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1. Deep slab collision during Miocene subduction causes uplift along crustal-scale reverse faults in Fiordland, New Zealand

   Keith Klepeis, Laura Webb (July 2019, GSA today)

   A new multidisciplinary project in southwest New Zealand that combines geological and geophysical data shows how and why deep lithospheric displacements were transferred vertically through the upper plate of an incipient ocean-continent subduction zone. A key discovery includes two zones of steep, downward-curving reverse faults that uplifted and imbricated large slices of Cretaceous lower, middle, and upper crust in the Late Miocene. Geochemical and structural analyses combined with 40Ar/39Ar geochronology and published images from seismic tomography suggest that the reverse faults formed at 8–7 Ma as a consequence of a deep (~100 km) collision between subducting oceanic lithosphere and previously subducted material. This collision localized shortening and reactivated two crustal-scale shear zones from the upper mantle to the Earth’s surface. The event, which is summarized in a new lithospheric-scale profile, is helping us answer some long-standing questions about the origin of Fiordland’s unique lower-crustal exposures and what they tell us about how inherited structures can transfer motion vertically through the lithosphere as subduction initiates. 

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

   https://doi.org/10.1130/GES02251.1  

   Blatchford, Hannah J.; Klepeis, Keith A.; Schwartz, Joshua J.; Jongens, Richard; Turnbull, Rose E.; Miranda, Elena A.; Coble, Matthew A.; Kylander-Clark, Andrew R. (August 2020, Geosphere)

   null (Ed.)

   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. 

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3. The initiation and growth of transpressional shear zones through continental arc lithosphere, southwest New Zealand

   Klepeis, K.A.; Schwartz, J.J.; Miranda, E.A.; Lindquist, P.; Jongens, R.; Turnbull, R.; and Stowell, H.H. (September 2022, Tectonics)

   Structural analyses combined with new U-Pb zircon and titanite geochronology show how two Early Cretaceous transpressional shear zones initiated and grew through a nearly complete section of continental arc crust during oblique convergence. Both shear zones reactivated Carboniferous faults that penetrated the upper mantle below Zealandia's Median Batholith but show opposite growth patterns and dissimilar relationships with respect to arc magmatism. The Grebe-Indecision Creek shear zone was magma-starved and first reactivated at ∼136 Ma as an oblique-reverse fault, along which an outboard batholith partially subducted beneath Gondwana. This system nucleated at or above ∼20 km depth and propagated downward at 2–3 mm yr−1, accumulating at least 35–45 km of horizontal (arc-normal) shortening by ∼124 Ma. In contrast, the magma-rich George Sound shear zone first reactivated in the lower crust (∼55 km depth) at ∼124 Ma and grew upward at ∼3 mm yr−1, reaching the upper crust by ∼110 Ma. In this latter system, magmatism influenced shear zone architecture and drove its growth while subduction and oblique convergence ended. As magma entered the roots of the system and began to solidify, deformation was driven out of the lower crust and into the middle crust where the system widened by a factor of three when fold-thrust belts formed on either side of a steep, central transpressional shear zone. This study illustrates how the reactivation of structural weaknesses localizes deformation at all depths in the lithosphere and shows how magma-deformation feedbacks influence shear zone connectivity and built a batholith from the bottom up. 

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4. Cenozoic kinematic histories of the Tidding and Lohit thrusts in the northern Indo-Burma Ranges: Implications for crustal thickening and exhumation of Gangdese lower arc crust along the Indus-Yarlung suture zone

   https://doi.org/10.1130/B36323.1  

   Haproff, Peter J.; Levy, Drew A.; Zuza, Andrew V.; Hooker, Julian D.; Heizler, Matthew T.; Stockli, Daniel F.; Braza, Mary (November 2022, GSA Bulletin)

   Crustal thickening has been a key process of collision-induced Cenozoic deformation along the Indus-Yarlung suture zone, yet the timing, geometric relationships, and along-strike continuities of major thrusts, such as the Great Counter thrust and Gangdese thrust, remain inadequately understood. In this study, we present findings of geologic mapping and thermo- and geochronologic, geochemical, microstructural, and geothermobarometric analyses from the easternmost Indus-Yarlung suture zone exposed in the northern Indo-Burma Ranges. Specifically, we investigate the Lohit and Tidding thrust shear zones and their respective hanging wall rocks of the Lohit Plutonic Complex and Tidding and Mayodia mélange complexes. Field observations are consistent with ductile deformation concentrated along the top-to-the-south Tidding thrust shear zone, which is in contrast to the top-to-the-north Great Counter thrust at the same structural position to the west. Upper amphibolite-facies metamorphism of mélange rocks at ∼9−10 kbar (∼34−39 km) occurred prior to ca. 36−30 Ma exhumation during slip along the Tidding thrust shear zone. To the north, the ∼5-km-wide Lohit thrust shear zone has a subvertical geometry and north-side-up kinematics. Cretaceous arc granitoids of the Lohit Plutonic Complex were emplaced at ∼32−40 km depth in crust estimated to be ∼38−52 km thick at that time. These rocks cooled from ca. 25 Ma to 10 Ma due to slip along the Lohit thrust shear zone. We demonstrate that the Lohit thrust shear zone, Gangdese thrust, and Yarlung-Tsangpo Canyon thrust have comparable hanging wall and footwall rocks, structural geometries, kinematics, and timing. Based on these similarities, we interpret that these thrusts formed segments of a laterally continuous thrust system, which served as the preeminent crustal thickening structure along the Neotethys-southern Lhasa terrane margin and exhumed Gangdese lower arc crust in Oligocene−Miocene time. 

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5. The Initiation and Growth of Transpressional Shear Zones Through Continental Arc Lithosphere, Southwest New Zealand

   https://doi.org/10.1029/2021TC007097  

   Klepeis, Keith A.; Schwartz, Joshua J.; Miranda, Elena; Lindquist, Peter; Jongens, Richard; Turnbull, Rose; Stowell, Harold (September 2022, Tectonics)

   Abstract Structural analyses combined with new U‐Pb zircon and titanite geochronology show how two Early Cretaceous transpressional shear zones initiated and grew through a nearly complete section of continental arc crust during oblique convergence. Both shear zones reactivated Carboniferous faults that penetrated the upper mantle below Zealandia's Median Batholith but show opposite growth patterns and dissimilar relationships with respect to arc magmatism. The Grebe‐Indecision Creek shear zone was magma‐starved and first reactivated at ∼136 Ma as an oblique‐reverse fault, along which an outboard batholith partially subducted beneath Gondwana. This system nucleated at or above ∼20 km depth and propagated downward at 2–3 mm yr⁻¹, accumulating at least 35–45 km of horizontal (arc‐normal) shortening by ∼124 Ma. In contrast, the magma‐rich George Sound shear zone first reactivated in the lower crust (∼55 km depth) at ∼124 Ma and grew upward at ∼3 mm yr⁻¹, reaching the upper crust by ∼110 Ma. In this latter system, magmatism influenced shear zone architecture and drove its growth while subduction and oblique convergence ended. As magma entered the roots of the system and began to solidify, deformation was driven out of the lower crust and into the middle crust where the system widened by a factor of three when fold‐thrust belts formed on either side of a steep, central transpressional shear zone. This study illustrates how the reactivation of structural weaknesses localizes deformation at all depths in the lithosphere and shows how magma‐deformation feedbacks influence shear zone connectivity and built a batholith from the bottom up. 

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