Page 22 - Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico

unconformity, nearly flat lying Santa Fe Group rocks. A second possibility is that these intrusive rocks represent younger, post- caldera andesitic volcanism that is as young as early Miocene (Futa and Ratté, 1989).
Olivine Basalt
Olivine basalt flows and vent rocks crop out on the divide south of Trujillo Canyon, west of State Highway 27. The vent is exposed about 3.2 km north of McClede Mountain. The flows unconformably overlie an east-dipping sequence of rocks rang­ ing from the Kneeling Nun Tuff to the Santa Fe Group. A thin layer of gravel, 1.5 m thick, not mapped separately, underlies basalt southwest of the vent. Similar basalt flows in the Hills­ boro quadrangle were dated at 4.2 and 4.5 ± 0.1 Ma by the K-Ar method (Seager and others, 1984).
Erosional remnants of a flow, east of the vent, cap the ridge south of Trujillo Canyon. Not more than about 6 m of basalt remain. Vent rocks consist of scoria, which overlies the basal flow, and a central plug of basalt. The scoria weathers brown and forms gentle slopes and flat areas of rubble around the topo­ graphically prominent plug.
The basalt flow is dark gray, aphanitic, and contains phe­ nocrysts of fresh, dark plagioclase (1–2 mm), pyroxene, and oli­ vine (both, 3–4 mm). Rock of the vent-filling plug is mottled dark and light gray, weathers pock-marked, and has an aphanitic texture. Chemical analyses of both vent and flow facies of the basalt are listed in table 1, samples 12 and 13, and figure 3B. The vent and outflow rocks have alkalic, as opposed to tholeiitic affinities, characteristic of rift-related basaltic volcanism adjacent to the central axis of the rift.
Structural Geology
A homoclinal sequence of eastward-dipping volcanic and sedimentary rocks in the southeastern Black Range is cut by pre- dominantly north trending, high-angle normal faults (Seager and others, 1982). The easterly dip of the rocks is interpreted as a response to the listric shape of normal faults that, although steeply dipping at the surface, must flatten at depth. Normal faults displace outflow from the Emory cauldron of early Oli­ gocene age and sedimentary basin-fill rocks of Pliocene to late Oligocene age; locally, they are concealed beneath pediment gravels of Pliocene and Pleistocene age and by younger alluvial deposits and landslides. The faults were formed during regional extension in Oligocene time, which in part overlapped with explosive volcanism and caldera formation, and were reactivated during Rio Grande rifting.
Two major normal fault systems, here designated the Lake Valley and Berrenda fault systems, cut through the map area from Lake Valley to Tierra Blanca Creek (pl. 1; fig. 2; fig. 7). Both fault systems downdrop volcanic and sedimentary rocks on the west side and rotate strata about 10°–20° easterly (pl. 1, cross sections A–A′, B–B′, C–C′). Rocks east of the Berrenda fault are also rotated down-to-the-east, indicating a third, con­ cealed, west-dipping fault beneath pediment gravels and
Miocene (?) and Pliocene (?) basalt east of the mapped area (pl. 1, section C–C′). Northwest of Lake Valley townsite, the Lake Valley and Berrenda fault systems intersect; one splay of the Berrenda fault system trends westerly and offsets the Lake Valley fault system; another splay trends southwest and abuts the Lake Valley fault system. To the north, along the west side of Sibley Mountain, the Berrenda fault system forms the east side of the Animas half-graben, which is the southern terminus of the Winston graben (fig. 2). The half-graben or basin is filled with interlayered volcanics and associated sedimentary rocks of the Thurman Formation and the Santa Fe Group. The Winston- Animas structural depression (fig. 2) can be traced from Lake Valley almost 80 km north (Lovering and Heyl, 1989).
Movement along the Berrenda and Lake Valley faults reflects the regional Tertiary tectonism of the Black Range, from mid-Tertiary regional extension coeval with volcanism and caul­ dron collapse, to younger late Tertiary Rio Grande rifting. The Tertiary movement on the southern part of the Lake Valley fault also appears to have localized deposition of the oldest volcanic flows by the reactivation of an older fault or zone of weakness in the Precambrian basement that is of inferred Laramide age.
Lake Valley Fault System
The Lake Valley fault system is divisible into two major segments: (1) a north-striking segment that extends from west of Berrenda Mountain past the west side of McClede Mountain, and (2) a northwest-striking segment that extends from south of Lake Valley townsite to Berrenda Mountain; displacement along this segment is near 360 m (cross section D–D′–D′′, pl. 1). Southwest of Berrenda Mountain, the two segments of the Lake Valley fault system are offset by small splays of the Berrenda fault system. Movement on the northern segment was domi­ nantly dip slip; on the southern segment, movement was oblique-slip, with slickenlines on most fractures indicating a component of right-slip. Based on geophysical resistivity mea­ surements (Chapter B, this report), the southern segment has a minimum displacement of about 50 m and a maximum of about 252 m. The southern segment passes northeast of Town Moun­ tain and is paired with a parallel antithetic fault located south of Town Mountain. The graben in which the Town Mountain dome is located was also detected in geophysical resistivity studies across the Lake Valley fault (see geoelectric measurements, Chapter B of this report).
Analysis of gravity and magnetic surveys of the Lake Val- ley and surrounding area conducted during this study (Chapter B of this report) suggests that although the Lake Valley fault sys­ tem is mapped as a continuous fault zone (pl. 1, fig. 2), two fun­ damentally different faults apparently have been tectonically linked in Tertiary time. The northern segment of the Lake Valley fault parallels the aeromagnetically defined structural margin of the Emory cauldron (Chapter B of this report), previously has been included in the ring fractures zone of the Emory cauldron (Elston, 1989), and appears to have partly controlled the intru­ sion of Mimbres Peak rhyolite intrusive-extrusive domes emplaced along the margins of the cauldron along Tierra Blanca Creek (pl. 1). The fault also clearly displaces the Mimbres Peak
Geology of the Lake Valley Area 13