EARL MP LOVEJOY Department of Geology, the University of Texas at El Paso, El Paso, Texas Mount Rose, Northern Carson Range
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EARL M. P. LOVEJOY Department of Geology, The University of Texas at El Paso, El Paso, Texas Mount Rose, Northern Carson Range, Nevada: New Light on the Late Cenozoic Tectonic History of the Sierra Nevada from a Classic Locality Abstract: The eastern border of the northern Carson Range, at Mount Rose, is important for the study of regional Sierran tectonic and faulting chronology. There Louderback indicated that more than 5000 feet of post-Miocene-Pliocene displacements occurred along the eastern front of the Carson Range at the Mount Rose cross section. He concluded that the entire Carson Range (hence the Sierra) was uplifted as a block along that frontal fault in post-late Miocene time. The structural evidence at Mount Rose has been re-examined. It may be reinterpreted as the result of formation of a strato-volcano over a rough terrain, and its subsequent deep erosion, as contrasted with the earlier interpretation involving major fault displacement after agglomerate and flow deposition. The new structural evidence and interpretation indicates that there has been little significant faulting along the Sierra Nevada front at Mount Rose since Miocene- Pliocene volcanism. Introduction problem of Basin and Range structure (Bate- man and Eaton, 1967, p. 1413). Mount Rose, elevation 10,778 feet, is the New observations of several kinds (Curry, highest peak in the northern Carson Range, a 1966; Bateman and Eaton, 1967, p. 1413; com- north-trending spur of the Sierra Nevada pare Billings, 1960, p. 391; Christensen, 1968; (Figs. 1, 2). This is the classic locality where Thompson and Sandberg, 1958, p. 1280; Louderback (1904) first advanced evidence Lovejoy, 1964) are changing somewhat our of major (5000 feet) Pliocene-Pleistocene concepts of the timing of the structural events. ("Cascadian") frontal faulting (compare Lind- A detailed study at McGee Mountain (Fig. 1) gren, 1897). Subsequently, Louderback (1907, on the Sierra front some 135 miles south- 1924) supported this temporal interpretation southeast of Mount Rose (Lovejoy, 1964,1966, with additional observations there and in the 1968a) shows that the Sierra front at that place Basin and Range Province. Within the present is no less than 2.6 m.y. old, and none of the decade his interpretation has been further evidence precludes its being much older. supported by Thompson and White (1964) This paper reports further on work done at and widely accepted (Hunt, 1967, p. 380-381; the classical Mount Rose locality (Figs. 3, 4). Burnett, 1968, p. 3; Morrison, 1965, p. 265). In 1959 the Steamboat Hills (Fig. 5) were In spite of this apparent agreement, my work mapped on a scale of 1 mile to 1 inch; the indicates that the question of the date of major Mount Rose summit block was also studied in uplift of the Sierra Nevada is still unsettled. 1959 and 1962. Hill 9400 on the Mount Rose Although most students of this problem agree cross section was mapped by Brunton compass that vigorous differential uplift occurred in late and pacing in 1963 and restudied and cross Pliocene and early Pleistocene (Hudson, 1960) sectioned, using a plane table, in 1966 (Lovejoy, the actual amount of vertical displacement is in 1968b; Figs. 6, 7). dispute (Lydon, 1962, p. 23). Not the existence but the age of faulting along the Sierran front Acknowledgments is questioned herein. This problem is also im- My two sons, Eric C. Lovejoy and Mark T. portant because the Cenozoic uplift of the Lovejoy, assisted with the plane-table map- Sierra Nevada may be part of the broader ping. Evans B. Mayo, William McAnulty, Geological Society of America Bulletin, v. 80, p. 1833-1842, 9 figs., September 1969 1833 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/9/1833/3417845/i0016-7606-80-9-1833.pdf by guest on 01 October 2021 1834 E. M. P. LOVEJOY-MOUNT ROSE, NORTHERN CARSON RANGE, NEVADA 0 5 10 20 30 40 50 Miles . MCGee Mtn. SCALE Figure 1. Regional index map of north-central Sierra Nevada and northwestern Nevada. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/9/1833/3417845/i0016-7606-80-9-1833.pdf by guest on 01 October 2021 NOTES AND DISCUSSIONS 1835 by a major fault. On the contrary, south of Tamarack Lake (Fig. 5) basal tuffs of the Kate Peak Formation bury a rugged granodiorite terrain with relief of several hundred to 1000 feet. In Galena Creek these tuffs were mapped as Truckee Formation by Thompson and White (1964, PI. 1; sec. 23). The Truckee Formation, however, overlies and intertongues the top of the Kate Peak volcanics; it does not underlie the flows; hence these tuffs must be the basal part of the Kate Peak. These basal tuffs extend south to the high ridge in the SE J4 sec. 23 but were not separately mapped as Truckee Formation there by Thompson and White. They also can be traced readily from the SE*4 sec. 23 to their position in Galena Creek. They have not been faulted; their difference in elevation between Galena Creek (8800 feet) and on the ridge in the SEM sec. 23 (9840 feet) is the result of deposition of the basal tuffs upon a very rugged terrain. The tuffs came from a volcano (see below) on Hill 9400. They covered the steep contact between the deeply eroded granodiorite and the andesites. Accordingly, fault No. 1 did not displace the Kate Peak Formation with respect Figure 2. Local index map of northern Carson to the granodiorite. The rugged subtuff Range, Washoe County, Nevada. granodiorite surface is evidence, however, that a preandesite scarp or cliff may have resulted from preandesite faulting extending along the western side of Mount Rose. Evidence of John Hills, and Aaron C. Waters made sug- significant postandesite displacements is lack- gestions that improved the manuscript. Miss ing; so, if a fault is present here, it must be Rachel Lopez typed it and Robert Sepulveda of pre-Kate Peak age. prepared most of the illustrations. Fault No. 2 (Thompson and White, 1964, p. A35, Fig. 15; Figs. 3, 4; this is the same Geologic Evidence along the structure described in 1904 by Louderback) was Mount Rose Cross Section also traced, but no structural evidence of a This section (Figs. 2, 5) has been constructed fault was found. The western wall similar to from the Mount Rose summit block east- many hill slopes in the granodiorite is a steep northeastward across Hill 9400 and Hill 7720 hillside (steeper than 35°) formed on gran- (Fig. 8) into sec. 2, T. 17 N., R. 19 E., hence odiorite, but grus covers the slope. At first east-southeastward across part of the Steam- glance, Hill 9400 (Figs. 3, 4) appears to be a boat Hills. The section thus consists of two down-dropped block composed of the same segments (D-D' and E-E', Fig. 5) that will be gently dipping volcanics as those seen on the discussed separately (Fig. 9). summit of Mount Rose. The dips in Hill 9400 Faults in the Carson Range. Fault No. 1 even appear to be gently westward, toward (Fig. 5) mapped by Thompson and White Mount Rose, as though the block had rotated (1964, PI. 1) extends 2 miles north-north- slightly as it dropped. This appearance is an westerly from a point in the NW^ sec. 24 to illusion; the hill itself is composed of steeply NWM sec. 11, T. 17 S., R. 18 E. I have fol- dipping dikes (Figs, 6, 7) of dense andesite. lowed this line from its southern end to the The andesite is not gently dipping flows, as trail west of Mount Rose. Unlike Thompson thought by Louderback. Moreover, there are and White, I found no evidence that the Kate andesite dikes in Hill 9400 that are dissimilar to Peak Formation, of late Miocene to early the andesite flows in the Mount Rose summit Pliocene (Mio-Pliocene) age, has been displaced block. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/9/1833/3417845/i0016-7606-80-9-1833.pdf by guest on 01 October 2021 1836 E. M. P. LOVEJOY—MOUNT ROSE, NORTHERN CARSON RANGE, NEVADA Figure 3. View toward northwest, approximately normal to Mount Rose-Hill 9400-Steamboat Hills cross section from northern flank of Slide Mountain. Slopes A and B are the scarps considered by Louder- back and Thompson and White to have resulted from post-Kate Peak faulting. Figure 4. View toward southwest along the Mount Rose-Hill 9400-Steamboat Hills cross section from base of Carson Range. Slopes A and B correspond to those shown in Figure 3. I interpret Hill 9400 as a deeply eroded dike- slope wash, which indicates a gently eastward- zone neck of a Kate Peak strato-volcano that dipping Kate Peak agglomerate-granodiorite was composed of tuffs and flows spread out contact rather than a steep contact. This beyond a breccia-agglomerate-dike complex seems to be another depositional contact slop- near the vent. Hematite veins, hematite- ing gently east eroded so as to form a cuestalike coated cinders, and andesite scoria abound in hill (7720). the top of Hill 9400. Fault No. 5 could not be found in the field. Thus, the only evidence of fault No. 2 ap- The outcrop pattern of the agglomerate on pears to be a geomorphic illusion. Thompson the granodiorite, as interpreted here, indicates and White (1964, p. A34) also state: "It is a depositional contact similar to that of the interesting to note that this fault, which was western fault but curved as the result of the described by Louderback . ., has little direct gentle eastward dip of the tuff-breccia ag- topographical expression." glomerates.