Brief Summary of the Geology
of Baja California
by
John Minch, Edwin Minch, and Jason Minch

1. Geologic Provinces of Baja California

BAJA CALIFORNIA can be divided into 5 provinces based on geologic landforms. They are:

1. the tilted granitic fault blocks of the main mountain ranges,
2. the broad flat coastal plains,
3. the isolated coastal mountains,
4. the fault block mountains and alluviated valleys of the Basin and Ranges, and
5. the plateaus of the Volcanic Tablelands.

The first three provinces are part of the Cretaceous collision of the North American and the Pacific Plate. The last two provinces are directly related to the opening of the Gulf of California.

1. The tilted granitic fault blocks are represented by the Sierra Juarez and the Sierra San Pedro Martir which form the main granitic ranges of the State of Baja California and the Sierra La Laguna of the Cape region mountains. The main rock types in this province are granitic, metamorphic, and metavolcanic.
   
   
2. The broad flat coastal plains are represented by the Vizcaino and Magdalena Plains in Baja California, Sur. This province is similar to the Great Valley of California with up to thirty thousand feet or more of sandstone, shale, and conglomerate deposited in a basin in a broad belt in the subduction zone.
   
   
3. The isolated coastal mountains form the mountains of Cedros Island, the Vizcaino Peninsula and the Magdalena Bay islands. It represents the scrapings from the sea floor of the Pacific Plate and mantle rocks (serpentine) which have been deeply buried, metamorphosed and brought back to the surface.
   
   
4. Fault block mountains and alluviated valleys form the Basin Ranges of the Gulf of California area east of the main Gulf escarpment. Nearly all of the blocky mountain ranges and alluviated valleys of Baja California belong to this province related to the opening of the Gulf of California.
   
   
5. The Volcanic Tableland is represented by broad plateaus and mesas in Baja California Sur. These rocks were formed as a result of volcanism associated with the passing of the East Pacific Rise under the continent and the beginning of the opening of the Gulf of California.

2. Baja Geologic History and Plate Tectonics

THE CRUST OF THE EARTH is a series of semirigid plates, moving about relative to each other, and "floating" on the mantle. When the plates pull apart they form Ridges or Rises in Oceanic Crust and rifts [Gulf of California] in the Continental Crust. When they push together they form Subduction Zones and Island Arcs. When they slide past each other they form Lateral [strike-slip] Faults.

It is the interaction of the plates, with each other, that is producing the major features on the earth's crust. The vast majority of this motion is very slow and literally occurring before our very eyes on a daily basis. An example is Cajon Pass in southern California. A surveyed point on the railroad through the pass is rising at a steady rate of 16"/100 years. That's 16,000 feet in a million years.

PALEOZOIC - The Quiet Time ( 600 - 230 million years ago )

The early history of Baja California is obscure, very fragmental, and not well documented. We know that what would later become the North American and Pacific Plates were together as part of a larger plate which was moving eastward pushing against other plates, closing an earlier Atlantic Ocean, on its way to a collision with Europe and Africa. The west coast was in the middle of this plate like the present eastern coast of North America is today. This stable edge of the continent was receiving sediments from the continental landmass while limestones were being deposited in shallow continental shelf and slope areas. This added a wedge of sediments to the continent.

The end of the Paleozoic was marked by the formation of the supercontinent of Pangaea which included all of the continents. In Baja metamorphosed remnants of Lower Paleozoic carbonates, shales and sandstones occur in at least one place while Upper Paleozoic rocks have been identified in a number of isolated and scattered areas.

MESOZOIC - The Big Squeeze ( 230 - 63 million years ago )

At the beginning of the Mesozoic the motions changed. As the supercontinent broke up and the Atlantic opened, North America began to move westward pushing against the thinner oceanic crust of the Pacific part of the plate. The thin edge of the continent buckled and the heavier oceanic crust was forced under the lighter continental crust forming a Subduction Zone and Island Arc with accompanying volcanoes.

As the Pacific plate was pushed under the continent, the friction caused melting and the formation of magmas. Oceanic crust was continually pushed into and added to the continent in this subduction zone resulting in uplift and the raising of the magmas closer to the surface. Some of them cooled miles below the surface to form the granitic rocks of the Peninsular Ranges Batholith with its accompanying metamorphic rocks. Some of the magma spewed out on the surface and formed volcanoes with the associated volcanic derived sedimentary basins.

Continual subduction and burial of these oceanic crustal rocks caused low grade metamorphism forming the widespread metavolcanic rocks. As the mountain mass rose, erosion carried much of the debris westward into a trench forming a large sedimentary basin (geosyncline) offshore. The subduction continued for a hundred million years. The fringes of this basin are exposed along the coastline in northern Baja and as a 30,000+ foot thick geosynclinal basin under the majority of southern Baja. The overriding of the East Pacific Rise by the North American plate resulted in the end of the subduction.

CENOZOIC - The Big Split and Rip-off ( 63 million years ago - present )

In the Early Cenozoic, the peninsula was again a relatively quiet place. The Sierras were worn down and a gently rolling erosion surface was developed on the exposed rocks. This surface stretched to the east well into Arizona and Sonora. Major rivers, bearing gravel, flowed across the area from central Arizona to the Pacific Ocean.

The North American Plate overrode the East Pacific Rise and began the great rip-off. Coastal California and Baja California began to slide northward along strike slip faults like the legendary San Andreas Fault in California and the San Miguel Fault, Agua Blanca Fault, Vizcaino Fault, and others in Baja California.

The middle Cenozoic opening of the rift, later to become the Gulf of California, took tens of millions of years. Great sheets of lava and pyroclastic rocks were spread over large areas of the peninsula during the Miocene and Pliocene and shallow Miocene seas spread across low areas of the southern part of the peninsula to fill tectonic basins opening in the Gulf area.

The present shape and form of Baja California was developed in the last 5-10 million years as the continent finally yielded to the stretching and opened successive areas of the Gulf, finally opening the mouth about 5 million years ago. The splitting of the continent tilted the peninsula westward forming the asymmetric fault blocks of the main ranges of the Sierra Juarez, Sierra San Pedro Martir, Sierra la Giganta and uplifting other ranges such as the Sierra la Asamblea and Sierra la Victoria. Coastal California and Baja California will eventually become an Island.