Thermochronology of extensional orogenic collapse in the deep crust of Zealandia

Abstract

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 amphibo lite facies metamorph...

Record of plate boundary metamorphism during Gondwana breakup from Lu–Hf garnet geochronology of the Alpine Schist, New Zealand

“…During the 97-75 Ma interval of garnet growth in the Alpine Schist, multiple lines of evidence exist for both widespread horizontal extension occurring in inboard regions, and horizontal shortening occurring near the trench. Widespread extension in the continental arc region, southern Alpine Schist and Otago Schist from as early as 105 Ma onwards (Figure 7; Bradshaw, 1989) is recorded by low-angle normal faulting and metamorphic core complex formation (Forster & Lister, 2003;Kula et al, 2007;Tulloch et al, 2009;Schwartz et al, 2016), A-type magmatism (Tulloch et al, 2009;Waight et al, 1998), extension-related intrusion of mafic dykes (Van der Meer et al, 2013;Waight et al, 1998), sedimentary basin formation (Bache et al, 2014;Bishop, 1992), and cooling and exhumation of the Otago Schist (Batt et al, 2000;Gray & Foster, 2004;Tippett & Kamp, 1993). Based on this strong evidence for extension and the apparent cessation of typical arc magmatism (Tulloch & Kimbrough, 2003), subduction at the Pacific-Gondwana plate boundary is widely assumed to have ceased by c. 105 Ma.…”

“…The tectonic processes responsible for the transition from crustal thickening to extensional collapse and lithospheric thinning in Zealandia are subject of ongoing investigation (e.g. Schwartz et al., ). Critical to understanding the tectonic processes responsible for horizontal extension and continental fragmentation in this former convergent margin setting are constraints on the timing, duration and driving mechanisms of high‐grade metamorphism at the plate margin.…”

“…108 Ma onwards (e.g. Schwartz et al., ). Current geochronological data from the Alpine Schist suggest that this former accretionary margin may preserve a record of the tectonic processes occurring during the transition from arc construction to widespread horizontal extension and separation of Zealandia from East Gondwana.…”

Temporal and spatial variations in magmatism and transpression in a Cretaceous arc, Median Batholith, Fiordland, New Zealand

“…A key feature of both middle-and lower-crustal ductile shear zones in Fiordland is that they contain titanite, which is increasingly recognized as a powerful tool with which to fingerprint the timing, temperature, and petrogenesis of igneous and metamorphic systems (Frost et al, 2001;Kohn and Corrie, 2011;Kylander-Clark et al, 2013;Spencer et al, 2013;Stearns et al, 2015;Scibiorski et al, 2019). Titanite occurs in a wide variety of crustal rocks, including quartzofeldspathic igneous rocks, such as the Separation Point Suite (Scott et al, 2009;this study), and in amphiboliteto granulite-facies prograde and retrograde metamorphic rocks such as those that characterize various shear zones in the middle and lower crust of Fiordland (e.g., Schwartz et al, 2016). It is an ideal phase for examining the timing and conditions of shear zone development because it incorporates abundant U (Essex and Gromet, 2000;Frost et al, 2001;Flowers et al, 2005;Schwartz et al, 2016), it partitions Zr as a function of pressure and temperature, enabling its use as a thermobarometer (Hayden et al, 2008), and it can crystallize and/or recrystallize during changing pressure and temperature events associated with reactions with Ca-and Al-bearing phases such as calcite, clinopyroxene, plagioclase, epidote, and hornblende (Franz and Spear, 1985;Essex and Gromet, 2000;Frost et al, 2001).…”

Thurston Island (West Antarctica) Between Gondwana Subduction and Continental Separation: A Multistage Evolution Revealed by Apatite Thermochronology

“…The remote location with mostly unstable weather conditions, a vast ice cover, and very limited rock exposure has only allowed for very few geoscientific studies on Thurston Island (e.g., Grunow et al, ; Leat et al, ; Pankhurst et al, ; Riley et al, ; Storey et al, ). However, unlike Marie Byrd Land and Zealandia, where mid‐Cretaceous crustal extension is constrained by exhumation of metamorphic core complexes, basin development, and extensive A‐type magmatism (Bache et al, ; McFadden et al, ; Schwartz et al, ; Siddoway, ; Tulloch et al, ; Weaver et al, ), rift‐related indications are scarce on the TIB. Consequently, the timing of extension has to be indirectly inferred.…”