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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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How is continental crust built? A study of how fault zone deformation controls vertical transport of magma during crustal growth in Fiordland, New Zealand
Subduction zones are sites where converging tectonic plates create magma that is transported upward by faults (acting as conduits) within the crust, incrementally building the continent over time. However, how faults and their deep, ductile counterparts (shear zones) transport magma across the entire thickness of the crust is not well understood. This is important to investigate because faults remain as zones of weakness and may trigger large magnitude earthquakes (Barnes et al., 2016). Fiordland, New Zealand, has a system of interconnected faults and shear zones that transect the world’s best-preserved section of crust produced in a subduction zone. We will focus on shear zone rocks from the middle crust, where the shear zone narrows in width as the composition of the crust changes. We hypothesize that the change in crustal composition acted as a strength boundary within the shear zone, changing the geometry of strain to allow for more localized movement of magma and deformation.
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- Award ID(s):
- 1650219
- PAR ID:
- 10232195
- Date Published:
- Journal Name:
- CSUNPosium: Annual Student Research and Creative Works Symposium
- Volume:
- 25
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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Structural analyses combined with U‐Pb zircon petrochronology show the influence of arc magmatism on the evolution of two transpressional shear zones in the deep root of the Late Cretaceous Southern California batholith. The mid-crustal Black Belt and lower-crustal Cucamonga shear zones (eastern San Gabriel Mountains) formed at ~84 Ma shortly after a large mass of tonalite and granodiorite intruded the lower crust. Both shear zones were active until at least ~74 Ma and probably until 72-70 Ma. In the mid-crustal shear zone, rheological contrasts between mingling magmas localized deformation at dike margins. The deformation began as hypersolidus flow in partially crystallized dikes and then transitioned to deformation below the solidus when alternations between viscous creep and brittle faulting produced interlayered pseudotachylyte, cataclasite, and mylonite. As the dikes solidified, strain hardening drove shear zone growth and created thin (10-30 m) high-strain zones and faults that are widely spaced across ~1 km. In contrast, the lower-crustal Cucamonga shear zone was magma-starved, lacks the variety of shear zone fabrics exhibited by its mid-crustal counterpart, and formed by the reactivation of a pre-existing fabric that records pure reverse displacements at 124-93 Ma. The two shear zones created a partitioned style of intra-arc transpression where sinistral-reverse (mostly arc-parallel with some arc-oblique) displacements were accommodated on moderately dipping faults and shear zones and arc-normal shortening was accommodated by coeval folds. This study shows how a magmatic surge influenced the architecture and style of Late Cretaceous transpression in the Southern California batholith, including the evolution of high-strain zones that record alternating episodes of brittle, ductile, and hypersolidus deformation. The results illustrate how magmatism localizes strain on deep-crustal faults during orogenesis and oblique convergence.more » « less
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The continental crust is rich in aluminosilicates and formed by the crystallization of arc magmas. However, the magma produced at sub-arc depths is often silica-poor. The chemical evolution of sub-arc magma from silica-poor to aluminosilicate-rich is perplexing. Magnetotelluric (MT) observations in subduction zones and complementary laboratory-based constraints of electrical conductivity (σ) are crucial to understanding this chemical evolution. The σ of a magma is sensitive to pressure (P), temperature (T), and chemistry (X). To date, laboratory-based measurements on the σ of silicate melts have helped to interpret MT observations at P ≤ 2 GPa. Yet, the melting in subduction zones could occur deeper, at P ≤ 6−7 GPa. The σ of melt at such pressures is poorly constrained. To address this, we performed experiments at P ≤ 6 GPa to examine the σ of basaltic to andesitic melts, which are common in subduction zones. We constrained the effects of silica, alumina, alkali, alkaline, and water (H2O) contents on the σ of melt. The activation volume of σ increases with silica contents. Hence, the σ of basaltic melt is overall greater than that of an andesitic counterpart. The σ of basaltic magma is also less sensitive to P than andesitic magma. Water lowers the activation energy and enhances σ for all melt compositions. Our results help constrain how the electrical properties of a magma change with an evolving composition in a subduction zone.more » « less
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