Point Reyes Peninsula: A Relentless Journey Northward

Point Reyes Peninsula
Richard L. Gunther

April 21, 2004

Professor James S. Aber

ES 767 Global Tectonics


Point Reyes Peninsula is a unique piece of continental crust attached to western Marin County in northern California. The peninsula is unique because it is vastly different geologically than its mainland neighbor. The reason for the geological differences is because Point Reyes is a long way from its source region. Point Reyes has been traveling for millions of years on the Pacific plate. At this time it is adjacent to Marin county that is part of the North American plate. The boundary between these two plates is the San Andreas fault.

The San Andreas fault is a strike-slip fault that separates Point Reyes Peninsula from Marin County. The fault zone travels through Bolinas Lagoon in the south through Olema Valley and out Tomales Bay in the north. In the famous earthquake of 1906 Point Reyes Peninsula moved approximately 5 meters north relative to Marin County.

The massive earthquake of 1906 and the unique geology of Point Reyes was not fully understood until the 1960's. During the 1960's the theory of plate tectonics became widely accepted. The processes of plate tectonics when applied to Point Reyes Peninsula and California brought a more comprehensive understanding to the processes that formed them.

With the acceptance of global tectonics in the 1960's geophysicists began to look closely at the west coast of North America. They realized that the west coast was actually made up of many large pieces of different sections of continental crust. The pieces of crust were named terranes. The terranes had traveled from various places around the Pacific theater. The first terrane added to the west coast is called the Sonomia terrane. Sonomia was added 375 million years ago. Next came Smartville about 225 million years ago. The Franciscan terrane began the accretion process 175 million years ago. The most current terrane is Point Reyes Peninsula. Point Reyes Peninsula had its origins many miles south of its current location.

Map of Point Reyes Peninsula
Numbered locations of photographs below
Point Reyes Peninsula is trianglular section of crust. Point Reyes extends out into the Pacific Ocean. Point Reyes is resting on the Pacific plate which is moving between 3 to 4 centimeters a year toward the northwest. There has not been a major earthquake in this region since 1906. So, considerable stress must be developing within the crust.

Evidence indicates a slow northward movement of Point Reyes Peninsula

Photographs taken by Rich Gunther in February and March of 2004

Pillow Lava in Marin County(1)
East of the San Andreas fault is an assemblage of rocks called the Franciscan Complex. Shale, chert, graywacke and pillow lava are common members of this complex. To understand how the Franciscan complex arrived in its present location we must look back in geologic time. The complex began forming in the late Jurassic or early Cretaceous approximately 136 million years ago. At that time the Franciscan subduction zone was located off the west coast of North America. The oceanic plate, the Farallon plate, was colliding with and subducting under the North American plate. Part of the Farallon plate was scraped off and then acreted to the North American plate. So, for millions of years the Franciscan complex was added to the west coast. The Farallon plate was slowly consumed by the subduction process. Eventually, the North America plate moved over the East Pacific Rise and the Pacific plate jammed the Franciscan subduction zone. With the plugging of the subduction zone a new boundary began to form. The Pacific plate was moving in a northwest direction. The Pacific plate captured a section of continental crust which sheared from the North American plate. The new boundary that formed between the plates was a transform boundary. This transform boundary is today's San Andreas fault.

Granite on Inverness Ridge(2)
The basement rock of Point Reyes Peninsula is granite. Most of the granite is buried beneath layers of sedimentary rock. However, the granite is visible along Inverness Ridge from Mount Whittenberg northward to Tomales Point. Granite is also visible at Sea Lion Overlook at the Point Reyes Headlands.

The granite is part of the large Salinian block that formed during the late Cretaceous. The Salinian block had its origin in the Franciscan subduction zone. One hundred million years ago the Farallon Plate was being carried down into the Franciscan subduction zone. The subducted material melted forming magma. The magma intruded upward into the crust but did not reach the surface. So, the magma cooled slowly forming large crystals. The age of the Salinian Block is 83 to 86 million years old. The Salinian block contains large distinctive crystals of feldspar, quartz, hornblende and mica.

Granite on Point Reyes Headlands(3)
The present location of the Salinian block of granite is displaced a considerable distance north of its site of formation. Geologic evidence points to a previous connection with the Tehachapi Mountains in Southern California. The Tehachapi's are located 300 miles south and east of the San Andreas fault. Point Reyes lies to the west of the San Andreas fault. This means that this portion of Salinian block traveled 300 miles northward along a right lateral fault to its present location.

Point Reyes Lighthouse(4)
The lighthouse on the Point Reyes headlands is built upon a formation of rock called Point Reyes Conglomerate. The Point Reyes Conglomerate rests atop the granite basement. The conglomerate is a mixture of sand, gravel and pebbles that was laid down on the seafloor between 50 and 60 million years ago. Evidence suggests tht the assemblage formed by turbidity currents. Turbidity currents occur when sediment quickly tumbles down the continental slope. Tumbling sand, gravel and pebbles flow until it reaches the deep seafloor. This rapid motion shapes and sorts the material by size and mass. The large pebbles stop first, the medium gravel second and finally, the fine grain sands. Layered beds of Point Reyes Conglomerate are easily observed along the walk leading down to the lighthouse.

Point Reyes Conglomerate(5)
Ninety miles south of Point Reyes is Point Lobos Marine Reserve that is close to Carmel in Monterey County. Point Lobos contains Carmelo Formation that is also a type of conglomerate. Point Reyes Conglomerate and Carmelo Formation share a similar assemblage of rocks. Within both assemblages is a special type of volcanic rock. This volcanic rock formed 140 million years ago in the region that is today the Sierra Nevada. The volcanic rock was eroded and deposited on the seafloor in the Carmel area 50 to 60 million years ago. This geologic evidence supports the idea that Point Reyes must have been in the Carmel region at that time of deposition. If that evidence is accurate, Point Reyes traveled along the coast of California a distance of 90 miles in the last 50 to 60 million years to reach its present location adjacent to Marin County.

Hugging the coast between Carmel and the southern tip of Point Reyes is the San Gregorio Fault. It appears that Point Reyes peninsula moved northward along this fault to reach its current position.

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Views of the San Andreas Fault zone

Bolinas lagoon looking north(6)
This photograph was taken on the westside of Bolinas Lagoon looking north. The hills to the right are located on the North American Plate. The San Andreas fault zone is under Bolinas Lagoon. The tall trees on the left are growing into the Pacific Plate. The valley in the distance is were the San Andreas fault heads north into Olema Valley.

Olema Valley(7)
The San Andreas fault zone passes through this valley. To the west of Olema valley the Inverness Ridge can be seen in the distance.

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Earthquake walk in Olema Valley(8)
Blue posts on the hillside mark the ground rupture that occurred in the 1906 earthquake. The rupture extended through a portion of the barn that can be seen in the distance. Displacement in this area was approximately 5 meters.

Strike of 1906 Earthquake
Old photograph showing the ground rupture that occurred in Olema Valley in 1906. Ground rupture measured 250 miles in the 1906 earthquake.

Tomales Bay looking south(9)
Tomales Bay is the location of the San Andreas fault zone to the north of Olema Valley. The hills to the left are on the North American plate. The coastline to the right is on the Pacific Plate.

Island of California ca.1650
Library of Congress
The cartographer who made this sketch ca. 1650 probably had the right idea but was off a few million years. Early cartographers sketched California as an island. Sailors moving northward along the west coast of Mexico would have encountered the opening into the Sea of Cortez. Sailing westward they would have come to Baja California. Not realizing they had discovered a very long peninsula the early cartographers sketched an island.

In a few million years the portion of California west of the San Andreas fault may be an island. The relentless drive of the Pacific plate will carry part of California towards Alaska. Perhaps California will be the next exotic terrane to be added to Alaska.


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