PLATE TECTONICS and Marin County

In the early part of this century, the German geographer Alfred Wegener began to assemble evidence which supported the idea that the continents of the world were not fixed, but rather that they moved and drifted over the surface of the earth over the course of time.

It was not until the mid-l960's, after the discovery of a magnetic striping preserved in the oceanic crust, that the idea of continental drift became widely accepted in the United States. It is now accepted that the earth's surface consists a number of variable-sized plates which float on a plastic interior zone and interact with each other. Not surprisingly, the boundaries between these plates are very geologically active areas. The US Geological Survey has a nice description of the concept of plate tectonics and their National Earthquake Information Center has a nice animation of the Earth's plate boundaries.

Fundamentally, there are three types of plate boundaries. The first is called a spreading center or divergent boundary. Here, along great unhealing wounds in the earth's surface, heated subsurface material wells up, cools and is added to the edges of the plates, which move away from one another on opposite sides of the spreading center. Spreading center boundaries are found today at the Mid-Atlantic Ridge, in the Gulf of California which separates Baja California from mainland Mexico and under Africa in the East African Rift Zone. Spreading center boundaries are the important in shaping the earth's surface and are the locations from which oceans are born. Legs 100-158 of the Deep Sea Drilling Project  included several investigations of the proposed ancient continental breakup zone in the Atlantic and provided valuable information on this type of boundary and other deepsea features. This effort continues today as part of the JOIDES Ocean Drilling Program. Some of the achievements of this effort are summarized here.

The second type of boundary occurs where two plates collide. When an oceanic plate collides with a continental plate, because the oceanic plate is denser and thinner, it bends down and is moves beneath the edge of the continent. Geologists call this process subduction. Major subduction zones exist today off the western Pacific coast from Cape Mendocino in Northern California to British Columbia, along the west coast of South America, most of the rest of the margins of the Pacific Ocean. Although this type of zone does not currently exists off the coast of California, except north of Cape Mendocino (Here is Humboldt State University's diagram of their local area, the tectonic setting and resultant earthquakes), this type of boundary has in the past played a significant role in the history of Marin County's rock record.

The San Andreas fault which today runs through the bay area and western Marin County represents the third major type of plate boundary, one in which the plates slide by one another along a major fracture system. These boundaries are called a transform boundaries.

The geologic record left by any of these particular types of boundaries may be preserved in the rock record even though that type of boundary may not play a role in the area's later geologic history. Thus, whereas the rock record of Marin County and the Franciscan rocks of California show signs of an initial origins at a sea-floor spreading location, there are clear indications of a subduction zone playing a role in what we see in the county today. In Marin and the area now occupied by California Coast Range, those processes are but distant memories contained in the rock record of these areas. But plate tectonics continues to play a role in the County's geology since the transform San Andreas fault system (the main branch of which lies approximately ten miles west of Ring Mountain) and the others of the system (Bolinas Mesa-San Gregorio-Sur/Nacimento; Hayward and Calaveras Faults) now dominate the area tectonically.

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