Summary

200 Ma, on: Subduction of the Farallon plate resulted in arc volcanism that produced a range of mountains along the west coast of North America. (These will eventually erode, and their batholiths will be exhumed and form today’s mountains and the eroded sediment will accumulate in basins forming soil and lithifying into sedimentary rock.)
20 Ma. As the Farallon plate completed its subduction, it broke into microplates, and
(1) SAFS. the East Pacific Rise subducted initating the transformation of a convergent margin into a transform margin and the San Andreas Fault system, and
(2) Cabo volcanoes. continued subduction of the microplates produced volcanism that can be seen around Cabl
~17 Ma. Complex tectonics, due to the SAF system, that I don’t really understand, generated the basin and range system farther north, and this in turn is thought to have channelized the stress that eventually opened the gulf of California.
~15 – 6 Ma. Tectonics lead to capture of Baja by Pacific plate, transitional shearing in the proto-Gulf of California, and at 6 Ma the beginning of the ‘unzipping’ of Baja.
~3.5 Ma. Development of spreading centers in the Gulf of California, and extrusion of mostly basaltic oceanic crust.

Geomorphology of Baja

What is visible to the eye”

San Jose del Cabo Fault: the margin along the mountains (the Lagunas) to the west of San Jose del CAbo.
San Jose del Cabo Basin: Flat area with alluvium that stretches from the Lagunas to the Trinidads (includes airport, SJdC, and Rio San Jose)
Cabo area volcanoes (~23 Ma): Due to Miocene volcanism resulting from subduction of Guadalupe and Magdalena microplates.
Gulf of California (~6 Ma). Begin unzipping at about 6 ma due to shearing that was produced by the San Andreas Fault system and possibly localized by the tectonic activity in the Basin and Range province to the north. You can see the ‘shadow’ of Baja on the coast of mainland Mexico to the south.
Gulf of California Spreading Centers (~3 Ma). In the seafloor there is an array of en enchelon transforms that link to a system of narrow spreading centers. The Gulf’s southernmost spreading center is the Alarcon rise, which is connected to the remnants of the East Pacific Rise by the Tamayo Fault Zone.
Riveria TPJ beyond the mouth of the Gulf. The Riviera triple point junction (Pacific plate, North American plate, Rivierar microplate) is ~ 100 miles south by south east of the mouth of the Gulf of California.

Q&A

Geology of the Gulf of California Seafloor and the East Pacific Rise

The presence of this young oceanic crust along the outer wallof the trench calls into question the evolution of the EastPacifc rise termination. This new information suggeststhat the East Paci®c rise reached the trench and possibly subducted beneath western Mexico, prior to the devel-opment of the large overlapping system which nowconnects the Rivera transform and the East Paci®c rise.Consequently, the shape of the Rivera plate subductionzone, the deformation of the upper plate, as well as theunusual magmatism of the western Mexican mainlandmay be related to the subduction of the East Paci®c risebefore it connected with the Rivera transform
(17) (PDF) Did the East Pacific rise subduct beneath the North America plate (western Mexico)?.
— Available from: https://www.researchgate.net/publication/225712142_Did_the_East_Pacific_rise_subduct_beneath_the_North_America_plate_western_Mexico[accessed Dec 09 2022].

Just south of the mouth of the Gulf of California, the East Pacific Rise changes from a series of spreading centers separated by transform faults to a series of transform faults separated by small spreading centers. The transform faults form a zigzag pattern below the seafloor of the Gulf. At the northern end of the Gulf, a single large transform fault continues up toward the northwest, where it connects with the San Andreas Fault system.

The East Pacific Rise is a “divergent” plate boundary, where huge slabs of the Earth’s crust (plates) are moving away from each other. Near the Gulf of California [at the Riviera Triple Junction?], the relative motion of the plates changes so that the two plates (the North American Plate and the Pacific Plate) are moving sideways, sliding past one another. It is this “sideways” motion that has created the many transform faults in the gulf, as well as the San Andreas Fault System in California. The combination of sideways motion and spreading also opened up many deep, elongated basins in the seafloor of the gulf.
– The Geologic Setting of the Gulf of California — https://www.mbari.org/news/the-geologic-setting-of-the-gulf-of-california/

Q: Why did Baja separate so quickly?

In the southern Gulf of California, sea-floor spreading commenced only ~6–10 m.y. after the formation of the oblique-divergent plate boundary at ca. 12.5 Ma. Three main factors caused this rapid rupture: (1) an inherited long, narrow belt of hot, weak crust from a volcanic arc that was active immediately before formation of the oblique-divergent plate boundary and that lay between two strong batholith belts; (2) relatively rapid plate motion resulting in high strain rates; and (3) a dominant role of strike-slip faulting in the highly oblique-divergent setting that formed large pull-apart basins with rapid and focused crustal thinning in a linked en-echelon system.
– GSA Today — https://www.geosociety.org/gsatoday/archive/21/11/article/i1052-5173-21-11-4.htm

Geological Development Sequence

~200 – 66 Ma: Subduction => Arc volcanism. In Jura-Cretaceous time the Farallan plate subducted beneath the North American plate, leading to
(1) volcanism that formed a range of volcanoes inland from the subduction zone; these stretched across the coastal margin of Mexico and Southern California
(2) creation of an accretionary prism scraped from the top of the Farallon plate
- ~66 – 23 Ma: Erosion/buoyancy=>batholith exhumation and a layer of sediments.
- `~20 Ma: Farallon subduction finishes and SAF system begins
  (1) remnants of Farallon fragment into micro-plates (Juan de Fuca; Guadalupe & Magdalena; Riveria & Cocos; Nazca) and
  (2) East Pacific Rise subducts, altering the convergent margin to a transform margin — the San Andreas fault (SAF) system.
  (3)Subduction of Guadalupe & Magdalena microplates => Cabo volacoes.
  acidic volcanism that covers norther Sierra Trinidad range of Los Cabos.
- ~17 Ma: Crustal extension => basin and range province. Back arc extension in the inland of the western US, plus gravitational collapse over over-thickened crust, and the SAF’s relaxation of the compression produced by the Farallon collision lead to extension and formation of the grabbens and horsts that characterize the basin and range province.
  - ~15-12 Ma: SAF Tectonics => Tectonic capture of Baja by Pacific plate.
    As the SAF system spreads north and south, the basin and range activity may have channelized extension in the region of the future Gulf of California, resulting in the detachment of Baja
12.3 Ma: Guadalupe and Magdalena microplates continue to subduct
12.3-7.8 Ma: Microplate subduction ceases and transitional shearing develops in the proto-Gulf as the Baja Microplate continues to move northwest.
~6 Ma: Oblique divergent rifting aligned with SAF begins to ‘unzip’ baja from the mainland.
3 – 3.5 Ma: Seafloor spreading => extrusion of largely basaltic lavas (oceanic crust)
50 Km Alacrcon rise becomes southernmost spreading center for gulf, connected to the remnant of the East Pacific Rise by the Tamayo fault zone. Riviera Triple point junction.