Keith A. Klepeis

Structure and kinematics of oblique continental convergence in northern Fiordland, New Zealand

Abstract Southeast of the Alpine Fault in Fiordland, New Zealand, Late Tertiary faults envelope exposures of Early Cretaceous high-P (12–16 kbar) granulite facies orthogneisses. These exposures allowed us to examine how pre-Cenozoic structures influenced variations in the degree and style of kinematic partitioning within a zone of oblique continental convergence. Fault-slip data and kinematic modelling show that oblique-slip and reverse displacements preferentially were partitioned east of the Alpine Fault onto curved NE-striking surfaces and moderately dipping (30–52°), N- and NW-striking surfaces, respectively. Approximately 3.0–5.5 km of exhumation and 6.5–7.0 km of oblique-dextral displacement occurred on faults that display palm tree-style geometries and subhorizontal detachments in profile. This heterogeneous style contrasts with the near horizontal slip on subvertical surfaces that define the Alpine Fault north of Milford Sound. The principal axes of instantaneous strain we determined using faults are compatible with principal stress axes derived from earthquakes. Our data indicate that mechanical anisotropies created by inherited structures controlled the locations and highly variable geometries of faulting since 7–9 Ma. The effects of these pre-existing structures and a focusing of contractional deformation along the northeast margin of Fiordland resulted in an unusually high degree of strike-slip partitioning across the southernmost onshore segment of the Alpine Fault.

Gneiss domes, vertical and horizontal mass transfer, and the initiation of extension in the hot lower-crustal root of a continental arc, Fiordland, New Zealand

Structural analyses and sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) zircon 206Pb/238U dates reveal the tectonic evolution of the deep (40–65 km) root of a Cretaceous continental arc as subduction beneath Gondwana ended and rifting began. By ca. 123 Ma, a dense root, composed partly of garnet pyroxenite and omphacite granulite, had formed. At 118–115 Ma, during regional contraction, a magma flare-up thermally and mechanically rejuvenated the base of the arc, resulting in widespread crustal melting, granulite-facies metamorphism, and the circulation of hot partially molten lower crust. By ca. 114 Ma, the flow formed two different styles of migmatitic gneiss domes. At the deepest (∼65 km) levels, the Breaksea domes record the upward flow of material in diapirs balanced by a sinking garnet pyroxenite root. At shallower (∼40 km) levels, the Malaspina domes record lateral flow in 1–2-km-thick channels beneath a roof of Paleozoic gneiss. At ca. 106 Ma, regional extension began with the formation of the Doubtful Sound shear zone (106–97 Ma), followed by the Resolution Island shear zone (95–89 Ma). These structures overprint migmatitic fabrics in the gneiss domes and record lower-crustal thinning, decompression with cooling (<730 °C), and horizontal flow oblique to the arc. They also show a migration of deformation toward the Gondwana interior that was driven by differences in the thermal structure and viscosity of the lower crust. In our model, gneiss doming and root detachment were precursors to rifting and triggered by magmatism. The evolution of extension occurred in three stages that reflect both the rheological structure of the lower crust and the influence of propagating spreading ridges.

The evolution of an exposed mid-lower crustal attachment zone in Fiordland, New Zealand

Abstract Studies of convergent margins suggest that large subhorizontal shear zones in the lower crust help regulate how displacements are transferred horizontally and vertically through the lithosphere. We present structural data from the Fiordland belt of SW New Zealand that illustrate the progressive evolution of a 25 km thick section of exhumed, Early Cretaceous middle and lower crust. The data show that the mechanisms by which displacements were relayed through the crust during a 25 Ma cycle of arc-related magmatism, high-grade metamorphism and contraction changed repeatedly. During the period 126–120 Ma, a ≥10 km thick batholith composed of gabbroic-dioritic magma was emplaced into the lower crust. Melt-enhanced shear zones evolved at the upper and lower contacts of the batholith where magma and steep temperature gradients created strength contrasts. By ∼ 120 Ma, partial melting of mafic-intermediate lower crust resulted in the formation of high-pressure (14–16 kbar) migmatite and steep, regionally extensive vein networks up to 10 km below the batholith. Melt segregation and transfer through and out of the lower crust were aided by melt-induced fracture arrays and ductile deformation in shear zones. During the period 116–105 Ma, differential shortening of the crust produced a network of subhorizontal and subvertical shear zones at different crustal depths. Near-vertical shear zones up to 15 km wide formed at the deepest part of the section. These shear zones cut upwards across the entire lower crust to merge with a gently dipping upper amphibolite facies fold-and-thrust zone that formed in the middle crust. A 1 km thick, subhorizontal shear zone underlies this mid-crustal fold-and-thrust zone and physically connected shear zones that formed at different crustal depths. Our data suggest that deformation above and below this mid-lower crustal attachment zone was coupled kinematically and accommodated subhorizontal arc-normal displacements in the middle crust and oblique sinistral displacements on steep shear zones in the lower crust. The steep lower crustal shear zones also record components of subhorizontal arc-normal shortening and vertical thickening. These results strongly suggest that large, kinematically coupled networks of flat and steep shear zones separated the Fiordland crust into distinctive structural domains and relayed displacements vertically and horizontally through the lithosphere during Early Cretaceous oblique convergence.

Thermochronology of extensional orogenic collapse in the deep crust of Zealandia

The exhumed Fiordland sector of Zealandia offers a deep-crustal view into the life cycle of a Cordilleran-type orogen from final magmatic construction to extensional orogenic collapse. We integrate U-Pb thermochronologic data from metamorphic zircon and titanite with structural observations from >2000 km 2 of central Fiordland to document the tempo and thermal evolution of the lower crust during the tectonic transition from arc construction and crustal thickening to crustal thinning and extensional collapse. Data reveal that garnet granulite facies metamorphism and partial melting in the lower crust partially overlapped with crustal thickening and batholith construction during emplacement of the Western Fiordland Orthogneiss (WFO) from 118 to 115 Ma. Metamorphic zircons in metasedimentary rocks yield 206 Pb/ 238 U (sensitive high-resolution ion microprobe–reverse geometry) dates of 116.3–112.0 Ma. Titanite laser ablation split stream inductively coupled plasma–mass spectrometry chronology from the same rocks yielded complex results, with relict Paleozoic 206 Pb/ 238 U dates preserved at the margins of the WFO. Within extensional shear zones that developed in the thermal aureole of the WFO, titanite dates range from 116.2 to 107.6 Ma and have zirconium-in-titanite temperatures of ∼900–750 °C. A minor population of metamorphic zircon rims and titanites in the Doubtful Sound region yield younger dates of 105.6–102.3 Ma with corresponding temperatures of 740–730 °C. Many samples record Cretaceous overdispersed dates with 5–10 m.y. ranges. Core-rim traverses and grain maps show complex chemical and temporal variations that cannot easily be attributed to thermally activated volume diffusion or simple core-rim crystallization. We interpret these Cretaceous titanites not as cooling ages, but rather as recording protracted growth and/or crystallization or recrystallization in response to fluid flow, deformation, and/or metamorphic reactions during the transition from garnet granulite to upper amphibolite facies metamorphism. We propose a thermotectonic model that integrates our results with structural observations. Our data reveal a clear tectonic break at 108–106 Ma that marks a change in processes deep within the arc. Prior to this break, arc construction processes dominated and involved (1) emplacement of mafic to intermediate magmas of the Malaspina and Misty plutons from 118 to 115 Ma, (2) contractional deformation at the roof of the Misty pluton in the Caswell Sound fold-thrust belt from 117 to 113 Ma, and (3) eclogite to garnet granulite facies metamorphism and partial melting over >8 m.y. from 116 to 108 Ma. These processes were accompanied by complex patterns of lower crustal flow involving both horizontal and vertical displacements. After this interval, extensional orogenic collapse initiated along upper amphibolite facies shear zones in the Doubtful Sound shear zone at 108–106 Ma. Zircon and titanite growth and/or crystallization or recrystallization at this time clearly link upper amphibolite facies metamorphism to mylonitic fabrics in shear zones. Our observations are significant in that they reveal the persistence of a hot and weak lower crust for ≥15 m.y. following arc magmatism in central Fiordland. We propose that the existence of a thermally weakened lower crust within the Median Batholith was a key factor in controlling the transition from crustal thickening to crustal thinning and extensional orogenic collapse of the Zealandia Cordillera.

Along-strike variation in crustal shortening and kinematic evolution of the base of a retroarc fold-and-thrust belt: Magallanes, Chile 53°S-54°S

The Late Cretaceous closure of the Late Jurassic–Early Cretaceous Rocas Verdes basin resulted in the development of the Patagonian fold-and-thrust belt and Magallanes foreland basin between 50°S and 54.5°S. New geologic maps, structural data, and two retrodeformed, line-balanced cross sections from the Magallanes region of Chile (53°S–54°S) constrain the kinematic evolution and along-strike correlations of deformation that occurred at the base of the fold-and-thrust belt near the brittle-ductile transition. The stratigraphic architecture of the predecessor basin controlled the position of regional decollement levels. During the initial stage of closure (Albian–Campanian), the floor of the Rocas Verdes basin was imbricated and thrust onto the continental margin to form a regional decollement within Jurassic–Lower Cretaceous shale. Continued shortening resulted in the deepening of the decollement to a ductile shear zone that formed

Influence of microscale weak zones on bulk strength

Shear zones have different rheological properties than the surrounding rocks, indicating that the bulk strength of regions containing shear zone networks cannot be determined by considering the the host rock rheology alone. We demonstrate the value of this concept at the microscale. We first consider the phase arrangements in naturally deformed rocks and document that weak phases exhibit little interconnection within a microstructure. Rather, three-dimensional weak zones, analogous to viscous shear zones, can interconnect or bridge weak phases. These zones typically form at high stress sites, comprise multiple minerals, and deform by mechanisms independent of those in the surrounding minerals. The presence of weak zones strongly affects the bulk strength of the rock, disproportionate to the mode of the weak zones. For example, the development of 1% mode of a weak zone at a high stress site can reduce the bulk strength of the rock nearly an order of magnitude. Calculation of the bulk strength of the rock by some averaging algorithm of the deformation mechanisms operating outside the weak zones will overestimate strength. Instead, accurate calculations and predictions of bulk strength require accounting for the presence and geometry of weak zones. For this reason, we advocate use of the scale-independent conceptual rheological model of interconnected weak zones or layers rather than that of interconnected weak phases. More generally, the way forward in improving quantification of the mechanical properties of the lithosphere requires recognizing and explicitly accounting for the spatial and temporal distribution of deformation mechanisms operating throughout a rock. This article is protected by copyright. All rights reserved.

The tempo of continental arc construction in the Mesozoic Median Batholith, Fiordland, New Zealand

We investigate the temporal record of magmatism in the Fiordland sector of the Median Batholith (New Zealand) with the goal of evaluating models for cyclic and episodic patterns of magmatism and deformation in continental arcs. We compare 20 U-Pb zircon ages from >2300 km2 of Mesozoic lower and middle crust of the Western Fiordland Orthogneiss to existing data from the Median Batholith to: (1) document the tempo of arc construction, (2) estimate rates of magmatic addition at various depths during arc construction, and (3) evaluate the role of cyclical feedbacks between magmatism and deformation during high and low magma addition rate events. Results from the Western Fiordland Orthogneiss indicate that the oldest dates are distributed in northern and southern extremities: the Worsley Pluton (123–121 Ma), eastern McKerr Intrusives (128–120 Ma), and Breaksea Orthogneiss (123 Ma). Dates within the interior of the Western Fiordland Orthogneiss (Misty and Malaspina Plutons, western McKerr Intrusives) primarily range from 118 to 115 Ma and signify a major flux of mafic to intermediate magmatism during which nearly 70% of the arc root was emplaced during a brief, ∼3 m.y., interval. The spatial distribution of dates reveals an inward-focusing, arc-parallel younging of magmatism within the Western Fiordland Orthogneiss during peak magmatic activity. Coupled with existing data from the wider Median Batholith, our data show that Mesozoic construction of the Median Batholith involved at least two high-flux magmatic events: a surge of low-Sr/Y plutonism in the Darran Suite from ca. 147 to 136 Ma, and a terminal surge of high-Sr/Y magmatism in the Separation Point Suite from 128 to 114 Ma, shortly before extensional collapse of the Zealandia Cordillera at 108–106 Ma. Separation Point Suite magmatism occurred at all structural levels, but was concentrated in the lower crust, where nearly 50% of the crust consists of Cretaceous arc-related plutonic rocks. Existing isotopic data suggest that the flare-up of high-Sr/Y magmatism was primarily sourced from the underlying mantle, indicating an externally triggered, dynamic mantle process for triggering the Zealandia high–magma addition rate event, with only limited contributions from upper plate materials.

Deformation and magma transport in a crystallizing plutonic complex, Coastal Batholith, central Chile

The Carboniferous–early Permian Santo Domingo complex in coastal Chile (33.5°S) preserves magmatic structures that allowed us to partially reconstruct and compare the deformation histories of two intrusive units within a mid-upper crustal zoned pluton. The oldest history is preserved in the Punta de Tralca tonalite, where microgranitoid enclaves record the emplacement and partial assimilation of mostly mafic magma into an intermediate host. Enclaves record early foliation development by a mechanical sorting and alignment of minerals during hypersolidus flow in melt-rich magma currents, followed by diffusion creep and sliding along melt-coated crystals. Structures in a weaker, tonalitic matrix record compaction, flattening, and near-solidus deformation as porous flow, aided by brittle deformation, drained residual melts. These processes produced penetrative S > L fabrics (i.e., planar more dominant that linear fabric) in an increasingly viscous, crystal-rich mush and promoted folding, fracturing, shearing, and crystal-plastic deformation as the mush approached its solidus. The deformation disrupted igneous layering and helped mobilize and concentrate melt-rich aggregates, forming diffuse patches and dikes that intruded previously deformed enclaves and matrix and aided pluton differentiation. A different deformation history is recorded by the Estero Cordoba dike, which intruded and interacted comagmatically with the Punta de Tralca tonalite. The dike records how magma flow near stiff boundaries resulted in velocity gradients that drove deformation during magma replenishment. This deformation reset inherited enclave fabrics, increased ductile stretching and winnowing, and formed linear (L > S) fabrics. This example illustrates how different styles of deformation assisted magma movement through a mid-upper crustal magma chamber and highlights the diverse origins and significance of structures generated by deformation in magmas of variable crystal-melt ratios.

Sm-Nd garnet ages for granulite and eclogite in the Breaksea Orthogneiss and widespread granulite facies metamorphism of the lower crust, Fiordland magmatic arc, New Zealand

Sm-Nd garnet and U-Pb zircon ages for eclogite and granulite from the Breaksea Orthogneiss provide a detailed chronology for pluton emplacement and subsequent thermal history of the lower arc crust exposed in Fiordland, New Zealand. The 147Sm-143Nd ages for ~1 cm garnet grains in eclogite yield a 108.2 ± 1.8 Ma (7 points) age and similar sized grains of garnet from granulite interlayered with eclogite yield a ca. 110.5 ± 1.6 Ma (8 points) age. Both samples retain sparse domains with older ages of 123–121 Ma. Distinct Ca, Lu, and Hf zoning in garnet indicate that eclogite and granulite cooled rapidly enough to negate significant diffusion. The Ca zoning is interpreted to indicate significant garnet recrystallization during the granulite facies event, ca. 110 Ma. The older garnet ages are indistinguishable from the oldest 206U/238Pb zircon ages, ca. 123 and 120 Ma, in granulite orthogneiss that yielded two age populations; these granulites have younger age populations of 111.1 ± 1.4 and 115.2 ± 1.3 Ma, respectively. Zircon from orthogneiss samples nearby yield single age populations indicating additional intrusions ca. 115 and late metamorphic zircon growth ca. 95 Ma. The zircon and garnet ages combined with pressure-temperaturetime paths document magma intrusion into the lowermost arc crust, near isothermal exhumation of Breaksea rocks at ~2.2 km/m.y. from ~65 km to 40–45 km depths, followed by continued high heat flow with granulite facies metamorphism. The latter high temperatures were synchronous with granulite facies metamorphism in the adjacent Malaspina pluton, indicating that high-temperature metamorphism affected >600 km2 of lower crust in the continental magmatic arc. The complex age results for U-Pb zircon and Sm-Nd garnet dating indicate the need for comprehensive data sets from multiple rocks for deciphering the intrusive and subsequent thermal history of the lower crust. The study detailed here clearly indicates that Sm-Nd garnet geochronology can provide useful ages for high-temperature rocks when large grains cool at rates of >10 °C/m.y. The geochronological results indicate that voluminous magmatism was closely followed by high-temperature metamorphism. This is a common phenomenon in the lower crust of magmatic arcs and a signature for high magmatic flux through the lower crust.