The Neogene Boca basin, located 15 km northeast of Truckee, California, records the depositional and deformational history for the late Miocene–Pliocene period in this part of the northern Sierra Nevada. This study consists of fine-scale analysis of the well-exposed Neogene sedimentary rocks in an otherwise poorly exposed area of the northern Sierra Nevada. The Neogene Boca basin sedimentary section is >500 m thick and dips generally west to southwest. Four distinct lithologic intervals are deposited unconformably over lahars and intermediate lavas of the Miocene Kate Peak Formation. An ~180-m-thick section of conglomerate and conglomeratic litharenite represents a generally southwest directed fluvial system that existed from at least 4.4 Ma (interval I). This is overlain by and locally interfingered with a ca. 4.38 Ma basalt flow of Boca Hill. Above this basalt, an ~107-m-thick section of quartz wacke and siltstone deposits represents a deltaic system controlled by local volcanic topography from ca. 4.4 to 4.1 Ma (interval II). Conformably above interval II, an ~122-m-thick section of silty diatomite deposits with interbedded tephra and litharenite represents a lacustrine environment from ca. 4.1 to 2.7 Ma (interval III). Overlying the diatomite along a disrupted surface, a >91-m-thick section of medium- to coarse-grained litharenite and cobble conglomerate represents an abrupt change in depositional environment, to a west directed fluvial system (interval IV). Pliocene westward tilting and change in base level began during deposition of interval IV (ca. 2.7 Ma) and prior to eruption of the Boca Ridge Formation (ca. 2.61 Ma).
Four orientations of large faults (>0.1 m displacement) are distributed evenly across the basin: (1) northeast to north-northeast striking sinistral faults; (2) northwest to north-northwest striking dextral faults; (3) west to west-northwest striking oblique-reverse faults; and (4) other fault orientations that have apparent motions not included in these categories. Strike-slip faulting is thought to have occurred during tilting of the Neogene section. The distributed conjugate strike-slip faults in the rocks of Boca basin accommodated east-southeast directed extension and south-southwest directed contraction.
These new stratigraphic and structural data provide information on late Miocene–Pliocene deformation at the eastern edge of the Sierra Nevada. The Boca basin appears to have been an isolated basin controlled by volcanic topography. A late Miocene deformation event is not recorded in Boca basin; however, a Pliocene event is recorded in the termination of deposition and deformation of the section through tilting, incision, and distributed faulting. Pliocene deformational style is consistent with generally east-west extension associated with westward encroachment of the Basin and Range or northward migration of normal faults at Lake Tahoe. The structural data cannot disprove migration of Walker Lane deformation into the Sierra Nevada but merely show that this did not occur in the area occupied by the Neogene Boca basin. The Pliocene deformation event coincided with local eruption of high-potassium lavas and a regional base-level change, and it may represent rollback of the Juan de Fuca plate after ca. 3 Ma.
