Twin Lakes terrane – central Sierra Nevada crest
Twin Lakes terrane – central Sierra Nevada crest
Early Jurassic and Triassic(?) to Paleozoic
Deformed and imbricated thrust slices of late Paleozoic deep-marine, pelagic, argillite, chert, pillow basal, and fault-bounded slivers of serpentinite and peridotite. Unit also includes local sandstone, quartzite, shale, conglomerate, and intermediate to silicic volcanic rocks, and silicic ash-flow tuff. Correlated in part by Schweickert and Lahren (1993, 2006) with late Paleozoic Golconda allochthon, a late Paleozoic thick sequence of metamorphosed basalt, siltstone, shale, chert and limestone. Samples with Jurassic detrital zircon MDAs may be from faulted slivers of Middle to Late Jurassic (221 to 151 Ma) volcanic rocks from the Goddard arc, Sierra Crest terrane (unit SCGV), whereas the Late Triassic and earliest Jurassic volcanics of this unit, along with parts of the High Sierra (unit HSV) and Sierra Crest (unit SCGV) terranes and the Scheelite Intrusive Suite (unit Gsc), make up the High Sierra arc.
In the Saddlebag Lake pendant, this unit was mapped by Chesterman (1975) as the late Paleozoic Twin Peaks sequence. Later detailed mapping by Cao and others (2015) shows the unit is formed of a lower section of Paleozoic well-bedded volcanic, volcaniclastic, and sedimentary rocks that they correlate with the Golconda allochthon, structurally overlain by Early Jurassic sedimentary rocks. The lower section has yielded a detrital zircon MDA of ~350 Ma and is intruded by a small granodiorite pluton that has yielded a zircon U-Pb age of ~233 Ma (Cao and others, 2015), probably a part of the Scheelite Intrusive Suite too small to map separately. The upper section has yielded detrital zircon MDAs of ~186 and ~187 Ma. At the northern tip of the pendant, Attia and others (2021) report a detrital zircon MDA of ~194 Ma from metasedimentary rocks that they assign to their Sawmill Canyon sequence, and a poorly constrained detrital zircon MDA of ~220 Ma from metasedimentary rocks they assign to their Koip sequence.
The two structural blocks of this unit to the east were mapped by Chesterman and Gray (1975) as metavolcanic and metasedimentary rocks that are part of the Jurassic(?) Virginia Lakes sequence, but this is precluded by intrusion of Late Triassic plutons of the Scheelite Intrusive Suite. These rocks were mapped as Triassic metavolcanics by Huber and others (1989). More recently Schweickert and Lahren (2006) have mapped the smaller block north of Virginia Lakes as metasedimentary rocks that are part of the Triassic Candelaria Formation, but the larger eastern block as metasedimentary and metavolcanic rocks, part of their Paleozoic Golconda thrust sheet.