ABSTRACT The early Miocene Markagunt (MGS) and late Oligocene Sevier (SGS) gravity slides in southwestern Utah, USA, exhibit the full range of structural features commonly seen in modern landslides, but on a gigantic scale—they are among Earth’s largest terrestrial landslides. The MGS, discovered in 2013, consists of four distinct structural segments: (1) a high-angle breakaway segment, (2) a bedding-plane segment, (3) a ramp segment where the slide cut up-section and the basal fault “daylighted,” and (4) a former land-surface segment where the upper plate moved at least 35 km over the Miocene landscape. The MGS remained undiscovered for so long precisely because of its gigantic size (>5000 km 2 , >95 km long, estimated volume 3000 km 3 ) and initially confusing mix of extensional, translational, and compressional structures overprinted by post-MGS basin-range tectonism. Preliminary mapping of the SGS, discovered in 2016, shows it to be smaller (>2000 km 2 ) and slightly older than the MGS. Both gravity slides are large contiguous sheets of andesitic lava flows, volcaniclastic rocks, source intrusions, and regional ash-flow tuffs that record southward, gravitationally induced catastrophic failure of the southern flank of the Oligocene to Miocene Marysvale volcanic field. Failure was preceded by slow gravitational spreading accommodated by the Paunsaugunt thrust fault system, which is rooted in Middle Jurassic evaporite-bearing strata at a depth of ~2 km; this thrust system deformed Middle Jurassic through lower Oligocene strata. MGS emplacement is presently constrained between ca. 23 and 21 Ma; SGS emplacement is presently constrained between ca. 25 and 23 Ma. The presence of basal and lateral cataclastic layers, injectites (clastic dikes), pseudotachylyte (frictionite), deformed clasts, and a variety of kinematic indicators suggests that each gravity slide represents a single catastrophic emplacement event from the north to the south; possibly the MGS comprises two gravity slides. The principal zone of failure was in mechanically weak, clay-rich sedimentary strata at the base of the volcanic section. The MGS and SGS are significant because they provide examples of lithified landslide structures so large that they may be mistaken for tectonic features. However, these gravity slides lie at right angles to regional compressional tectonic structures and are cut longitudinally by modern basin-range normal faults, and thus offer compelling case studies for how to differentiate features resulting from surficial verses tectonic processes. Here we offer a history of MGS and SGS discovery, our current understanding of the gravity slides as of late 2018 (which are the focus of ongoing research), and a guide to locations of particularly instructive exposures where we document our conclusions about size, distinctive structural features, emplacement ages, and interpreted emplacement mechanisms.

We estimated the timing of paleodrainage connections in the Colorado River Basin using mitochondrial deoxyribonucleic acid (DNA) sequence divergences among populations of the speckled dace, Rhinichthys osculus . Cytochrome b and ND4L sequences were analyzed by maximum likelihood methods to estimate phylogenetic branch lengths, which were calibrated to geological time with a fossil age estimate. We assume that heterogeneity in rate of evolution of mitochondrial DNA is caused in part by differences in body size, temperature, and correlated life-history traits; therefore, branch lengths are used directly to calculate rates of nucleotide substitution and ages of nodes on the phylogenetic tree. Rhinichthys osculus is estimated (by the corrected age of the oldest fossil) to have diverged from its sister species at 6.3 Ma. We estimate that speckled dace have been in the Colorado drainage for 3.6 m.y., and they have dispersed through the drainage and to former connectives, such as the Los Angeles Basin, in the past 1.9 m.y. Divergence among lineages of the upper and lower Colorado River drainages (above and below Grand Canyon) is estimated to have occurred ca. 1.9–1.3 Ma. Genetic divergence of allopatric lineages in the lower Colorado River drainage was accompanied by morphological adaptations to different stream gradients, but small genetic distances among these forms indicate recent gene flow and lack of reproductive isolation.