A Senior Project. presented to. California Polytechnic State University, San Luis Obispo. In Partial Fulfillment. of the Requirements for the Degree

Transcription

1 Determining Slip Sense along the Sur-Nacimiento Fault through the use of Detrital Zircon Geochronology on the Nacimiento Block at Cerro Alto, California A Senior Project presented to the Faculty of the Natural Resources Management and Environmental Science California Polytechnic State University, San Luis Obispo In Partial Fulfillment of the Requirements for the Degree Bachelor of Science, Earth Science by Karen Decker March, Karen Decker

2 Senior Project Advisor: Dr. Scott Johnston Department Head: Dr. Douglas D. Piirto ii

3 Abstract The Cretaceous geology of California represents an archetypical convergent margin assemblage. Studying the faulting patterns of the convergent margin will indicate the way in which the margin deformed as subduction occurred. The Sur- Nacimiento Fault in California is located to the west of the San Andreas Fault. Slip along the Sur-Nacimiento Fault occurred in the late Cretaceous during the time that subduction was occurring. The Sur-Nacimento Fault caused the juxtaposition of the subduction complex and magmatic arc of the convergent margin assemblage. This is unusual because the fault removed the forearc basin that is normally found in a convergent margin assemblage. Through the use of detrital zircon geochronology samples of rock are dated to determine where the sediment was when it was formed. This will aid in explaining how the Sur-Nacimiento Fault moved when deformation was happening. Detrital zircons were extracted from quartzite clasts found in conglomerates and were analyzed through the use of laser ablation inductively coupled plasma mass spectrometry to determine the age of individual zircon grains. The results of this process show that there are age peaks around 1.1 Ga, 1.4 Ga, and 1.8 Ga and indicates that these forearc sediments that were derived from the North American miogeocline. Although further analyses are necessary, this work is consistent with the interpretation that the Sur-Nacimiento Fault was a thrust fault that developed as the California Mesozoic forearc was underthrust below the developing arc. iii

4 Acknowledgements First and foremost, I would like to thank Brittany Brookshire for being my partner in this long, crazy journey. Her constant support, encouragement, and all around positive spirit made the process more than bearable, she made it fun. Scott Johnston has been an incredible advisor for this project. His enthusiasm and zeal for all things geology has been contagious and made this project exciting. I would like to thank him for this opportunity to not only further my knowledge of California geology, but for the opportunity to learn more about the processes behind the research papers I ve learned to love. Scott has also been an incredible teacher in the classroom and the field. I d like to also thank Dr. Tony Garcia for being my first college geology professor and the reason I started in on the Geology minor. If it weren t for his enthusiasm for an introductory geology class, I might never have started this journey. I ve had the opportunity to partake in his upper level classes and he never failed to amaze me with his knowledge and excitement. Dr. Lynn Moody has been my academic advisor for my entire college career and I don t think I d have made it to this point without her guidance. Her understanding and willingness to help has been crucial to my higher education. I d like to thank my fiancé, Dylan, for his unending support, even when the stress drove me a little nuts. Mostly, I d just like to thank him for putting up with me and my tears. I d like to thank my grandparents, Charles and Betty Stolte, for their financial and emotion support. They never gave up on me, so I promised not to give up on myself. And finally, I d like to thank my family, Todd, Penny, Wes, and Wade, for encouraging me to never back down. I d like to thank my parents for enabling me to go to my (expensive) dream school and for being proud of me even when I had a hard time being proud of myself. iv

5 Table of Contents Page Abstract.iii Acknowledgements...iv List of figures vi Introduction..1 Geologic background...1 Methods and materials...6 Results...8 Discussion.8 References 11 v

6 List of Figures Page 1. Regional map of California.4 2. Local geologic map Methods of obtaining detrital zircons Diagram of laser ablation ICMPS 8 5. Relative probability chart of all samples..10 vi

7 Introduction Convergence along California occurred during the Cretaceous and the terrane assemblage formed represents a classic example of the convergent margin tectonic setting. The assemblage created is preserved in California today and is useful for determining the way that similar convergent margins have formed and evolved. Subduction creates three main formations that are typical of convergent margins. The accretionary prism is created closest to the convergent margin and a magmatic arc is formed farther away from the margin. Between these formations is the forearc basin, where sediments eroded off of the magmatic arc collect. The accretionary prism is typically composed of mélange. In California the accretionary prism is represented by the Franciscan Complex. The forearc basin is represented in California as the Great Valley Group. The magmatic arc is a zone of upwelling magma that creates magmatic plutons. The Sierra Nevadas are representative of the magmatic arc in California. Using detrital zircon geochronology to determine the age of the sediments will aid in determining the sense of slip of the Sur-Nacimeinto Fault. The Sur-Nacimiento Fault formed during subduction along the California coast and juxtaposes the magmatic arc with the accretionary prism. Determining the sense of slip along this fault will give valuable information as to how the California convergent margin deformed as subduction was occurring. The Sur-Nacimiento Fault is likely a thrust fault formed during subduction that underthrust the Mesozoic forearc below the developing arc. Geologic background The California convergent margin is representative of a Cretaceous convergent margin assemblage. The Franciscan Complex, Great Valley Group, and Sierra Nevada batholith characterize the California convergent margin northeast of the San Andreas Fault. This assemblage is representative of the accretionary prism, forearc basin, and magmatic arc, respectively (Wright, Wyld, 2007). The Franciscan Complex formed as an accretionary prism as the Farallon Plate subducted beneath the west coast of North America during the Cretaceous. Subducted material was heated as it was subducted, released water entrained in minerals structure, and wet melting in the mantle caused the 1

8 formation of magma. This melted material subsequently rose to the surface and formed the Sierra Nevada batholith. The sediment of the Great Valley Group, between the Franciscan to the west and the Sierra Nevadas to the east, formed due to erosion of the Sierra Nevadas. The Franciscan Complex is represented in the area of the Sur-Nacimiento Fault as the Franciscan mélange. This mélange consists of greywacke, argillite, chert, greenstone, serpentinite, and some limestone, among other material. The Great Valley Group is composed of clasts formed from the granitic plutons of the Sierra Nevada batholith as well as clasts from the metamorphic foothills. In some places the Great Valley Group shows evidence of turbidites, as it overlies the Coast Range ophiolite. The Sierra Nevadas are composed of granitic plutons formed during subduction in the Cretaceous (Fig. 1). The Sur-Nacimiento Fault is located to the east of the San Andreas Fault in California. The Sur-Nacimiento Fault separates the Salinian block to the east and the Nacimiento block to the west. The Salinian block consists of rock type that is consistent with igneous and metamorphic rocks of the Sierra Nevada batholith (Dickinson, 2005). The rock type that forms the Nacimiento block to the west of the Sur-Nacimiento Fault is consistent with the mélange that forms the Franciscan Complex (Dickinson, 1983, 2005). The juxtaposition of the Salinian block and the Nacimiento block means that the Sur- Nacimiento Fault moved in such a way that the Great Valley Group has almost been completely removed. Slip along the Sur-Nacimiento fault occurred during the late Cretaceous and has been reactivated during Cenozoic deformation, so determining the sense of slip during the Cretaceous has been difficult. Slip along the Sur-Nacimiento fault occurred during the late Cretaceous. Determining where the Great Valley Group was formed, through the use of detrital zircon geochronology, will help determine which of the three predominant models best explains the slip along the Sur-Nacimiento Fault. The first model studied suggested that the Sur-Nacimiento Fault was a sinistral strike slip fault. Slip along the fault would have occurred in the Cretaceous prior to any slip along the San Andreas Fault. Samples supporting this theory would show age signatures similar to that of rocks with miogeoclinal origins. Sinistral slip along the Sur- Nacimiento Fault would have brought the Nacimiento block south of where it lies today. 2

9 Then dextral slip along the San Andreas Fault would have brought the Salinian block and the Nacimiento block back to their present day location after faulting had halted along the Sur-Nacimiento Fault (Dickinson, 1983). The second model suggests that the Sur-Nacimiento Fault was a dextral strike slip fault with origins in Mexico in the Oaxaca region. Samples supporting this theory would show a significant Grenville age peak on a relative probability chart with the peak just before the 1 Ga mark (Wright, 2007). The third model supports the theory that the Sur-Nacimiento Fault was a thrust fault that thrust the Sierra Nevadan batholith over the Franciscan complex, thereby removing the Great Valley Group. This would have occurred near the present-day location of San Diego, California. The samples would show miogeoclinal origins and the relative probability chart would show Grenville peaks just above 1 Ga and other peaks around 1.8 Ga. After thrusting along the Sur-Nacimiento Fault, the Nacimiento block and the Salinian block would have been transported dextrally to their present-day location by the San Andreas Fault (Hall, 1991). In order to determine the slip sense along the fault, samples of rock were collected from the Nacimiento block. The samples were processed to extract detrital zircons. In order to get accurate and abundant zircons from the rock, these samples were taken from a small section that was consistent with sediment from the Great Valley Group, though found on the Nacimiento block of the Sur-Nacimiento Fault, and consisted of quartzite clasts and sandstones. Samples also had to be collected from a formation of rocks that would have formed before slip along the fault. The formation containing the samples is called the Toro Formation near Atascadero, California (Fig. 2). The Toro Formation is the lower part of the Great Valley Group stratigraphically and has yielded fossils from the early Jurassic and the late Cretaceous (Seiders, 1983). This means that it was formed before slip along the Sur-Nacimiento Fault, so dating these rocks will give a better determination of where they were deposited prior to slip. The Toro Formation consists of sandstone and mudstone with lenses of pebble to cobble conglomerate with clasts of chert, quartzite, mudstone, and sandstone (Seiders, 1983). It also rests on a thin layer of radiolarian chert that dates to the Tithonian and this layer of chert lies on top of the Coast Range ophiolite (Seiders, 1983). 3

10 Figure 1. Regional map of California depictingthe rock types of a typical subduction complex. Franciscan mélange represents the accretioanry prism, the Great Valley Group represents the forearc basin, and the Sierra Nevada Batholith represents the magmatic arc. 4

11 Figure 2. Local geologic map of the area between Morro Bay, California and Atascadero, California where rock samples were collected. 5

12 Methods and Materials Samples of rock were collected from the Toro Formation near Atascadero, California through the use of rock hammers and sledge hammers. Each sample was noted and photographed. The three samples were quartzite clasts collected from conglomerates. These were chosen because of the abundance of zircons and reliability of zircons collected. In order to extract the detrital zircons from the rocks, the rocks must be crushed to sand size, or smaller, particles. Before the crushing process could occur, the area where the crushing was to be performed was cleaned thoroughly as well as the materials themselves. This was done to prevent contamination from other stray zircons. The actual crushing of the rocks was performed through the use of a stainless steel mortar and pestle (Fig. 3). During the crushing process the rocks were put through a 300-micron sieve to separate the smaller particles. These samples were labeled in beakers and stored in a clean area. Sample J11212A2-1 yielded approximately 150 ml, sample J11212A2-2 yielded approximately 150 ml, and sample J11212A2-4 yielded approximately 150 ml. These crushed samples contained more sand sized rock particles than zircons, so they had to be further processed to concentrate the zircons. First, the samples had to be panned to rid the samples of very small dust-sized particles as well as to remove a lot of the extraneous material (Fig. 3). This was done through the use of gold pans and water. Because zircons have a higher density than the rock they are contained in, the zircons sink and excess minerals are removed with the wastewater. This still yielded fairly large samples, however. Because these samples still contained a large portion of other minerals, further reduction was necessary. This was done through the use of heavy liquids (Fig. 3). Lithium metatungstate (LMT) was used because it is has a higher density than quartz and feldspar. However, because the zircons are so dense, they sink through the LMT while the other rock material floats toward the top. This makes it very easy to decant the unnecessary material and retain the important zircons. Other particles in the rock could have a density similar to the zircons, such as magnetic minerals so the next step in the process is to remove the magnetic material. The small samples containing zircons and magnetic material were processed through an LB-1 Barrier Frantz electromagnet to separate the magnetic material from the zircons. Finally 6

13 the samples contained more zircons than other materials and were ready to be mounted. The samples were mounted in epoxy in order to put them through the laser ablation. Before the samples could be put in the laser ablation, they had to be polished to expose the zircons more fully to remove chances of lead loss contamination. The samples were then mounted in the laser ablation inductively coupled plasma mass spectrometry (ICPMS) and were shot with a laser to measure the levels of 206/207-lead in relation to 238-uranium for the purposes of dating the samples (Fig. 4). Figure 3. Methods of obtaining detrital zircons. A) Stainless steel mortar and pestle for grinding rocks. B) Panning materials to separate unnecessary rock material. C) Lithium Metatungstate used for heavy liquid separation. 7

14 Figure 4. Representative diagram of the laser ablation inductively coupled plasma mass spectrometer (ICPMS). Results The sampled quartzite clasts were collected from conglomerates found along a stream bed. The conglomerates were about 1 meter in size, but the clasts themselves were approximately 10 cm in size. The conglomerates were matrix supported conglomerate. The results of the laser ablation ICPMS were put into relative probability charts to better show the age signatures of the arenite quartzite clasts collected. The clasts Sample J11212A2-1 showed a significant peak at 1.8 Ga and less significant peaks around 2.8 Ga, 3.0 Ga, and 3.8 Ga. Sample J11212A2-2 yielded an age signature with peaks at approximately Ga and 1.8 Ga. Sample J11212A2-4 resulted in a relative probability chart that showed age signature peaks around 1.1 Ga, 1.4 Ga, and 1.8 Ga. Discussion The three models suggest that the Great Valley Group and the Franciscan complex formed at a certain location that can be identified through the age signature of the rocks sampled. If the rocks were derived Mexico, the Grenville peak in the relative probability chart would have been slightly under 1.0 Ga, though it is shown that the Grenville peak for these rocks is slightly older than 1.0 Ga (Fig. 5). This inconsistency 8

15 reveals that the rocks were not likely to have been transported from that region. If the rocks were eugeoclinal, there would be a peak around Ga shown on the relative probability chart. These samples, however, do not show a peak at that mark and so we can discount the eugeoclinal theory of massive sinistral slip along the Sur-Nacimiento Fault (Fig. 5). By comparing the relative probability chart derived from the samples taken near Atascadero, CA and other relative probability charts taken from previous research, it is evident that the samples yielded dates similar to miogeoclinal origins. Miogeoclinal origins suggest that there would be a Grenville peak slightly above 1.0 Ga, which is what is seen in the relative probability chart taken from the samples. Another indication of miogeoclinal origins is a peak in the relative probability chart just before 2.0 Ga at around Ga. This peak is shown in the relative probability chart taken from the samples collected (Fig. 5). The miogeoclinal theory suggests that the Sur-Nacimiento Fault was once a thrust fault that was located in the present day location near San Diego, California. Thrusting would have occurred during the late Cretaceous and ended before slip along the San Andreas Fault occurred. This new juxtaposition of Franciscan mélange against Sierra Nevadan batholith was then carried north through dextral slip along the San Andreas Fault to its present day location near Big Sur, California. 9

16 Figure 5. Relative probability chart showing the age signatures of the quartzite samples collected, as well as the sandstone collected from the same area. Locations for the distinct age signature peaks are indicated.

17 References Barbeau, D.L., U-Pb detrital-zircon geochronology of northern Salinian basement and cover rocks. Accessed April 17, Dickinson, W.R., Cretaceous Sinistral Strike Slip Along Nacimiento Fault in Coastal California: The American Association of Petroleum Geologists Bulletin, v. 67, No. 4, p Dickinson, W.R., Ducea, M., Rosenberg, L.I., Greene, H.G., Graham, S.A., Clark, J.C., Weber, G.E., Kidder, S., Ernst, W.G., and Brabb, E.E., Net dextral slip, Neogene San Gregorio-Hosgri fault zone, coastal California: Geologic evidence and tectonic implications: Geological Society of America Special Paper 391, p Hall, Clarence A., Geology of the Point Sur-Lopez Point region, Coast Ranges, California: A part of the Southern California allochthon. Geological Society of America, Special Paper 266, p Jacobson, Carl E., Late Cretaceous-early Cenozoic tectonic evolution of the southern California margin inferred from provenance of trench and forarc sediments. Accessed January 27, Seiders, Victor M., Correlation and provenance of upper Mesozoic chert-rich conglomerate of California: Geological Society of America Bulletin, v. 94, p Wright, James E. and Wyld, Sandra J., Alternative tectonic model for Late Jurassic through Early Cretaceous evolution of the Great Valley Group, California. Geologic Society of America, Special Paper 419, p