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FIGURE 35. Simplified structure map of the Desert Peak area.

Desert Peak Fault (fig. 35) drops lower Desert Peak Formation on the west against upper Chloropagus Formation volcanics on the east. Displacement on this fault may be as much as much as 1000 feet, based on lithologic data from strat. test no. 4. This fault is significant in that it may be part of the boundary system for the Desert Peak geothermal field. Although the age of this fault is uncertain, it probably postdates the younger volcanic unit.

The Desert Queen Fault (informal name, fig. 35) trends about N25°E, dips 50°E east (where measured), and drops andesite ash flows on the east against lower Truckee Formation limestone on the west. The total displacement at the southern portion of the fault is roughly 100 feet, but displacement increases northward to the vicinity of the Desert Queen Mine, where andesite ash flows on the east are dropped against the unnamed rhyolite unit. Erosion prior to faulting in addition to faulting prior to deposition of the andesite account for some apparent displacement, but the dip-slip is still at least 500 feet. The increase in displacement northward along the fault is supported by detailed gravity work discussed later, which shows a gradual but definite increase in gravity relief northward along the trace of the fault. Because the Desert Queen Fault cuts the andesite sequence, the latest movement must be younger than 2.3 to 4.6 m.y.

Other faults in the southeast portion of the mapped area have displaced the Truckee Formation and older rocks more than the overlying andesite ash flows. The andesite sequence characteristically has been stepped down to the east, usually in increments of 50 to 100 feet. The underlying Truckee, Desert Peak, and Chloropagus Formations have been displaced in a similar sense, but the dip-slip ranges from 100 to 500 feet. Clearly, faulting was initiated prior to deposition of the andesite sequence. Continued similar stress during and after andesite deposition was accomodated along essentially the same fault planes.

The distribution of rocks in the northern Hot Springs Mountains suggests that the area is a highly faulted and tilted fault block which trends north-northeast. Older rocks of the unnamed rhyolite unit are exposed in the central portion of the area and in the vicinity of the geothermal anomaly, and increasingly younger rocks are exposed outward, both to the east and west (pi. 13; Willden and Speed, 1974). Differential movement of blocks has left the area between the Desert Peak Fault (informal name) and the Desert Queen Fault structurally elevated relative to surrounding areas. Blocks are incrementally stepped down to the west and to the east west and to the east (fig. 36).

The second and subordinate fault set occurs as a vaguely defined zone which is first seen in S29,T22N,R27E, trending approximately N60°E to the vicinity of the Desert Queen Mine. Faults within this zone trend from N50°E to N76°E. Most fault traces can be followed or inferred for only short distances, usually about 1 mile, and they appear to predate the basin-andrange faults, although the apparent termination of a few of the basin-and-range faults by N60°E faults implies renewed or concurrent movement on N60°E faults. Unfortunately, traces of the N60°E faults are mostly con cealed beneath alluvium, and the actual relationship between N25°E and N60°E fault sets is inferential at best. Slickensides observed at the north end of Rhyolite Ridge (informal name, pi. 13) rake 52°SE, indicating that at least the last motion of the N60°E faults had an oblique component. Thus it appears that some amount of horizontal shear contributed to the formation of the zone.

Surface evidence suggests that the area in the N60°E zone is a horst. Faults at the southeastern edge of the zone have moved down on the south, whereas faults along the northwestern margin of the zone have moved down to the north. Well relationships suggest that central portions of the horst (S14,15,21,22,T22N.R27E) may be structurally elevated over the surrounding terrain by as much as 2000 feet, although movement on each individual fault is probably only 200 to 500 feet. For example, the base of the Cholorpagus Formation in well 29-1 is nearly 1500 feet lower than the formation base in well B21-1 and approximately 2700 feet lower than in well B21-2, although some of the apparent structural high is undoubtedly the result of differential erosion, dipping rock sequences, and later movement along N25°E-trending faults. Nevertheless, the N60°E zone crudely defines a horst system within the zone boundaries.

The area within the N60°E-trending fault zone was apparently elevated during the latter stages of deposition of the unnamed rhyolitic unit, as indicated by sporadic occurrences of gravels in the upper part of the rhyolite section in the wells. Additional support for the elevated nature of this area includes a decrease in thickness of the overlying Chloropagus Formation and a local, uncharacteristic lack of sedimentary intercalations which are prevalent in the Chloropagus Formation elsewhere in the area. This suggests that uplift along N60°E-trending faults occurred in late Miocene time. A second uplift occurred later, probably in late Pliocene, along N25°E-trending faults, principally the Desert Queen and Desert Peak Faults. A late Pliocene age is implied by the unconformable relationship of the andesite ash-flow sequence with the underlying rocks, which were folded, faulted, and eroded prior to andesite deposition.

The result of these two distinct faulting episodes is the creation of an en echelon, rhombohedral series of horst blocks defined by the N25°E and N60°E faults (fig. 37). The en echelon horsts occupy the area from S29,T22N,R27E to Cinnabar Hill, S6,T22N,R28E. The east and west boundaries of the horst complex appear to be the Desert Queen and Desert Peak Faults. The north and south boundaries are the outlying faults of the N60°E fault zone.

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