Mapping on the west side of the Raft River-Grouse Creek core complex has shown that Upper Permian, Triassic, and Middle to Late Miocene rocks compose an extensive allochthonous sheet that was displaced laterally as well as down-section onto the complex. The sheet is underlain locally by two thin allochthonous sheets of Permian rocks that were displaced at about the same time. That all these sheets moved either northeastward or southwestward is suggested by the trends of fold hinge-lines and by the strike of strike-slip faults confined to the sheets. The Miocene sequence is at least 2000 m thick and consists of poorly sorted fluvial conglomerate intercalated with numerous thick beds of silicic vitric tuff and smaller amounts of monolithologic breccia, lacustrine limestone, basalt, and intrusive as well as extrusive rhyolite. K-Ar dates on a basalt flow and a rhyolite plug show that the Triassic and part of the Permian rocks were infaulted with the Miocene rocks between 14.4 and 11.7 m.y. ago, and that the composite sheet was displaced later than 11.7 m.y. ago.

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.

The Matlin Mountains, an area of low hills on the northern edge of the Great Salt Lake Desert, expose a rootless sequence of upper Paleozoic, lower Mesozoic, and upper Miocene rocks that have been affected by folds and by older-on-younger low-angle faults. The area lies 5 to 10 km east of the Grouse Creek Mountains, the southernmost range of a metamorphic core complex which extends from south-central Idaho into northwestern Utah. In this core complex, metamorphism that was accompanied by intrusion, younger-on-older low-angle faulting, recumbent folding, and horizontal extension of rock units did not end until Miocene time. The Matlin Mountains expose the coarse Tertiary sedimentary rocks and the displaced sheets that were shed from uplifts that followed metamorphism and intrusion in the core complex. These structurally interlayered sedimentary deposits and rock sheets overlie the zone of transition between metamorphically attenuated Paleozoic rocks in the core complex and unmetamorphosed, undeformed Paleozoic formations of normal thickness less than 15 km to the east. The Matlin Mountains consist of two structurally distinctive terranes: an eastern rooted terrane in which Miocene(?) sedimentary rocks unconformably overlie upper Paleozoic carbonate rocks, and a western displaced terrane that is composed of five brecciated displaced sheets of pre-Tertiary rocks, each resting upon Tertiary sedimentary rocks; the lowest sheet overlies the rooted terrane. All of the Tertiary sedimentary rocks contain abundant silicic volcanic ash probably derived from nearby volcanic centers. The displaced sheets strike north-northeast, are nearly flat-lying, and consist of steeply dipping, locally folded strata. Two major low-angle faults are inferred to divide the four lowest sheets into two superimposed composite plates, each of which has a unique lithologic, metamorphic, and structural character. The fifth and highest displaced sheet was emplaced after both of these plates were in their present position. The Miocene(?) sedimentary rocks and displaced sheets of the structurally lower composite plate probably were derived from an ancestral range in the site of the Grouse Creek Mountains and moved from west to east in late Miocene time. The higher plate may once have been continuous with a complex upper allochthon of late Miocene age that crops out widely on the west side of the Grouse Creek and Raft River Mountains. The source of the upper Miocene sedimentary rocks and the displaced sheets of the higher composite plate lay somewhere west of the Grouse Creek Mountains and the plate apparently travelled eastward at some time after 11.1 to 10.5 m.y. B.P. Ductile structures that formed during, and after, brecciation in the most completely exposed displaced sheet suggest that the sheets moved initially by slow creep under moderate load. In spite of locally intense brecciation, the orientation in the sheets of sedimentary and metamorphic structures that were inherited from the source areas remained remarkably consistent. In the Matlin Mountains, the sheets apparently moved over an eroded Tertiary surface and (or) into a depositional basin. Clastic dikes preserved in the brecciated base of one sheet suggest that fractures were propagated by relatively high fluid pressures derived from underlying sedimentary water, thus allowing the sheet to disintegrate in place (hydrofracturing). Locally, movement of the sheets was facilitated by thin layers of silicic volcanic ash and (or) tuffaceous lacustrine beds. Little is known about the source areas of the displaced sheets, but compositions, textures, and fabrics of the associated upper Miocene sedimentary rocks and the lithologies and geometries of the sheets point to the existence of postmetamorphic, broad domal uplifts whose locations shifted temporally and spatially but generally lay to the west, near the Utah-Nevada state line. The postmetamorphic low-angle faults in the core complex cut down-section to the Precambrian basement as well as carrying displaced sheets laterally for tens of kilometers. The presence of rugged highlands to the west suggests a gravitational sliding mechanism and the close spatial and temporal association of volcanic activity suggests that uplifts were genetically related to subjacent intrusions. The older-on-younger geometry of the Matlin sheets and their local tight folding and reverse faulting indicate that this area represents the distal, eastern toe of a distending complex allochthon to the west. Basin and Range high-angle faulting did not begin in the area until Pliocene time, after postmetamorphic uplifts and low-angle faulting had ceased.

A gneiss complex in the Grouse Creek Mountains, northwestern Utah, consists of 2.5-b.y.-old adamellite that was remobilized and intruded younger Precambrian and Paleozoic cover rocks 25 m.y. ago during an extended period of synkinematic metamorphism. One or more structural domes formed in the region during metamorphism. The remains of a middle Tertiary culmination in the central Grouse Creek Mountains is an asymmetrical welt of schistose Precambrian adamellite that protrudes into greatly attenuated autochthonous and allochthonous cover rocks. Fine- to medium-grained gneiss and schist of the upper 200 m of the welt, or dome, grade upward to retrograde phengite-quartz-albite schist that intruded the lowermost beds of upper Precambrian(?) quartzite. The age of the schistose Precambrian adamellite and its geographic continuity with less-metamorphosed Precambrian adamellite lying unconformably beneath the quartzite indicate that remobilization of the former was post-Precambrian. Simultaneously, cover rocks were thinned to one-fifth their original thickness by metamorphic flattening, extension, and low-angle faulting. A 25-m.y.-old pluton underlies the area of remobilization. Its outer shell bears metamorphic folds related to a second deformation. These folds also predominate in a metasomatic aureole in the Precambrian adamellite surrounding the Tertiary pluton and in the upper, schistose part of the gneiss dome. Thus, development of the dome was closely related to the second deformation and to the intrusion 25 m.y. ago. Late-metamorphic folds and low-angle faults chiefly affected allochthonous cover rocks, and postmetamorphic movements carried parts of these rocks over 1 l-m.y.-old sedimentary rocks in the adjoining basin. Isotopic data on autochthonous rocks suggest that metamorphic temperatures persisted into Miocene time and provided the potential for continuing uplift and basinward shedding of sheets which ended after 11 m.y. ago.

A composite section of eight Grande Ronde Basalt flows delineates the margin of the Columbia River Basalt Group on this portion of the eastern flank of the Cascade Range. The Grande Ronde Basalt flows belong to the following units (in descending stratigraphic order): Sentinel Bluffs Member (Basalt of Museum 2 and Museum 1; Basalt of Stember Creek; and upper and lower flows of the Basalt of McCoy Canyon), Ortley member (informal), Grouse Creek member (informal “Meeks Table” flow), and Wapshilla Ridge Member. All these Grande Ronde Basalt flows display similar intraflow structures (cooling joint patterns) and lithology, but they are separable by chemical compositions (i.e., TiO 2 , MgO, P 2 O 5 , Cr, Ba, and Zr). Individual Grande Ronde Basalt flows can range in thickness from 8 to 180 m, with the maximum total thickness of the Grande Ronde Basalt section being 555 m. As the Grande Ronde Basalt flows advanced into the map area, they covered plains, filled stream-cut valleys and canyons up to 160 m deep, and surrounded extinct volcanoes 750 m tall. In post–Grande Ronde Basalt time, the Grande Ronde Basalt flows were deformed into a series of ENE-striking anticlines, synclines, and associated faults that define this portion of the Yakima Fold Belt. During this same time, transpressional deformation activity increased folding and thrust faulting in the Cle Elum–Wallula Lineament, a structural segment of the Olympic-Wallowa Lineament. In addition, series of NNW-striking, dextral strike-slip and normal faults were developed with displacements up to 4.5 km on the strike-slip faults and 1 km on the normal faults. The N-striking Goat Creek and NW-striking Indian Flat and Devils Slide faults merge with the White River fault to the west. These faults, along with the E-NE–striking Bethel Ridge anticline and NNW-striking Cleman Mountain anticline, form the major structures in this area.