One of the things I ran across putting together the posts on recent volcanic activity in Nevada was something called the Walker Lane. This is defined as a series of transform faults working its way north from the Salton Sea in southern California parallel with the California – Nevada border into southern Oregon.
Several papers describe the Walker Lane as an incipient rift in North America, taking perhaps 20 – 25% of the total movement of the Pacific Plate northward. The San Andreas Fault takes the rest of the movement.
Any time you have a rift, you also have a weakness in the continental crust that allows magma to reach the surface. Combine this with a mantle plume / hot spot, and you end up with a Large Igneous Province (flood basalt). Happily, while there has been volcanic activity along the new rift, some of it significant in volume, that activity has not expressed itself as a LIP, though there are indeed volcanic fields along the new tectonic field which I will describe in the next posts.
But first, let’s take a look at what is going on tectonically in eastern California. The next section will review North American rift systems.
North America is home to several rifts, some complete, some failed, and at least one active. The complete rift is called the Eastern North America Rift Basin. It is a series of aborted rifts created some 220 Ma where Pangea started rifting between NE North America and NW Africa. There is no new activity along this rift system.
The Midcontinent Rift System is much older at 1,100 Ma. It is a failed rift, though it does express some geologic activity in the New Madrid fault system in the central US.
We previously discussed the Rio Grande Rift system, another failed rift in posts on volcanic activity in New Mexico. It dates 35 – 29 Ma and supports significant intraplate volcanic activity. https://volcanocafe.wordpress.com/2014/01/21/the-raton-clayton-volcanic-field/
The active rift in North America is working its way up from Mexico, through the Salton Trough where it splits, the western branch becomes the San Andreas Fault and the eastern branch in the process of forming a rift connecting numerous faults in the Walker Lane. The Sierra Nevada Micro Plate and Sierra Nevada sit between these two active rift arms.
The tectonics of western North America was driven by the subduction of the Farallon Plate beneath it starting some 165 Ma. While the west to east subduction was going on, the plate delivered a number of terranes onto western North America, the last delivered perhaps 50 Ma. The Cocos Plate and Juan de Fuca Plate are remainders of the Farallon Plate.
The subducted Farallon Plate traveled under North America a significant distance as flat plate subduction before resuming its downward motion perhaps 38 Ma, fueling a significant caldera / ignimbrite outbreak from Nevada to Colorado lasting until 18 Ma. https://volcanohotspot.wordpress.com/2016/10/18/the-utah-nevada-ignimbrite-flareup-of-30-ma/
The Pacific Plate followed the Farallon into North America, though its motion was different. Where the Farallon was a subduction impact, the Pacific was moving generally to the NNW, so its impact transitioned to a transverse impact. Once the subduction push stopped, the western portion of North America relaxed and the Basin and Range Province started its formation.
The boundary between the Pacific and Farallon Plates was a spreading center called the East Pacific Spreading Center / East Pacific Rise. It arrived at North America some 45 Ma and extension in the vicinity of the spreading center began some 10 Ma later. This trough finally created the Baja California rift some 5.6 Ma. As this spreading center worked its way north, a complex region of faults near its northern end coalesced into the San Andreas Fault system to the west.
There is another complex region of faults to the east of the Sierrian Microplate and the Sierra Nevada in the process of coalescing into a parallel transverse fault system. This is the Walker Lane, and it is working its way north into Oregon. Today, the strike – slip motion of the Pacific Plate against North America is divided between the two regions. 75% of it is carried by the San Andreas Fault System. The rest of it by the Walker Lane.
Baja California has moved 300 km NW from mainland Mexico over the last 7 Ma. The San Diego – LA – Santa Barbara region of southern California is moving north and will be abeam San Francisco in 12 Ma. Whether or not portions of North America between the two rift systems is likewise carved off and transferred to the Pacific Plate remains to be seen, though some geologists believe this is likely.
The region was initially called The Eastern California Shear Zone and began on the margins of the Mojave Desert. Actual displacement along the system began 10 – 6 Ma. All faults are active with earthquakes into the M 7+ range being recorded.
The Southern Walker Lane is estimated to have offsets in the range of 40 – 100 km between the two sides of the lane. This movement is expressed through the actions of multiple faults in the region. By contrast, total movement along the San Andreas system since its formation is no more than 150 km and perhaps less than 100 km. The southern region of the Lane includes Death Valley.
The Central Walker Lane is described as a block. Its total offset is estimated at 48 – 60 km over the last 10 – 15 Ma. This region has significant historic volcanic activity over that period.
The Northern Walker Lane includes at least two blocks, one of which has rotated over the last 9 – 13 Ma. Large earthquakes are also possible with movement being somewhat less than that of southern regions. The slip rate of northern segment is being accommodated by rotation of crustal blocks bounded by strike – slip faults.
Finally, there is a poorly defined Northern California Shear Zone that extends northward into the Lassen and Shasta volcanic centers of the Cascade Volcanic Arc.
Once you get deep, active faulting coupled with block rotation, you open avenues for magmas to reach the surface. When that region is at the boundary of an active extension province like the Basin and Range, the stage is set for significant volcanic activity.
Sierra Nevada Microplate
There are at least two calderas and a recently discovered stratovolcano associated with the growth of the Walker Lane, the calving of the Sierra Nevada microplate, and the extension associated with the Basin and Range Province. These include the Little Walker Caldera, 11 – 9 Ma, the Markleyville Center, 6 Ma, and the recently identified Ebbetts Pass stratovolcano at 6 – 4 Ma.
The Little Walker caldera was formed at the same time the Sierra Nevada microplate was formed, some 11 Ma. The other two volcanic features were formed during a subsequent volcanic pulse 7 – 6 Ma. Location of the volcanic features was controlled by faults which allowed the magmas to reach the surface.
Regional faulting along the eastern face of the Sierra Nevada began 16 – 15 Ma due to Basin and Range extension. It was the weakest of three pulses and took place in a region of thermal uplift. The second phase took place 11 Ma and was immediately followed by extensive volcanism. This phase marked the creation of a future plate boundary along the eastern margin of the Sierra Nevada. The final pulse took place 8 – 7 Ma. It reactivated some faults and offset volcanoes.
I will discuss the volcanic fields in Part 2.