The Ricardo Group is a 1,700-m-thick sequence of Miocene volcanic rocks and continental sedimentary rocks deposited between ∼ 19 and 7 Ma in the El Paso basin, near the junction of the Garlock fault and the Sierra Nevada frontal fault in southern California. The combination of stratigraphic and structural data from the Ricardo Group with chronologic and rotational histories derived from magnetostratigraphic and radiometric studies provides some new constraints for the morphotectonic emergence of the Sierra Nevada, the initiation of strike- slip movement on the Garlock fault, and related east-west extension in the southern Basin and Range region. The Miocene sequence documents (1) volcanism and north-south tension without net extension at 18-15 Ma (2) relative uplift about 15-13.5 Ma, (3) the beginning of sinistral slip on the Garlock fault and east-west extension in the southwestern Basin and Range region about 10-9 Ma, and (4) emergence of the Sierra Nevada as a source area and topographic high by 8 Ma.
Stratigraphic evidence from the Ricardo Group does not indicate significant Miocene dip-slip movement along the large-scale, down-to-the-north fault that has been previously hypothesized to have preceded the present Garlock fault in the same location during the early Tertiary. The evolution of this sequence, however, can be explained by tectonic models which propose a time-space linkage between basin evolution, east-west extension in southern California, and the Mendocino triple-junction system.