The track of the Yellowstone hot spot is represented by a systematic northeast-trending linear belt of silicic, caldera-forming volcanism that arrived at Yellowstone 2 Ma, was near American Falls, Idaho about 10 Ma, and started about 16 Ma near the Nevada-Oregon-Idaho border. From 16 to 10 Ma, particularly 16 to 14 Ma, volcanism was widely dispersed around the inferred hot-spot track in a region that now forms a moderately high volcanic plateau. From 10 to 2 Ma, silicic volcanism migrated N54°E toward Yellowstone at about 3 cm/year, leaving in its wake the topographic and structural depression of the eastern Snake River Plain (SRP). This <10-Ma hot-spot track has the same rate and direction as that predicted by motion of the North American plate over a thermal plume fixed in the mantle. The eastern SRP is a linear, mountain-bounded, 90-km-wide trench almost entirely(?) floored by calderas that are thinly covered by basalt flows. The current hot-spot position at Yellowstone is spatially related to active faulting and uplift. Basin-and-range faults in the Yellowstone-SRP region are classified into six types based on both recency of offset and height of the associated bedrock escarpment. The distribution of these fault types permits definition of three adjoining belts of faults and a pattern of waxing, culminating, and waning fault activity. The central belt, Belt II, is the most active and is characterized by faults active since 15 ka on range fronts >700 m high. Belt II has two arms forming a V that joins at Yellowstone: One arm of Belt II trends south to the Wasatch front; the other arm trends west and includes the sites of the 1959 Hebgen Lake and 1983 Borah Peak earthquakes. Fault Belt I is farthest away from the SRP and contains relatively new and reactivated faults that have not produced new bedrock escarpments higher than 200 m during the present episode of faulting. Belt III is the innermost active belt near the SRP. It contains faults that have moved since 15 to 120 ka and that have been active long enough to produce range fronts more than 500 m high. A belt with inactive faults, belt IV, occurs only south of the SRP and contains range-front faults that experienced high rates of activity coincident with hot-spot volcanism in the late Tertiary on the adjacent SRP. Comparison of these belts of fault activity with historic seismic activity reveals similarities but differences in detail. That uplift migrated outward from the hot-spot track is suggested by (1) the Yellowstone crescent of high terrain that is about 0.5 km higher than the surrounding terrain, is about 350 km across at Yellowstone, wraps around Yellowstone like a bow wave, and has arms that extend 400 km southerly and westerly from its apex; (2) readily erodible rocks forming young, high mountains in parts of this crescent; (3) geodetic surveys and paleotopographic reconstructions that indicate young uplift near the axis of the Yellowstone crescent; (4) the fact that on the outer slope of this crescent glaciers during the last glaciation were anomalously long compared with those of the preceding glaciation, suggesting uplift during the intervening interglaciation; (5) lateral migration of streams, apparent tilting of stream terraces away from Yellow-stone, and for increasingly younger terrace pairs, migration away from Yellowstone of their divergent-convergent inflection point; and (6) a geoid dome that centers on Yellowstone and has a diameter and height similar to those of oceanic hot spots. We conclude that the neotectonic fault belts and the Yellowstone crescent of high terrain reflect heating that is associated with the hot-spot track but has been transferred outward for distances of as much as 200 km from the eastern SRP in 10 m.y. The only practical mechanism for such heat transport would be flow of hot material within the asthenosphere, most likely by a thermal mantle plume rising to the base of the lithosphere and flowing outward horizontally for at least such 200-km distances. The changes in the volcanic track between 16 to 10 Ma and 10 to 2 Ma is readily explained by first the head (300-km diameter) and then the chimney (10 to 20 km across) phases of a thermal mantle plume rising to the base of the southwest-moving North American plate. About 16 Ma, the bulbous plume head intercepted the base of the lithosphere and mushroomed out, resulting in widespread magmatism and tectonism centered near the common borders of Nevada, Oregon, and Idaho. Starting about 10 Ma near American Falls and progressing to Yellowstone, the chimney penetrated through its stagnating but still warm head and spread outward at the base of the lithosphere, adding basaltic magma and heat to the overriding southwest-moving lithospheric plate, leaving in its wake the eastern SRP-Yellowstone track of calderas, and forming the outward-moving belts of active faulting and uplift ahead and outward from this track. We favor a mantle-plume explanation for the hot-spot track and associated tectonism and note the following problems with competing hypotheses: (1) for a rift origin, faulting and extension directions are at nearly right angles to that appropriate for a rift; (2) for a transform origin, geologic evidence requires neither a crustal flaw nor differential extension across the eastern SRP, and volcanic alignments on the SRP do not indicate a right-lateral shear across the SRP; and (3) for a meteorite impact origin, evidence expected to accompany such an impact near the Oregon-Nevada border has not been found. The southern Oregon rhyolite zone is not analogous to the eastern SRP and therefore does not disprove formation of the Yellowstone hot-spot track by a mantle plume. The postulated rise of a mantle-plume head into the mantle lithosphere about 16 Ma corresponds in both time and space with the following geologic changes: (1) the start of the present pattern of basin-range extension, (2) intrusion of basalt and rhyolite along the 1,100-km-long Nevada-Oregon rift zone, (3) the main phases of flood basalt volcanism of the Columbia River and Oregon plateaus, and (4) a change from calc-alkaline volcanism of intermediate to silicic composition to basaltic and bimodal rhyolite/basalt volcanism.

Abstract Detailed geologic mapping and new SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon, Ar/Ar, Lu-Hf, 14 C, luminescence (optically stimulated), thermochronology (fission-track), and palynology reveal the complex Mesoproterozoic to Quaternary geology along the ~350 km length of the Blue Ridge Parkway in Virginia. Traversing the boundary of the central and southern Appalachians, rocks along the parkway showcase the transition from the para-autochthonous Blue Ridge anticlinorium of northern and central Virginia to the allochthonous eastern Blue Ridge in southern Virginia. From mile post (MP) 0 near Waynesboro, Virginia, to ~MP 124 at Roanoke, the parkway crosses the unconformable to faulted boundary between Mesoproterozoic basement in the core of the Blue Ridge anticlinorium and Neoproterozoic to Cambrian metasedimentary and metavolcanic cover rocks on the western limb of the structure. Mesoproterozoic basement rocks comprise two groups based on SHRIMP U-Pb zircon geochronology: Group I rocks (1.2-1.14 Ga) are strongly foliated orthogneisses, and Group II rocks (1.08-1.00 Ga) are granitoids that mostly lack obvious Mesoproterozoic deformational features. Neoproterozoic to Cambrian cover rocks on the west limb of the anticlinorium include the Swift Run and Catoctin Formations, and constituent formations of the Chilhowee Group. These rocks unconformably overlie basement, or abut basement along steep reverse faults. Rocks of the Chilhowee Group are juxtaposed against Cambrian rocks of the Valley and Ridge province along southeast- and northwest-dipping, high-angle reverse faults. South of the James River (MP 64), Chilhowee Group and basement rocks occupy the hanging wall of the nearly flat-lying Blue Ridge thrust fault and associated splays. South of the Red Valley high-strain zone (MP 144.5), the parkway crosses into the wholly allochthonous eastern Blue Ridge, comprising metasedimentary and meta-igneous rocks assigned to the Wills Ridge, Ashe, and Alligator Back Formations. These rocks are bound by numerous faults, including the Rock Castle Creek fault that separates Ashe Formation rocks from Alligator Back Formation rocks in the core of the Ararat River synclinorium. The lack of unequivocal paleontologic or geochronologic ages for any of these rock sequences, combined with fundamental and conflicting differences in tectonogenetic models, compound the problem of regional correlation with Blue Ridge cover rocks to the north. The geologic transition from the central to southern Appalachians is also marked by a profound change in landscape and surficial deposits. In central Virginia, the Blue Ridge consists of narrow ridges that are held up by resistant but contrasting basement and cover lithologies. These ridges have shed eroded material from their crests to the base of the mountain fronts in the form of talus slopes, debris flows, and alluvial-colluvial fans for perhaps 10 m.y. South of Roanoke, however, ridges transition into a broad hilly plateau, flanked on the east by the Blue Ridge escarpment and the eastern Continental Divide. Here, deposits of rounded pebbles, cobbles, and boulders preserve remnants of ancestral west-flowing drainage systems. Both bedrock and surficial geologic processes provide an array of economic deposits along the length of the Blue Ridge Parkway corridor in Virginia, including base and precious metals and industrial minerals. However, common stone was the most important commodity for creating the Blue Ridge Parkway, which yielded building stone for overlooks and tunnels, or crushed stone for road base and pavement.

Abstract: In coastal areas of the SW part of the island of Rhodes, Greece, eolian oolitic sediments represent the latest depositional phase, and are presumed to have a Late Pleistocene to Early Holocene age. The ooids have sand-size nuclei and are lightly cemented by vadose meniscus cement. In a road cut, supplemented by minor sections in small, ancient stone quarries nearby, the sedimentary architecture and trace fossils are visible. The dunes have ramp morphology and contain three horizons of paleosols that divide the eolian sediments into three units. The paleosols contain rhizoliths and poorly preserved invertebrate bioturbation. Vertical sections allowed representative measurements of 79 tracks and limited horizontal surfaces supplied four measured tracks. Five size classes of tracks are distinguished. The mode of preservation of the tracks is poor, probably on account of the oolitic nature of the substrate. The three smallest size classes are probably of artiodactyls. The largest class probably was produced by proboscidians. A bedding-plane view of one track indicates that the next-largest class may be the work of camels. If this is the case, and the bedding-plane specimen is convincing, it is the first record of Pleistocene or Early Holocene camels on Rhodes. The combination of size groups of tracks differs in the three units, demonstrating differences in faunal composition in the different periods of deposition of the oolite.

Abstract The 23-year evaluation history of the Diavik kimberlites (1994–2017) commenced with elevated microdiamond stone counts from small samples of drill core. These stone counts equated with high sample grades, grades that were higher than anyone had expected (2–5 carats per tonne [ct/t]), and caused many experienced explorers to change their opinions on the prospectivity of small kimberlites (1–2 ha in surface area). The diamond size-frequency distributions were coarser trending, and the diamond quality was “mixed,” resulting in >1-mm diamond prices of about US$100 per ct, slightly better than the world average. This grade and price combination gave ore values ranging from US$200 to US$500 per tonne and, with four kimberlites, the Diavik joint venture partners estimated that there would be sufficient ore to underpin a 20-year mine life at a treatment rate of 2.2 million tonnes per annum. Primary diamond deposits are usually sampled on 50- or 100-m grids. Grade drilling at Diavik was conducted on 25-m centers, allowing for 12- × 6-in holes in each kimberlite. The kimberlites are located beneath Lac de Gras and had to be bulk sampled underground to generate diamond parcels (approximately 12,000 and 8,000 ct) for sorting and valuation. Three of the kimberlites transitioned from open-pit to underground mines (two sublevel retreats and one cut and fill). The fourth kimberlite narrows at depth and is unlikely to be mined from underground. Grade reconciliation has presented challenges; each kimberlite comprises three to four geologic units and plant feed can comprise ore from three kimberlites concurrently. Careful ore management and tracking has overcome these difficulties, meaning that grade, size-frequency, and size-quality reconciliations have been meaningful and, for the most part, have varied less than ±5% on an annual basis. The Diavik mine is neither the largest nor the smallest deposit in the world. However, by the end of its life it will have produced about 7 million carats of diamonds per annum for 22 years at an average grade of 3.4 ct/t. It represents an exceptional diamond resource for kimberlites ~100 m in diameter, considering that it is also located in one of the coldest and remotest locations on Earth.