ABSTRACT The Basin and Range Province is considered to be one of the most iconic continental rift provinces that postdates a prolonged orogeny. Here, I present evidence that challenges all the assumptions that lead to the long-held notion that gravitational collapse of thickened (55–65-km-thick) continental crust was a major driver of Basin and Range extension. This study focused on integrating the regional tectonic and magmatic history of the northeastern region of the Basin and Range (centered on the Albion–Raft River–Grouse Creek metamorphic core complex) and combines insights from a compilation of data from metamorphic core complexes worldwide to illustrate the effect of accounting for the magmatic histories when estimating pre-extensional crustal thickness. In the region of the Albion–Raft River–Grouse Creek metamorphic core complex, there is evidence of three Cenozoic extensional events and three coeval magmatic events. By taking into account the regional magmatic activity during the Cenozoic (Paleogene, Neogene, and Quaternary magmatism), and the inferred mantle-derived magmatic volume added to the crust during the process of extension, it is shown that the pre-extensional crustal thickness cannot have been more than ~53 km, and it was more likely close to ~46 km. This estimate is consistent with Eocene igneous geochemistry estimates of crustal thickness and with crustal thickness estimates from shortening of ~30-km-thick mid-Jurassic crust. During the Cenozoic evolution of the northeastern Basin and Range, the crust in the area of study thinned from ~46 km to ~32 km, and the elevation of the pre-extensional plateau collapsed from ~2.5 km to its present-day average of ~1.8 km. This study concludes that an alternative mechanism to predominantly gravitational crustal collapse is required to explain the extension in the region of the Albion–Raft River–Grouse Creek metamorphic core complex. I support recent interpretations that this mechanism involved the complex interaction of the removal of the Farallon flat slab (by slab roll-back or delamination of the slab) with the impingement of the Snake River Plain–Yellowstone mantle anomaly. The switch in the stress regime from compression (during the slab subduction) to a complex regime during slab roll-back, followed by extension (in the middle Miocene), and the associated mantle-derived magmatism, led to the thinning of the subcontinental lithospheric mantle, thermal weakening of the crust, and the thinning of the crust during the Cenozoic. This crustal extension is expressed as regional Basin and Range normal faulting and local vertical flow and exhumation of the mobilized middle crust at metamorphic core complexes like the Albion–Raft River–Grouse Creek complex.

ABSTRACT The most common and widespread sedimentary facies of Pleistocene Lake Bonneville, in the eastern Great Basin of North America, is marl, which consists of a mixture of fine-grained endogenic calcium carbonate that precipitated in the epilimnion of the lake and then settled onto the lake floor and mixed with fine-grained clastic sediments. Primary sources of clastic sediment were inflowing rivers, wave activity in shore zones, and ice rafting. The thickness of deposits in cores and outcrops is largely dependent on the proportion of clastic sediment, although the rate of endogenic calcium carbonate precipitation probably also varied temporally and spatially. Net sediment-accumulation rate in the marl, as measured in outcrops and cores, ranges from a low of 4 cm/1000 yr, in the middle of the lake basin far from sources of clastic input, to over 100 cm/1000 yr near clastic-sediment sources. Underflow deposits, derived from higher-density river water loaded with suspended sediment, are thick and extensive near the mouths of major rivers that drained glaciated mountains. Net sediment-accumulation rates in suspended-load underflow deposits were much greater than those in contemporaneously deposited marl. The largest underflow-sediment accumulations, which have a fan shape in plan view, have been referred to as deltas (as at the mouths of the Sevier, Provo, Weber, and Bear Rivers). True Gilbert-type deltas composed of gravel, with topset, foreset, and bottomset beds, are uncommon in the basin. Variability in the sedimentary characteristics of the Bonneville deposits is determined by geomorphic factors, such as wave energy, composition of surficial material in the shore zone (e.g., resistant bedrock vs. unconsolidated alluvium), slope, and proximity to river mouths and active shore zones.

ABSTRACT On this field trip we visit three sites in the Salt Lake Valley, Utah, USA, where we examine the geomorphology of the Bonneville shoreline, the history of glaciation in the Wasatch Range, and shorezone geomorphology of Great Salt Lake. Stop 1 is at Steep Mountain bench, adjacent to Point of the Mountain in the Traverse Mountains, where the Bonneville shoreline is well developed and we can examine geomorphic evidence for the behavior of Lake Bonneville at its highest levels. At Stop 2 at the mouths of Little Cottonwood and Bells Canyons in the Wasatch Range, we examine geochronologic and geomorphic evidence for the interaction of mountain glaciers with Lake Bonneville. At the Great Salt Lake at Stop 3, we can examine modern processes and evidence of the Holocene history of the lake, and appreciate how Lake Bonneville and Great Salt Lake are two end members of a long-lived lacustrine system in one of the tectonically generated basins of the Great Basin.