Progressive derangement of ancient (Mesozoic) east-west Nevadaplano paleochannels into modern (Miocene–Holocene) north-northwest trends in the Walker Lane Belt, central Sierra Nevada

Eocene to Pliocene paleochannels of the Sierra Nevada (California, USA) were first exploited for gold placer deposits during the California gold rush (1848), and then mapped in surveys more than century ago. The surveys showed that the paleochannels flowed westward, like the modern rivers of the range; it then was assumed that the heads of the paleochannels were at the modern range crest. A first paradigm shift occurred ∼50 yr ago, when it was recognized that at least some of the paleochannel fill was sourced from the region of the current state of Nevada, and it was proposed that the Sierra Nevada range was younger than the paleochannels (younger than 6 Ma). More recent work has demonstrated that Sierran paleochannels are ancient features that formed on the shoulder of a broad high uplift (the Nevadaplano) formed during Cretaceous crustal shortening; the headwaters were in central Nevada prior to disruption of the plateau by Basin and Range extension. A second paradigm shift occurred in the past decade: the Sierra Nevada range front is formed of north-northwest transtensional structures of the younger than 12 Ma Walker Lane belt, not north-south to north-northeast–south-southwest extensional structures of the Basin and Range. In this paper we use detailed geologic mapping to reconstruct the paleogeographic evolution of three Oligocene to Pliocene east-west paleochannels in the central Sierra Nevada, and their progressive south to north derangement by Walker Lane structures: the Stanislaus in the south, the Cataract in the middle, and the Mokelumne in the north. Previous work has shown that east-west Nevadaplano paleochannels in the central Sierra have four stratigraphic sequences floored by erosional unconformities; we describe distinguishing characteristics between the ancient Nevadaplano paleochannels and the north-northwest–deranged paleochannels of the Walker Lane grabens. In the east-west paleochannels unconformity 1 is the deepest, eroded into mesozonal Cretaceous plutons; it is overlain by the Oligocene to early Miocene Valley Springs Formation (sequence 1), consisting of ignimbrites erupted ∼250 km to the east in Nevada. Sequence 1 is the most useful for tracing the courses of the paleochannels because it was deposited before faulting began; however, it is incompletely preserved, due to erosion along unconformity 2 (with as much as 500 m of relief) as well as later erosional events. Sequence 2 consists of ca. 16–12 Ma andesitic volcaniclastic rocks referred to as the Relief Peak Formation; it occurs in all three paleochannels (Stanislaus, Cataract, and Mokelumne) as stratified fluvial and debris flow deposits, with abundant cut and fill structures. However, we show for the first time that Relief Peak Formation also forms the basal fill of a Walker Lane transtensional basin system that began to form by ca. 12 Ma, in a full graben along what is now the Sierra Crest, and in transfer zone basins and half-grabens on what is now the eastern range front. The Relief Peak Formation in the Walker Lane transtensional basins consists of massive (nonstratified) andesitic debris flow deposits and debris avalanche deposits, with slabs as much as 2 km long, including slabs of the Valley Springs Formation. Sequence 3 in the Nevadaplano paleochannels consists of distinctive, voluminous high-K lavas and ignimbrites of the Stanislaus Group. The lavas were erupted from fissures in the transtensional Sierra Crest graben-vent system, which beheaded the Stanislaus paleochannel prior to development of unconformity 3 and eruption of the voluminous basal lavas, referred to as the Table Mountain Latite (TML). In the Cataract paleochannel, TML lavas are inset as much as 100 m into the Relief Peak Formation along unconformity 3, indicating fluvial reincision within the paleochannel; TML lavas were ponded in the graben-vent system to thicknesses 6 times greater than the paleochannel fill, with no reincision surfaces. Sequence 3 ignimbrites of the Stanislaus Group (Eureka Valley Tuff) were erupted from the Little Walker caldera, and mark the course of all three paleochannels, with channel reincision surfaces between them (but not in the grabens). Sequence 3 lavas in the paleochannels differ from those in the grabens by having interstratified fluvial deposits, stretched vesicles parallel to the paleochannels, tree molds, peperitic bases, and kuppaberg (cobble jointed) tops, which form when water penetrates into a cooling lava along vertical joints, allowing secondary joints to form perpendicular to them. The Cataract paleochannel was deranged from its ancient (Mesozoic) east-west Nevadaplano trend into the north-northwest Walker Lane tectonic trend prior to development of unconformity 4 and deposition of sequence 4 (Disaster Peak Formation). The north-northwest–deranged Cataract paleochannel is along the Sierra Crest between the Stanislaus and Mokelumne paleochannels, with fluvial deposits indicating northward flow; this paleochannel is perpendicular to the ancient east-west Nevadaplano paleochannels, and parallel to modern Walker Lane drainages, indicating tectonic reorganization of the landscape ca. 9–5 Ma. This derangement was followed by progressive beheading of the Mokelumne paleochannel, development of the Ebbetts Pass pull-apart basin (ca. 6 Ma) and the Ebbetts Pass stratovolcano within it (ca. 5–4 Ma), which fed lava into the relict Mokelumne paleochannel. The derangement of central Sierran paleochannels proceeded as follows, from south to north: (1) the Stanislaus paleochannel was beheaded by ca. 11 Ma; (2) the Cataract paleochannel became deranged from an east-west Nevadaplano trend into a north-northwest Walker Lane trend by ca. 9 Ma, now exposed along the Sierran crest; and (3) the Mokelumne paleochannel was beheaded by ca. 6–5 Ma, and the Carson Pass–Kirkwood paleochannel several kilometers to the north was deranged from east-west into the north-northwest Hope Valley graben ca. 6 Ma. The next paleochannel to the north is in the southern part of the northern Sierra at Lake Tahoe, and based on published descriptions was beheaded ca. 3 Ma. The timing of paleochannel beheading corresponds to the northward migration of the Mendocino Triple Junction and northward propagation of the Walker Lane transtensional strain regime. This paper illustrates in detail the interplay between tectonics and drainage development, exportable to a very broad variety of settings.