Late Palaeocene uplift of the Beartooth Range in northwestern Wyoming and southwestern Montana generated the Beartooth Conglomerate along the eastern and northeastern flanks of the range. Systematic unroofing sequences and intraformational unconformities, folds, and faults in the conglomerate attest to deposition during uplift. Along the eastern flank, at least three ancient alluvial‐fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies. The two southern fans consist of 700+ m of sedimentary‐clast conglomerate and subordinate sandstone, dominated by hyperconcentrated‐flow and stream‐flow facies. The next fan to the north is dominated by plutonic and metamorphic clasts and contains abundant mud‐matrix‐supported debris‐flow facies, as well as stream‐flow facies. The northernmost depositional system consists of arkosic, channellized fluvial conglomerate and sandstone, overbank mudstone, and crevasse‐splay sandstone units. Palaeocurrent data indicate eastward dispersal, away from the Beartooth Range. Outstanding exposure of the Beartooth Conglomerate allows facies to be mapped on lateral photographic mosaics. A seven‐fold hierarchy of bounding surfaces and enclosed lithosomes exists in the Beartooth Conglomerate. First‐ through fourth‐order surfaces are analogous to first‐ through fourth‐order surfaces that recently have been documented in sandy fluvial facies, with one exception: sediment gravity flows are bounded by first‐order surfaces. Fifth‐order surfaces are either erosional (e.g. lateral migration of fanhead trench) or accretionary (e.g. aggradation of fan surface during backfilling of trench, and construction of lobes on lower fan during entrenchment on upper fan). Some fifth‐order surfaces coincide with intraformational angular unconformities and are thus the result of long‐term fanhead entrenchment following uplift of the upper part of the fan. Sixth‐order surfaces bound individual fan packages that are several hundred metres thick and ∼ 10 km2 in area. The enclosed sixth‐order lithosomes are distinguishable in terms of petrofacies and lithofacies. A single seventh‐order surface bounds the entire Beartooth Conglomerate. Lower‐order lithosomes are produced by intrinsic processes of fan construction. Fifth‐order lithosomes can be attributed to both extrinsic and intrinsic controls. Sixth‐ and seventh‐order lithosomes are generated by extrinsic controls.

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The Caribou Creek volcanic fi eld lies along the continent-side edge of forearc basin rocks in south-central Alaska and consists of over 1000 m of shallow-dipping basalt and andesite lavas with minor mafi c pyroclastic deposits. Dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas. The basalts are tholeiitic and strongly depleted in the light rare earth elements (La/ Yb = 0.18-1.5), with concentrations of high fi eld strength elements (e.g., Zr, Hf, Ti, Y) similar to mid-ocean-ridge basalt and with variable enrichment in fl uid-mobile elements (e.g., Cs, Ba, and Pb). Intermediate and felsic rocks show enrichment in the rare earth elements and fl uid-mobile elements plus Rb and K, but retain low La/Yb ratios (0.48-3.6). A few andesite and dacite samples are strongly depleted in the heavy rare earth elements and are geochemically similar to adakites (e.g., Sr/Y up to 52). Ten 40 Ar/ 39 Ar ages for the Caribou Creek volcanic rocks range from 49.4 ± 2.2 to 35.6 ± 0.2 Ma. An adakite-like tuff beneath the other volcanic rocks yields an age of 59.0 ± 0.4 Ma.Caribou Creek basalts were derived from mid-ocean-ridge-like depleted mantle that was emplaced beneath the southern margin of Alaska through a slab window following spreading ridge subduction. Caribou Creek volcanism was coeval with oblique subduc- tion, oroclinal bending, and right-lateral strike-slip faulting in south-central Alaska, all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas. The basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of Jurassic arc basement rocks to form the intermediate and felsic magmas.Enrichment of the basaltic parent magmas in fl uid-mobile elements occurred by contamination from the Jurassic arc rocks and/or by contamination with metasomatic mantle remnant from preceding subduction. High heat fl ow through the slab window induced partial melting of garnet-bearing mafi c parts of the Jurassic arc basement to form the adakite-like rocks. The Caribou Creek volcanic rocks demonstrate that slab windows can directly infl uence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime (e.g., crustal extension) and that the infl uence of a slab window on continental margin magmatism can be long-lived (>20 m.y.).

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Abstract. The Cantwell basin was formed during Late

Episodes of middle Cenozoic near-trench volcanism in California occurred during the transition from convergent to transform plate boundaries as segments of the East Pacific Rise intersected a subduction zone along western North America. Geochemical features of volcanic rocks from the Coast Range Province and Santa Maria Province, which represent two near-trench volcanic episodes, indicate that magmas from each province were derived from depleted mantle and evolved by assimilation-fractional crystallization processes to form predominantly bimodal suites. Basalt and basaltic andesite from both provinces yielded Nd (t) values between ؉9.3 and ؉2.4 and 87 Sr/ 86 Sr(t) ratios of 0.702 58-0.706 72. The observed Nd (t) values that cluster around ؉9 and the 87 Sr/ 86 Sr(t) ratios <0.7029 imply a source of depleted mantle, analogous to mid-ocean-ridge basalt (MORB) sources, for these rocks. Th/Ta and Ba/Ta ratios as low as 0.49 and 35.78, respectively, for the basalt are similar to those of MORB and also suggest a magma source from depleted mantle. Acidic rocks, including rhyolite, dacite, and trachyte samples have Nd (t) values between ؉6.3 and ؊3.2 and 87 Sr/ 86 Sr(t) ratios of 0.703 93 to 0.711 31. The variation among the Coast Range and Santa Maria Provinces volcanic rocks in Nd-Sr isotope ratio space suggests that mixing occurred between the depleted mantle-derived basaltic end-member and an incompatible-element-enriched crustal reservoir through which these rocks erupted. The observed negative correlation of Nd (t) and positive correlation of 87 Sr/ 86 Sr(t) ratios with SiO 2 , respectively, also suggest assimilation of an isotopically distinct crustal component by depleted mantle-derived melts. The ages and paleogeographic distributions of these volcanic rocks indicate that they were erupted during episodes when segments of the East Pacific Rise intersected southern California. Depleted mantle that was emplaced beneath the continental margin during ridge subduction became a source of magma for the episodes of near-trench volcanism as a new strike-slip regime evolved along the continental margin. TABLE 1. AGES OF SOME OLIGOCENE-MIOCENE VOLCANIC UNITS IN WESTERN CALIFORNIA Volcanic unit Age (Ma)* References Coast Range Province (CRP) volcanic centers Halfmoon Bay basalt HMB* Lower Miocene Stanley (1987) Pescadero Beach basalt PB* 22.0 Ϯ 0.7 Taylor (1990) Mindego basalt and related volcanic rocks M † 20.2 Ϯ 1.2 to 23.7 Ϯ 0.7 Turner (1970) Carmel basalt CM † 27.0 Ϯ 0.

The Alaska Range suture zone exposes Cretaceous to Quaternary marine and nonmarine sedimentary and volcanic rocks sandwiched between oceanic rocks of the accreted Wrangellia composite terrane to the south and older continental terranes to the north. New U-Pb zircon ages, 40Ar/39Ar, ZHe, and AFT cooling ages, geochemical compositions, and geological field observations from these rocks provide improved constraints on the timing of Cretaceous to Miocene magmatism, sedimentation, and deformation within the collisional suture zone. Our results bear on the unclear displacement history of the seismically active Denali fault, which bisects the suture zone. Newly identified tuffs north of the Denali fault in sedimentary strata of the Cantwell Formation yield ca. 72 to ca. 68 Ma U-Pb zircon ages. Lavas sampled south of the Denali fault yield ca. 69 Ma 40Ar/39Ar ages and geochemical compositions typical of arc assemblages, ranging from basalt-andesite-trachyte, relatively high-K, and high concentrations of incompatible elements attributed to slab contribution (e.g., high Cs, Ba, and Th). The Late Cretaceous lavas and bentonites, together with regionally extensive coeval calc-alkaline plutons, record arc magmatism during contractional deformation and metamorphism within the suture zone. Latest Cretaceous volcanic and sedimentary strata are locally overlain by Eocene Teklanika Formation volcanic rocks with geochemical compositions transitional between arc and intraplate affinity. New detrital-zircon data from the modern Teklanika River indicate peak Teklanika volcanism at ca. 57 Ma, which is also reflected in zircon Pb loss in Cantwell Formation bentonites. Teklanika Formation volcanism may reflect hypothesized slab break-off and a Paleocene–Eocene period of a transform margin configuration. Mafic dike swarms were emplaced along the Denali fault from ca. 38 to ca. 25 Ma based on new 40Ar/39Ar ages. Diking along the Denali fault may have been localized by strike-slip extension following a change in direction of the subducting oceanic plate beneath southern Alaska from N-NE to NW at ca. 46–40 Ma. Diking represents the last recorded episode of significant magmatism in the central and eastern Alaska Range, including along the Denali fault. Two tectonic models may explain emplacement of more primitive and less extensive Eocene–Oligocene magmas: delamination of the Late Cretaceous–Paleocene arc root and/or thickened suture zone lithosphere, or a slab window created during possible Paleocene slab break-off. Fluvial strata exposed just south of the Denali fault in the central Alaska Range record synorogenic sedimentation coeval with diking and inferred strike-slip displacement. Deposition occurred ca. 29 Ma based on palynomorphs and the youngest detrital zircons. U-Pb detrital-zircon geochronology and clast compositional data indicate the fluvial strata were derived from sedimentary and igneous bedrock presently exposed within the Alaska Range, including Cretaceous sources presently exposed on the opposite (north) side of the fault. The provenance data may indicate ∼150 km or more of dextral offset of the ca. 29 Ma strata from inferred sediment sources, but different amounts of slip are feasible. Together, the dike swarms and fluvial strata are interpreted to record Oligocene strike-slip movement along the Denali fault system, coeval with strike-slip basin development along other segments of the fault. Diking and sedimentation occurred just prior to the onset of rapid and persistent exhumation ca. 25 Ma across the Alaska Range. This phase of reactivation of the suture zone is interpreted to reflect the translation along and convergence of southern Alaska across the Denali fault driven by highly coupled flat-slab subduction of the Yakutat microplate, which continues to accrete to the southern margin of Alaska. Furthermore, a change in Pacific plate direction and velocity at ca. 25 Ma created a more convergent regime along the apex of the Denali fault curve, likely contributing to the shutting off of near-fault extension-facilitated arc magmatism along this section of the fault system and increased exhumation rates.

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