Geochronology of Catalina Island Schist and Future Work. Island Schist, we look into geochronological data to constrain the timeline of various

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1 Maria delos Angeles Cuevas May 4, 2015 Professor John Platt Geochronology of Catalina Island Schist and Future Work Abstract Taking into consideration the various issues involving the exhumation of Catalina Island Schist, we look into geochronological data to constrain the timeline of various units including timing of metamorphism and exhumation. While current data is lacking in the exhumation history, (U-Th)-He zircon dating would clarify its history. Introduction A variety of models have been presented to explain the exhumation of the Santa Catalina Schist. Most of the Catalina Schist is found on the Island and has a genetic relationship to the Franciscan complex located throughout coastal and central California (See Figure 1). While there is a relatively good understanding of most of the Franciscan complex, the Catalina schist is a little more difficult to understand in terms of its exhumation history. The two models that most closely describe this involve the rotation of the Transverse Ranges block and exhumation during the subduction of the Farallon Plate under the North American plate. The first model suggests that the Catalina Schist was exhumed during Late Oligocene to Early Miocene rotation of the Transverse Ranges block. The tectonic history of the Transverse Ranges can be divided into three parts; subduction, transtension and transpression, all representing a different stage in the developing of the Pacific-North American plate boundary. This initiates with the subduction of the Farallon plate under

2 Cuevas - 2 the North American plate during the Miocene (Atwater, 1998). During the mid-cenozoic, the spreading center between the Farallon plate and the Pacific plate approached the North American plate. At this point the Farallon plate broke up into smaller plates that then attached themselves to the North American or Pacific plates and followed that new movement. Once the spreading center reached the North American plate (around 28my), the new motion became transtensional. This new motion caused the movement of the broken up Farallon pieces between the Pacific and North American boundaries. One of these smaller blocks is the Transverse Ranges block. The Transverse Ranges block is thought to have rotated between degrees clockwise (Kamerling and Luyendyk, 1985) caused by a corner of the block getting stuck while it continued moving with the Pacific plate. This allowed for crustal thinning and crustal extension that eventually allowed for the exhumation of Franciscan rocks, such as the Catalina schist, through underplating and rising in a ductile manner. (Atwater, 1998). Finally, transpression occurred in the last 5my when the Pacific plate captured the last remnants of the Farallon plate in the south and consequentially forming the San Andreas Fault system (Nicholson et at., 1994). (See Figure 2) The alternative model for the exhumation of the Catalina Schist assumes that it occurred during the subduction of the Farallon plate within the accretionary wedge. The Franciscan Complex is made up of three primary belts: the eastern belt, the central belt, and the coastal belt. These belts get progressively older and more deformed toward the east. This model assumes that the Franciscan Complex was subducted beneath the Great Valley Belt where the high-pressure metamorphism occurred. Because of continued subduction, underplating of the wedge occurred forcing what is now the Catalina Schist

3 Cuevas - 3 to mid-crustal levels or right below the Great Valley belt by the Late Cretaceous. Through rifting of the Great Valley belt and its basement rocks, the Catalina schist was completely unroofed and exposed to erosion. This sequence is recorded in the sedimentary deposits on Santa Rosa and Santa Cruz Islands (Suppe and Crouch, 1993). See Figure 3. Catalina Schist: The Santa Catalina Island Schist is made up of three tectonic units; distinguishable by the mineral assemblages. The Catalina Blueschist Unit is structurally the lowest and is made up of metagraywacke, metachert, mafic metavolcanic rocks and ultramafic rock, all of which were uniformly metamorphosed under blueschist-facies conditions (Platt, 1975). The Catalina Greenschist Unit is structurally above the Blueschist Unit on the island. The Catalina Greenschist Unit is similar to that of the Blueschist unit, but contains a large proportion of mafic volcanic rocks, and was metamorphosed under high-pressure greenschist facies conditions. This unit has also undergone extensive deformation, losing its primary texture. In addition, there seems to be second deformation period followed by two separate periods of folding that further complicate the unit. The last unit is the Catalina Amphibolite Unit. This unit is structurally above both the Blueschist and Greenschist Units and consists mostly of green hornblende-zoisite schist. Above this there is another unit of serpentinite. This unit seems to have undergone serpentinization post metamorphism in the amphibolite facies because it contains minerals that are unstable in the amphibolite facies (Platt, 1975).

4 Cuevas - 4 Previous Geochronological Work: Various efforts have been made to constrain the time period for the prograde history as well at peak temperatures for each of these units. While the main point of interest is the exhumation history, looking at the initial history can also give us clues into the exhumation history. The Lu-Hf dating method was applied to various schists in the Franciscan complex. Specifically, Anzkiewicz and others (2004) dated rocks from the Catalina Amphibolite Unit. The ages obtained from this study for this unit were of /- 0.8 Ma. This is within error of a previous study done by Mattinson in 1986 in which U-Pb dating of garnet-amphibolite-sphene-apatite was obtained at /-1.5 Ma.. Additionally, Ar-Ar dating suggests that underplating occurred during the formation of the subduction zone around Ma (Grove, 1995). In this case, the younger Ar-Ar ages reflect cooling below the closing temperature for the K-Ar system. Additionally, Grove performed U-Pb detrital zircon dating on blueschist facies metagraywackes collected from Santa Catalina Island. This resulted in ages of 85 Ma, placing an upper limit on the timing of the subduction suggesting the subduction and accretion on the Island lasted a minimum of 20 million years (Grove, 2008). Methods: For the previously mentioned Lu-Hf dates, the following process was used to date various schists from the Franciscan Complex. Lu-Hf dating on garnets is very ideal as garnet highly fractionates Lu relative to Hf. This allows for very high Lu-Hf ratios and therefore highly precise ages are obtained. In order to produce highly precise ages, the garnets must be very clean. Sometimes garnets may have zircon intrusions and therefore

5 Cuevas - 5 in order to obtain accurate results in Lu-Hf dating, the mineral fractions are dissolved on a hot plate. (Anczkiewicz, 2004) Additionally, sulphuric acid leaching was used to remove any possible inclusions that may have been nearly invisible in the garnets to provide an age that was about 40% younger. Geochronological Work for Thesis Three samples from the Santa Catalina Island Blueschist facies were used to undergo the (U-Th)/He dating method. Each sample underwent mineral separation beginning with crushing, pulverizing and sieving. Then each sample (106 microns- 333 microns) underwent the Frantz magnetic separation at 0.9A with a tilt of 5 degrees and a slope of twenty degrees. Lastly, the non-magnetic portion was separated via heavy liquid and the more dense portion was analyzed for zircons. Unfortunately, the zircons obtained from these samples lacked a crystal structure and therefore we were unable to date them. The proposed method involved determining the He levels, followed by the U-Th ratio and lastly using this information to come up with an age. In order to determine the He levels, single zircon crystals were heated with a focused beamed to degrees Celsius in fifteen-minute intervals. To determine the U-Th ratio the zircon crystals were spiked and heated at 225 degrees for 72 hours, dissolved in HCl at 200 degrees Celsius for twenty-four hours and the re-heated until dried. Then it was dissolved in 6% HNO 3 or 0.8%HF. Some problems with dating zircons via this method are the potential zonation of the U and the Th within each zircon. This affects the ratios as well as the alpha ejection correction that needs to be made for shape of the zircons.

6 Cuevas - 6 Conclusion: Using the (U-Th)/He method would allow us to constrain the time period for the exhumation of the Catalina Schist. Knowing if this occurred in the Miocene versus the Late Cretaceous can help determine which of the aforementioned models most closely represent the actual history of the Catalina Schist. Although finding zircons in this type of metamorphosed environment can be difficult, it is an endeavor that should be pursued in order to answer some of the current questions. Figures Figure 1: General Map of the Franciscan Complex with Santa Catalina Island in the southwest. Figure taken from Irwin, 1990

7 Cuevas - 7 Figure 2 [A] Farallon-North America subduction showing collision of the Pacific and North America plates and fragmentation of the Farallon plate into the Arguello (AP) and Monterey (MP) microplates. [B]Beginning of transtensional phase. Capture of the Monterey and Arguello microplates by the Pacific plate. Separation of the Transverse Ranges (WTR), Santa Lucia Banks (SLB), and Outer Borderlands (OB) from North America and the beginning of rotation. [C] Continued rotation of the Transverse Ranges causes slab windows to open, and intrusion of the Inner continental Borderlands. The Farallon plate continues to fragmemtn into the Magdalena (MP) and Guadalupe (GP) microplates. [D] Spreading begins in the Gulf of California. Transform boundary well established on the Pacific side of Baja. [E] Capture of the Magdalena and Guadalupe microplates and Baja California by the Pacific plate. Plate boundary realigns through southern California. [F] Baja is transported northwest, pressing its northern end against southern California and shifting to a transpressional tectonic regime with the Transverse Ranges being extruded around the larger transpressional bend of the San Andrea and shortened north-south. Figure taken from Nicholson et at., 1994.

8 Cuevas - 8 Figure 3 Possible tectonic setting [after Gastil et al., 1981]. (a) Earliest Cretaceous shows the existence of fringing volcanic arc and back arc basin, and relatively slow NW drift of the North American plate.(b) Suturing of the fringing arc to the continent in the Mid-Early Cretaceous resulting in closure of the back arc basin and plate deformation is shown. Deformed character of Western Peninsular Ranges Batholith (PRB) synkinematic plutons reflects intrusion during continued shortening. (c) Step out of trench and initiation of new subduction. This stage coincides with the earliest metamorphism of the Catalina Schist (recorded by the higher-grade units) in the newly formed subduction zone. Continued subduction and associated underplating/cooling produce the lower-grade units of the Catalina Schist. Higher-grade units have cooled to below 350øC by 105 Ma. Intrusion of the Western PRB continues. The abandoned slab beneath the PRB heats. (d) Continued subduction and associated underplating/cooling in the Early-Late Cretaceous lead to partial tectonic exhumation of the Catalina Schist by extension. To the east, melting of the slab in the abandoned subduction zone, perhaps generating the Eastern PRB postkinematic plutons Ctonalitic welt"), results from heating during thermal reequilibration of the abandoned slab - 20 Ma after the cessation of subduction.

9 Cuevas - 9 References Atwater, T., 1998, Plate Tectonic History of Southern California with emphasis on the Western Transverse Ranges and Santa Rosa Island, in Weigand, P. W., ed., Contributions to the geology of the Northern Channel Islands, Southern California: American Association of Petroleum Geologists, Pacific Section, MP 45, p. 1-8 Cameron A. Snow, John Wakabayashi, W. G. Ernst and Joseph L. Wooden Detrital zircon evidence for progressive underthrusting in Franciscan metagraywackes, west-central California: Geological Society of America Bulletin (2010), 122(1-2): Crouch, J.K., 1979, Neogene Tectonic Evolution of the California Continental Borderland and the Western Transverse Ranges: Geological Society of America Bulletin, v. 90, p Crouch, J. K., and Suppe, J., 1993, Late Cenozoic tectonic evolution of the Los Angeles basin and Inner California borderland: A model for core complex-like crustal extension: Geological Society of America Bulletin, v. 105, p Grove, M., and Bebout, G.E., 1995, Cretaceous tectonic evolution of coastal southern California: Insights from the Catalina Schist: Tectonics, v. 14 p Grove, M., 2008, Implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary (non-collisional) and collisional (suturing) orogens, Geological Society, London, Special Publications, 318, p Irwin, William P. (1990). "Geology and plate-tectonic development". In Robert E. Wallace, ed. The San Andreas Fault System, California. U.S. Geological Survey Professional Paper p Mattinson, M., J., and Echeverria, L., M., Ortigalita Peak gabbro, Franciscan Complex; U-Pb dates of intrusion and high-pressure-low-temperature metamorphism Geology (Boulder)(December 1980), 8(12): Nicholson, C., Sorlien, C.C., Atwater, T., Crowell, J.C., and Luyendyk, B.P.,1994, Microplate capture, rotation of the western Transverse Ranges, and initiation of the San Andreas transform as a low-angle fault system: Geology, v.22 p doi: / (1994)022<0491:MCROTW>2.3.CO;2. Platt, J., John, Metamorphic and deformational processes in the Franciscan Complex, California: some insights from the Catalina Schist terrane: Geological Society of America Bulletin, 86 (1975), pp

10 Cuevas - 10 Robert Anczkiewicz, John P. Platt, Matthew F. Thirlwall, John Wakabayashi, Franciscan subduction off to a slow start: evidence from high-precision Lu Hf garnet ages on high grade-blocks, Earth and Planetary Science Letters, Volume 225, Issues 1 2, 30 August 2004, Pages