Evolution of the Sonora Coast

Some 30 million years ago, an old Mid Oceanic Ridge was getting close to the old Sonora coast, subsiding under Sonora.

The compression forces, together with the subsiding plate produced a chain of volcanoes along the whole of the Sonora coast.

 

then, some 25 million years ago, the Oceanic ridge arrived at the edge of Sonora.

When it began subsiding under the Continent, the old compression strains transformed into the opposite, now of tension, forcing Sonora to "stretch" towards the West.

This "stretching" of Sonora towards the coast formed the series of alternate ranges and valleys when the plates fractured, giving Sonora it´s current orography: ranges separated by valleys, all with a direction Northwest - Southwest.

Besides this, a strip of land separated from the continental slab, and began traveling towards the Northwest. This originated both, the Gulf of California as well as the Peninsula.

 

The birth of paralley mountain ranges and valleys:

The following movie shows all of this process, reconstructing the last 36 million years: The separation and emigration of the Lower California Peninsula, the birth of the Gulf of California, and the "stretching" of Sonora in a direction East - West.

 

Now, you can read the Geologic History of the Region of Both Nogales

Or, if there´s some concept you don´t understand, you can read the page on Plate Tectonics