayan sequence is ductily extruded to the south while ductily accreting the subjacent lower portion of the Greater Himalayan sequence across a metamorphic discontinuity. 40Ar/39Ar thermochronology indicates Miocene homogeneous cooling of the Greater Himalayan sequence. Cooling rates of the GHS and the homogeneous cooling profile suggest east-west extensional exhumation followed peak-metamorphism and south-directed shearing and supports the hypothesis of the southeast propagation of the Gurla-Mandhata-Humla fault system into the Karnali valley. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2010-09-20 09:23:07.103

Field mapping and, structural, microstructural, and chronological analyses confirm the existence of a segment of the Gurla-Mandhata-Humla fault, an orogen-parallel strike-slip dominated shear zone in the upper Karnali valley of northwestern Nepal. This shear zone forms the upper contact of, and cuts obliquely across the Greater Himalayan Sequence (GHS). Data from this study reveal two phases of GHS deformation. Phase 1 is characterized by U-Th-Pb monazite crystallization ages (~26–12 Ma, peak ~18–15 Ma), consistent with typical Neohimalayan metamorphic ages, and the final stages of south-directed extrusion of the GHS. Phase 2 is characterized by south-dipping high-strain foliations and intensely developed ESE-WNW trending, shallowly plunging mineral elongation lineations, indicating orogen-parallel extension. Thermochronology of muscovite defining these fabrics implies that the area was cooling and experiencing orogen-parallel extension by ~15–9 Ma. Mineral deformation mechanisms and quartz c-axis patterns of these fabrics record a rapid increase in temperature from ~350°C along the shear zone, to ~650°C at ~2.5 structural km below the shear zone. Such temperature gradients may be remnants of telescoped and/or flattened isotherms generated during south-directed extrusion of the GHS. Overprinting ESE-WNW fabrics record progressive deformation of the GHS at lower temperatures. Progressive deformation included a significant component of pure shear, as indicated by symmetric high-temperature quartz c-axis fabrics and a lower-temperature vorticity estimate (~59% pure shear). A transition in c-axis fabrics from type I to type II cross-girdles at ~ 1.2 km below the fault could indicate a transition from plane strain towards constriction. Together, these data suggest orogen-parallel extension was occurring as a result of transtension. This study reveals a transition from south-directed extrusion of the GHS to orogen-parallel extension between ~15–13 Ma. Comparing these data with tectonic events across the Himalaya reveals an orogen-wide middle Miocene transition, coeval with the uplift of eastern Tibet. This is consistent with interpretations invoking radial spreading of Tibet and east-directed lower-crustal flow to explain orogen-parallel extension. Our study leads to the suggestion that a transition affecting mid- to lower-crustal processes may be responsible for the cessation of south-directed extrusion of the GHS and onset of east-directed lower-crustal flow. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2012-09-23 02:16:09.326

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This thesis investigates normal faulting and its influence on fluid flow over a wide range of spatial and temporal scales using tunnel engineering geological logs, outcrop, surface fault traces, earthquakes, gravity, and volcanic ages. These data have been used to investigate the impact of faults on fluid flow (chapter 2), the geometry and kinematics of the Taupo Rift (chapter 3), the hydration and dehydration of the subducting Pacific plate and its influence on the Taupo Volcanic Zone (chapter 4), the migration of arc volcanism across the North Island over the 16 Myr and the associated changes in slab geometry (chapter 5) and the Pacific-Australia relative plate motion vectors since 38 Ma and their implications for arc volcanism and deformation along the Hikurangi margin (chapter 6). The results for each of these five chapters are presented in the five paragraphs below. Tunnels excavated along the margins of the southern Taupo Rift at depths < 500 m provide data on the spatial relationships between faulting and ground water flow. The geometry and hydraulic properties of fault-zones for Mesozoic basement and Miocene strata vary by several orders of magnitude approximating power-law distributions with the dimensions of these zones dependent on many factors including displacement, hostrock type and fault geometries. Despite fault-zones accounting for a small proportion of the total sample length (≤ 15%), localised flow of ground water into the tunnels occurs almost exclusively (≥ 91%) within, and immediately adjacent to, these zones. The spatial distribution and rate of flow from fault-zones are highly variable with typically ≤ 50% of fault-zones in any given orientation flowing. The entire basement dataset shows that 81% of the flow-rate occurs from fault-zones ≥ 10 m wide, with a third of the total flow-rate originating from a single fault-zone (i.e. the golden fracture). The higher flow rates for the largest faults are interpreted to arise because these structures are the most connected to other faults and to the ground surface. The structural geometry and kinematics of rifting is constrained by earthquake focal mechanisms and by geological slip and fault mapping. Comparison of present day geometry and kinematics of normal faulting in the Taupo Rift (α=76-84°) with intra-arc rifting in the Taranaki Basin and southern Havre Trough show, that for at least the last 4 Myr, the slab and the associated changes in its geometry have exerted a first-order control on the location, geometry, and extension direction of intra-arc rifting in the North Island. Second-order features of rifting in the central North Island include a clockwise ~20° northwards change in the strike of normal faults and trend of the extension direction. In the southern rift normal faults are parallel to, and potentially reactivate, Mesozoic basement fabric (e.g., faults and bedding). By contrast, in the northern rift faults diverge from basement fabric by up to 55° where focal mechanisms indicate that extension is achieved by oblique to right-lateral strike-slip along basement fabric and dip-slip on rift faults. Hydration and dehydration of the subducting Pacific plate is elucidated by earthquake densities and focal mechanisms within the slab. The hydration of the subducting plate varies spatially and is an important determinant for the location of arc volcanism in the overriding plate. The location and high volcanic productivity of the TVZ can be linked to the subduction water cycle, where hydration and subsequent dehydration of the subducting oceanic lithosphere is primarily accomplished by normal-faulting earthquakes. The anomalously high heat flow and volcanic productivity of the TVZ is spatially associated with high rates of seismicity in the underlying slab mantle at depths of 130-210 km which can be tracked back to high rates of deeply penetrating shallow intraplate seismicity at the trench in proximity to oceanic fluids. Dehydration of the slab mantle correlates with the location and productivity of active North Island volcanic centres, indicating this volcanism is controlled by fluids fluxing from the subducting plate. The ages and locations of arc volcanoes provide constraints on the migration of volcanism across the North Island over the last 20 Myr. Arc-front volcanoes have migrated southeast by 150 km in the last 8 Ma (185 km since 16 Ma) sub-parallel to the present active arc. Migration of the arc is interpreted to mainly reflect slab steepening and rollback. The strike of the Pacific plate beneath the North Island, imaged by Benioff zone seismicity (50-200 km) and positive mantle velocity anomalies (200-600 km) is parallel to the northeast trend of arc-front volcanism. Arc parallelism since 16 Ma is consistent with the view that the subducting plate beneath the North Island has not rotated clockwise about vertical axes which is in contrast to overriding plate vertical-axis rotations of ≥ 30º. Acceleration of arc-front migration rates (~4 mm/yr to ~18 mm/yr), eruption of high Mg# andesites, increasing eruption frequency and size, and uplift of the over-riding plate indicate an increase in the hydration, temperature, and size of the mantle wedge beneath the central North Island from ~7 Ma. Seafloor spreading data in conjunction with GPlates have been used to generate relative plate motion vectors across the Hikurangi margin since 38 Ma. Tracking the southern and down-dip limits of the seismically imaged Pacific slab beneath the New Zealand indicates arc volcanism in Northland from ~23 Ma and the Taranaki Basin between ~20 and 11 Ma requires Pacific plate subduction from at (or beyond) the northern North Island continental margin from at least 38 Ma to the present. Pacific plate motion in a west dipping subduction model shows a minimum horizontal transport distance of 285 km preceding the initiation of arc volcanism along the Northland-arc normal to the motion vector, a distance more than sufficient for self-sustaining subduction to occur. Arc-normal convergence rates along the Hikurangi margin doubled from 11 to 23 mm/yr between 20 and 16 Ma, increasing again by approximately a third between 8 and 6 Ma. This latest increase in arc-normal rates coincided with changes in relative plate motions along the entire SW Pacific plate boundary and steepening/rollback of the Pacific plate.

Detailed structural and petrographic analysis of the Greenbrier Fault (GBF) reveal different fold and fabric styles and generations preserved in the Great Smoky Group (GSG) hanging wall and Snowbird Group (SG) footwall. Four planar fabrics (S0, S1, S2, and S3) are completely overprinted within meters of the contact by shear zone-related fabrics. Bedding (S0) is defined by planar laminations in the SG siltstones. S1 is weak, not associated with folding of S0, and defined locally by sub-parallel alignment of biotite. S2 (slaty cleavage) is deflected into a disjunctive planar (in GSG) or continuous planar (in SG) S3 foliation characterized by mica formation and dynamic recrystallization of quartz. Metamorphic microstructures indicate lower greenschist to upper amphibolite facies Taconian metamorphism is syn- to post-S2, and pre-S3. Local lower greenschist facies retrograde metamorphism precedes S3 formation. A meter scale, ductile mesoscopic shear zone in SG at the GSG-SG contact is characterized by S/C fabric; this is the youngest deformational event and postdates retrograde mineral assemblages indicating postmetamorphic motion along the contact. Premetamorphic fault fabrics indicative of GSG thrusting onto the SG were absent or completely reconstituted during metamorphism and deformation. The Metcalf phyllite and Pigeon siltstone were also compared to test the hypothesis that the Metcalf phyllite is tectonized Pigeon siltstone. Major and trace element abundances are similar between the lithologies, with the exception of depletion of Ca, Na and Zr in the Metcalf. The system appears to have been open with respect to these elements. It is concluded that the Metcalf phyllite is the tectonized equivalent of the Pigeon siltstone based on lateral continuity, the strong macroscopic and microscopic resemblance of weakly deformed Metcalf to the Pigeon, similar mean values and ranges in major, minor, and trace elements, and identical rock densities.

The structural framework of the northern Gulf of Mexico coastal zone is characterized by numerous growth fault systems. Neotectonic processes in coastal marshes in this region have been shown to be important drivers of relative sea-level rise as well as having significant influence on marsh accretion processes. One active growth fault has been identified at East Matagorda Peninsula, Texas. To characterize the Holocene behavior of this fault and the consequent sedimentary responses, a suite of fallout radionuclides (7Be, 137Cs, 210Pb) and radiocarbon, supplemented by sediment physical property data have been used to determine sediment mixing depths, rates of accumulation, and geochronology. Correlation of time-equivalent stratigraphic boundaries reveals a maximum total Holocene offset of ~1 meter. Determination of slip rates from these values reveals a linear trend of displacement as a function of distance along the fault trace with maximum slip occurring to the southwest and minimum slip to the northeast. Sediment accumulation rates from the downthrown station nearest to the fault trace display a dramatic increase over the last 30 years. Sediment bulk density and grain size data suggest an interaction between fault-driven geomorphic change and sedimentation where a migrating land-water interface has influenced the type of sediment accumulation here.

This thesis investigates the timing and mechanisms involved in the formation of evaporite piercement structures on Ellef Ringnes Island, Canadian Arctic Archipelago. The study includes the interpretation of industry seismic reflection and borehole data to characterize the geometry of the domes, 1D backstripping of wells to investigate the role of tectonic influences on diapirism, and analogue modelling to better understand the mechanisms that drive diapirs with dense anhydrite caps. I propose that basement structures played a significant role in the formation of evaporite domes by triggering and directing salt movement. The domes developed during the Mesozoic by passive growth driven by the differential loading of salt on adjacent fault blocks, which led to their present day asymmetric geometries. Diapir growth rates in the Mesozoic were closely linked to the rate of sedimentation, which was greatly influenced by the amount of accommodation space provided by tectonic subsidence of the basin.

Salt tectonics is a key player in the evolution of many worldwide sedimentary basins on rifted continental margins. For more than a century, the evolving structures have been studied; but focus remained primarily on the onshore and shallow-water regions. The evolution of the poorly studied deep-water salt-tectonic structures is the focus of this thesis. Investigations are performed using 2D numerical models that comprise a viscous salt layer overlain by a frictional-plastic passive margin sedimentary sequence from shelf to deep water. This thesis addresses multiple salt-tectonic processes (gravity spreading, evolution of fold belts and salt canopies, diapirism) in a general context but with special focus on the structural evolution of the northwestern Gulf of Mexico (GoM). Here, multiple phases of gravity-spreading induced salt mobilization and thin-skinned deformation occurred throughout the Cenozoic. During the latest, late Oligocene-Miocene phase, the Perdido Fold Belt (PFB) formed from a 4.5km thick pre-kinematic section as a prominent salt-cored deep-water structure above the pinch-out of the autochthonous salt. It is here demonstrated with analytical as well as numerical calculations that the folding of the PFB can have formed by gravity spreading alone without basement tectonics. A requirement for this deformation is very high pore-fluid pressure in the sediments, which effectively reduces the sediments' mechanical strength. These values are refined using numerical models that couple compaction-induced fluid pressure to mechanical deformation. It is shown that very high fluid pressure is only necessary at the landward base of the deforming system; fluid pressure in other regions may remain moderate. This study shows, for the first time, the regional and dynamic evolution of pore-fluid pressure in a continental margin sedimentary system above salt. Additionally, the contribution of `lateral compaction' during fold-belt evolution is addressed. Landward of the PFB, a large-scale canopy developed during the Eocene. Its evolution is studied by investigating three different concepts of canopy evolution that have been proposed in the scientific literature. A canopy evolving via the mechanism of squeezed diapirs is most similar to the Eocene canopy of the northwestern GoM. A canopy evolving via the mechanism of breached anticlines is similar to that observed above the landward end of the PFB. Dynamic diapir growth is addressed in a neutral stress regime under uneven sedimentation employing a new mechanism of diapir initiation and evolution.