Interpretation of Anomalous and Previously Unmapped Fransican Complex in the Gualala Block, northern California Coast.

Dave Allderdice, Class of 2000

Department of Geology, Whitman College, Walla Walla, WA 99362

There are many fascinating puzzles that arise in geology. I have had the privilege to research one of these puzzles over the last summer and hope to turn it into a scientifically significant study and formal honors thesis. The Gualala Block in northern California is unique terrain bounded by the Pacific Ocean and the San Andreas Fault (SAF) to either side. It is rides on the Pacific Plate, while the rest of California is part of the N. American Plate. The Franciscan Complex is the mélange of rock that makes up the majority of California and all rock to the E. of the Gualala Block. Franciscan Complex has never been formally mapped in the Gualala block and is not considered any part of the common stratigraphy of that area. The Gualala Block originated hundred of miles south of the Franciscan and the block has only been brought to where it lies now by millions of year of movement along the San Andreas Fault. So why on earth was I able to find significant outcrops of in place Franciscan rock on the West side of the SAF and within the Gualala Block? I think this is a very good question, and one whose answer could lead to many new ideas about the SAF, the Franciscan Complex and the local geology of the area. My Thesis will address, the origin and mechanism of emplacement for the Franciscan rocks found in the Gualala Block through field and lab work. This involves the mineralogy of the different types of rocks sampled, their relationship to each other as well as applying the current body of knowledge on SAF and other local geology, understood geologic processes and fundament theories, to the question posed. Through these avenues, I hope to unravel the puzzle of the anomalous Franciscan rock found in the Gualala Block.

There are three main theories that could explain the origin of the Franciscan rocks in the Gualala Block. The first is that over the past few million years, the main trace of the San Andreas Fault has been jumping inland periodically. This may have allowed the fault to capture the Franciscan rock between the old trace and the newly formed inland trace and incorporated it into the Gualala block. The second theory is that an offshore and poorly understood terrain know as the Vischaino Block is though to have Franciscan Complex as it's Basement and it is believed to come on shore in the exact area of my field locality. This may be what I'm seeing. Thirdly, there may be underground fault structures such as flower structures that have carried this Franciscan from the East side of the fault via thrusting to the West side where we now observe it. All of these theories have support for them as well as ideas that could completely discount them. This is where my research comes in.

There are some main tools I will use to figure out the best theory on the origin of the Franciscan rocks in question. Mineralogical and petrologic study of thin sections made of the rocks will give an area of origin within the larger Franciscan belt of rocks that spans over 300 miles of the California Coast. An important tool for me to use here will be K-feldspar staining of these rocks. I will also study maps and air photos of the area to understand structural control that might be influencing the placement of Franciscan rock. Thirdly, I will research the literature and consult scientist who have helped me on this project to come to some conclusions about the data I've collected. In this scientific manner I hope to come to some definitive answer about the origin of the Franciscan Complex found in the Gualala Block.

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