The Early and Middle Proterozoic rocks in the Needle Mountains include three distinct rock sequences (1) multiply deformed bimodal metavolcanic rocks, related sedimentary rocks, and plutonio units, all metamorphosed to medium grade; (2) multiply deformed clastic sedimentary rocks metamorphosed to low grade; and (3) weakly foliated to unfoliated plutonic rocks. Although many important questions remain unresolved, work by a number of individuals suggests the following history. Mafic and silicic volcanics, sediments, and intrusives of the Irving Formation and Twilight Gneiss accumulated at least 1,760 Ma and were multiply deformed and metamorphosed shortly after accumulation. Small post-tectonic plutons of the Tenmile and Bakers Bridge Granites invaded the metavolcanic sequence between 1,680 and 1,700 Ma. The timing of deposition of the conglomerates, clean quartzites, and pelites of the Vallecito Conglomerate and the Uncompahgre Formation with respect to events in the metavolcanic sequence is poorly constrained, largely because shear zones lie along crucial contacts. Recent work suggests that much if not all of this clastic sedimentary sequence may be parautochthonous and was probably deposited sometime after 1,680 to 1,700 Ma. Despite uncertainties in timing of deposition, it is clear that a second event of deformation and metamorphism affected the region, producing polyphase features in both the metavolcanic sequence and the clastic sedimentary sequence, as well as a weak foliation in the Tenmile Granite. The Eolus Granite and related intrusions, ranging in age from 1,320 to 1,450 Ma, crosscut all structural features in the region and provide a minimum age limit for the latest deformation in the region. This younger event is recorded nowhere else in Colorado but may correlate with events in northern New Mexico.

Abstract The Southern Mountain region includes the San Gabriel Mountains which are one of the Transverse Ranges, and the Peninsular Ranges which include the Santa Ana, San Jacinto, Santa Rosa, Vallecito, and Laguna Mountains. The northern and eastern boundary of the region is essentially the San Andreas fault zone, and the southwestern boundary is the contact of Cretaceous-Tertiary sedimentary rocks on the crystalline rocks of the mountain region (Fig. 38). The San Gabriel Mountains consist of a complex series of closely related plutonic rocks comprising part of the batholith of the Transverse Ranges, possibly a continuation of the Sierra Nevada batholith at the north and possibly continuing in the southern California batholith in the Peninsular Ranges toward the south. The principal rock groups in the San Gabriel Mountains are gabbro, diorite, monzonite, anorthosite, and granite. They include numerous fragments of older intrusive rocks and of an older metasedimentary series possibly Paleozoic in age. The range is flanked on the north by a thick series of continental sandstones and gravels interbedded with volcanic rocks, probably Miocene in age. Intensely folded marine Pliocene and continental lower Pleistocene gravels lie on the crystalline rocks along the southwest border of the range. Marine Paleocene Martinez sandstones and conglomerates occur in slivers along the San Gabriel fault. The remaining part of the Southern Mountain region is made up of the Peninsular Ranges in which the crystalline rocks of the great Lower Cretaceous(?) batholith of southern and Lower California are exposed (Fig. 39). The batholith is extremely