An integrated structural, stratigraphic, geochronological, and geochemical investigation of Cenozoic volcanic and sedimentary rocks within a highly extended part of the eastern Great Basin sheds light on the interplay between magmatism and extensional tectonism. Tertiary rocks in east-central Nevada and west-central Utah can be divided into three broad groups: (1) 40 to 35 Ma, locally derived sequences of andesite and rhyolite lava flows and ash-flow tuffs; (2) the voluminous 35 Ma Kalamazoo volcanic rocks, including the compositionally zoned (rhyolite to dacite) Kalamazoo Tuff, crystalrich hornblende dacite lavas, and the K-rich dacite tuff of North Creek and associated lavas; and (3) 35 to 20(?) Ma, predominantly sedimentary sequences. Crosscutting relations between faults and subvolcanic intrusions, decreasing tilts upward within the Tertiary sections, and sedimentologic evidence for rapid unroofing of deep structural levels demonstrate that rapid, large-magnitude extension in this region began at least 36 Ma during some of the earliest eruptions, was ongoing at 35 Ma during the culminating eruptions of Kalamazoo volcanic rocks, and continued after volcanism had largely ceased. These synextensional volcanic rocks constitute a high-K calc-alkaline andesite to rhyolite series, and closely resemble suites from the central Andes rather than the bimodal or alkalic suites commonly associated with continental rifts. Trace-element systematics and reconnaissance Sr and Nd isotopic data suggest that the suite formed by extensive contamination of mantle-derived basalt by crustal partial melts in the deep crust, followed by relatively minor wall-rock assimilation during fractionation from andesite to rhyolite, presumably at shallower levels. Modeling of the isotopic data suggests that the most voluminous rock type, hornblende dacite, consists of 30 to 50 percent mantle material. Thus, intrusions associated with Cenozoic volcanic rocks represent a significant addition of new mantle-derived material to the continental crust.A comparison of the eastern Great Basin with other highly extended parts of the Basin and Range province reveals striking similarities in eruptive and extensional histories, despite important regional variations in absolute timing. These similarities are best explained by an active rifting model that invokes a flux of basaltic magma into the crust, hybridization and mixing of these magmas with crustal melts to produce intermediate magmas that differentiate in shallower magma reservoirs, and magmatically induced thermal weakening of the crust culminating in brittle failure of the upper crust and ductile flow at depth. This model helps explain (1) the close spatial and temporal association between the onset of large-magnitude extension and voluminous volcanism throughout the province; (2) the general decrease in extensional strain rates through time; (3) the typical progression of magma compositions from early, intermediate to silicic rocks to late, relatively primitive basaltic or bimodal suites; and (4) ...

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In the Mt. Olympos region of northeastern Greece, continental margin strata and basement rocks were subducted and metamorphosed under blueschist facies conditions, and thrust over carbonate platform strata during Alpine orogenesis. Subsequent exposure of the subducted basement rocks by normal faulting has allowed an integrated study of the timing of metamorphism, its relationship to deformation, and the thermal history of the subducted terrane. Alpine low‐grade metamorphic assemblages occur at four structural levels. Three thrust sheets composed of Paleozoic granitic basement and Mesozoic metasedimentary cover were thrust over Mesozoic carbonate rocks and Eocene flysch; thrusting and metamorphism occurred first in the highest thrust sheets and progressed downward as units were imbricated from NE to SW. 40Ar/39Ar spectra from hornblende, white mica, and biotite samples indicate that the upper two units preserve evidence of four distinct thermal events: (1) 293–302 Ma crystallization of granites, with cooling from >550°C to <325°C by 284 Ma; (2) 98–100 Ma greenschist to blueschist‐greenschist transition facies metamorphism (T∼350–500°C) and imbrication of continental thrust sheets; (3) 53–61 Ma blueschist facies metamorphism and deformation of the basement and continental margin units at T<350–400°C; (4) 36–40 Ma thrusting of blueschists over the carbonate platform, and metamorphism at T∼200–350°C. Only the Eocene and younger events affected the lower two structural packages. A fifth event, indicated by diffusive loss profiles in microcline spectra, reflects the beginning of uplift and cooling to T<100–150°C at 16–23 Ma, associated with normal faulting which continued until Quaternary time. Incomplete resetting of mica ages in all units constrains the temperature of metamorphism during continental subduction to T≤350°C, the closure temperature for Ar in muscovite. The diffusive loss profiles in micas and K‐feldspars enable us to “see through” the younger events to older events in the high‐T parts of the release spectra. Micas grown during earlier metamorphic events lost relatively small amounts of Ar during subsequent high pressure‐low temperature metamorphism. Release spectra from phengites grown during Eocene metamorphism and deformation record the ages of the Ar‐loss events. Alpine deformation in northern Greece occurred over a long time span (∼90 Ma), and involved subduction and episodic imbrication of continental basement before, during, and after the collision of the Apulian and Eurasian plates. Syn‐subduction uplift and cooling probably combined with intermittently higher cooling rates during extensional events to preserve the blueschist facies mineral assemblages as they were exhumed from depths of >20 km. Extension in the Olympos region was synchronous with extension in the Mesohellenic trough and the Aegean back‐arc, and concurrent with westward‐progressing shortening in the external Hellenides.

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In the Fort Irwin region of the northern Mojave desert, late Cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age. Kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral‐thrust slip on northwest striking, moderately dipping segments at the east ends of the blocks. Crustal blocks ∼7–10 km wide by ∼50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block. Faulting initiated after ∼11 Ma, and Quaternary deposits are faulted and folded. We document a minimum of 13 km cumulative sinistral offset in a north‐south transect from south of the Bicycle Lake fault to north of the Drinkwater Lake fault. Paleomagnetic results from 50 sites reveal two direction groups in early and middle Miocene rocks. The north‐to‐northwest declinations of the first group are close to the middle Miocene reference pole. However, rock magnetic studies suggest that both primary and remagnetized directions are present in this group. The northeast declinations of the second group are interpreted as primary and 63.5° ± 7.6° clockwise from the reference pole and suggest net post middle Miocene clockwise rotation of several of the east trending blocks in the northeast Mojave domain. The Jurassic Independence Dike Swarm in Fort Irwin may be rotated 25–80° clockwise relative to the swarm north of the Garlock fault, thus supporting the inference of clockwise rotation. Using a simple‐shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks, we predict ∼23° clockwise rotation using the observed fault slip, or one‐third that inferred from the paleomagnetic results. The discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks, where most of our paleomagnetic sites are located. However, at least 25°–40° of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible “rigid‐body” rotation of the region evidenced by clockwise bending of northwest striking domain‐bounding faults. Our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the Eastern California Shear Zone over the last 10 m.y. occurred within the northeast Mojave Domain. The remainder must be accommodated in adjacent structural domains, e.g., east of the Avawatz Mountains and west of the Goldstone Lake fault. Supporting Appendices 1 and 2 are available on diskette or via Anonymous FTP from kosmos.agu.org, directory APEND (Username ‐‐ anonymous, Password = guest). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W, Washington, DC 20009 or by phone at 800‐966‐2481; $15.00. Payment must accompany order.