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- Creator:
- Clements, Brian
- Description:
- Plate reconstructions document a polyphase rifting history of the Zealandia microcontinent, from Cretaceous continental breakup, to Tertiary intraplate rifting (Lamarche et al., 1997; Gaina et al., 1998; Sutherland, 1999; Cande and Stock, 2004; Schellart et al., 2006; Furlong and Kamp, 2013). Cretaceous extension led to collapse of a thickened arc in Zealandia (Klepeis et al., 2004; Stowell et al., 2014), followed by opening of the Tasman Sea (Gaina et al., 1998). Following a ~60 Ma period of tectonic quiescence, paleogeographic maps illustrate a rift system that bifurcated around an undeforming, rigid Fiordland block (Nathan et al., 1986; Turnbull et al., 1993; King and Thrasher, 1996; Turnbull et al., 2010). However, identification of several major and minor intra-Fiordland transtensional faults (Turnbull et al., 2010; Newman, 2014) suggest that the Fiordland block is much more complex than previously thought, although the amount of slip and direct relation to exhumation remain unknown. Notwithstanding a >60 Ma tectonic history of progressive rifting and crustal thinning in Zealandia, we still have a limited understanding of how the lower-crustal portion of Fiordland responded to polyphase rifting. The objective of this study is to use LASS rutile U-Pb chronology to investigate the thermal evolution of the lower crust of Western Fiordland in response to Cretaceous continental breakup and Tertiary rifting. Results from three-hundred and eighty-seven rutile 206Pb/238U dates are classified into two groups: 1) Eocene-Oligocene homogeneous age populations (group EO), and 2) overdispersed populations in which 206Pb/238U dates span >> 10 Ma and do not define a single age population (group OD). Integrating these dates with diffusion equations for simple cooling (Dodson, 1973) and reheating (Watson and Cherniak, 2013) demonstrate that the Group EO rutile U-Pb system was totally reset at T ≥ 655 °C during propagation of Tertiary intraplate rifting. At distances of 7-60 km from Group EO, the Group OD rutile U-Pb system experienced partial resetting during open-system behavior over temperatures of ~460-585 °C. Although Tertiary paleodepths remain uncertain, rutile U-Pb dates and calculated reheating temperatures suggest that the lower crust of Western Fiordland was not fully exhumed during the Cretaceous, and illustrates the importance of investigating lower-crustal exhumation with respect to major intra-Fiordland transtensional faults (re)activated in the Tertiary.
- Resource Type:
- Thesis
- Campus Tesim:
- Northridge
- Department:
- Geological Sciences
- Creator:
- Kylander-Clark, Andrew, Stowell, Harold H., Shen, R., Schwartz, Joshua J., Hacker, Bradley, Tulloch, Andy, Klepeis, Keith A., Lin, Z.X., Ren, Y.J., Deng, W.Y., Sheng, Donna, Coble, Matthew, Xing, D.Y., and Sheng, L.
- Description:
- 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 km2 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 206Pb/238U (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 206Pb/238U 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.
- Resource Type:
- Article
- Identifier:
- 1553-040X
- Campus Tesim:
- Northridge