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- Creator:
- Erickson, Rolfe and Erickson, Rolfe
- Resource Type:
- Dissertation
- Campus Tesim:
- Sonoma
- Creator:
- Lackey, Jade Star
- Description:
- The Fine Gold Intrusive Suite is one of the largest (>2000 km2) and oldest intrusive complexes in the Sierra Nevada batholith (California, USA), and therefore contains a wealth of information about nascent magmatic processes in a convergent margin arc. Because the suite intrudes both accreted oceanic and/or island-arc terranes and continental crust, it provides perspective on how convergent margin magmatism recycles existing crust versus reworking of fringing island arcs into continental crust. Such insight informs our understanding of how continental crust formation may have operated in the Phanerozoic as compared to earlier in Earth history.New zircon U-Pb geochronology shows that the largely tonalitic suite was emplaced over ?19 m.y. (124-105 Ma), in three pulses that young from west to east. The most recent domain is nested within the previous ones, such that lobes of magma protruding from the main bodies of the Bass Lake Tonalite (the primary member of the Fine Gold Intrusive Suite) are older than interior areas. Zircon ?18O (6.1‰-8.0‰) and ?Hf (-4.7 + 6.4) show temporal trends indicating that early magmas were source mixtures of mantle with as much as 45% Paleozoic to Mesozoic oceanic and/or arc rocks, whereas later magmas contain greater inputs (to 50%) of Proterozoic North American crust. Older domains in the suite were likely generated from isolated sources, including initial high Sr/Y (to ?90), high Na2O magmas consistent with garnet-bearing sources inferred to be relatively deep. Higher 87Sr/86Sr, lower ?Hf, and higher Rb/Sr values in younger plutons show a source that tapped greater proportions of North American crust and was presumably more organized and larger, given its more homogeneous isotopic and trace element traits. Our findings also show that expression of the 87Sr/86Sr = 0.706 isopleth in arc magmas may be delayed until magma sources are sufficiently vigorous to melt and incorporate aged continental crust. Therefore, Sri values of older stitching plutons may better record the position of discrete terrane boundaries, whereas younger plutons will record the magmatically average position of terrane boundaries. Although the Fine Gold Intrusive Suite is comparable to the Late Cretaceous voluminous intrusive suite of eastern Sierran suites in terms of duration and age zoning of magmatism, the influence of preexisting basement compositions and differing degrees of organization of the magma sources with age is more pronounced. In addition, the findings show that recycling of fringing arc terranes into continental crust is relatively rapid and that estimates of the growth of Phanerozoic continental crust from such reprocessing should be revised upward.
- Resource Type:
- Article
- Identifier:
- 1553-040X
- Campus Tesim:
- Northridge
- 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
- Creator:
- Gehrels, George E., Marsaglia, Kathleen M., Barth, Andrew P., Koch, Lauren G., Lessard, Sarah J., Stephenson, Erin J., Hawley, John A., Rivas, Donato A., Yaspelkis III, Benedict B., Pedrick, Jane N., Nourse, Jonathan A., Grove, Marty, Britton, Steven L., and Jacobson, Carl E.
- Description:
- During the Late Cretaceous to early Cenozoic, southern California was impacted by two anomalous tectonic events: (1) underplating of the oceanic Pelona-Orocopia-Rand schists beneath North American arc crust and craton; and (2) removal of the western margin of the arc and inner part of the forearc basin along the Nacimiento fault. The Pelona-Orocopia-Rand schists crop out along a belt extending from the southern Sierra Nevada to southwestern Arizona. Protolith and emplacement ages decrease from >90 Ma in the northwest to <60 Ma in the southeast. Detrital zircon U-Pb ages imply that meta-sandstones in the older schists originated primarily from the western belt of the Sierran-Peninsular Ranges arc. Younger units were apparently derived by erosion of progressively more inboard regions, including the southwestern edge of the North American craton. The oldest Pelona-Orocopia-Rand schists overlap in age and provenance with the youngest part of the Catalina Schist of the southern California inner continental borderland, suggesting that the two units are broadly correlative. The Pelona-Orocopia-Rand-Catalina schists, in turn, share a common provenance with forearc sequences of southern California and the associated Salinian and Nacimiento blocks of the central Coast Ranges. This observation is most readily explained if the schists were derived from trench sediments complementary to the forearc basin. The schists and forearc units are inferred to record an evolution from normal subduction prior to the early Late Cretaceous to flat subduction extending into the early Cenozoic. The transition from outboard to inboard sediment sources appears to have coincided with removal of arc and forearc terranes along the Nacimiento fault, which most likely involved either thrusting or sinistral strike slip. The strike-slip interpretation has not been widely accepted but can be understood in terms of tectonic escape driven by subduction of an aseismic ridge, and it provides a compelling explanation for the progressively younger ages of the Pelona-Orocopia-Rand schists from northwest to southeast.
- Resource Type:
- Article
- Identifier:
- 0016-7606, 1943-2674
- Campus Tesim:
- Northridge