The uppermost 2 km of the oceanic crust created at the fast spreading (135 mm yr À1 , full rate) equatorial East Pacific Rise (EPR) is exposed for tens of kilometers along escarpments bounding the Hess Deep Rift. Mosaics of large-scale digital images from the remotely operated vehicle (ROV) Argo II and direct observations from the submersible Alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading. Dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units. These rock units are characterized by extensive faulting, fine-scale fracturing, and rotations of coherent crustal blocks meters to tens of meters across. The uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces. Through most of the basaltic lava unit, however, lava flow contacts dip (208-708W) toward the EPR and generally increase in dip downward in the section. Dikes cutting the lavas and in the underlying sheeted dike unit generally dip (908 -408E) away from the EPR. Deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units. We interpret this upper crustal structure as the result of subaxial subsidence within 1-2 km of the EPR that accommodated the thickening of the basaltic lava unit to $500 m. Variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply. These results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the Hess Deep Rift. If this type of structure is typical of uppermost oceanic crust generated at the EPR, these processes may be common along fast spreading mid-ocean ridges.

Mesozoic strata of the northwestern Talkeetna Mountains are located in a regional suture zone between the allochthonous Wrangellia composite terrane and the former Mesozoic continental margin of North America (i.e., the Yukon-Tanana terrane). New geologic mapping, measured stratigraphic sections, and provenance data define a distinct three-part stratigraphy for these strata. The lowermost unit is greater than 290 m thick and consists of Upper Triassic-Lower Jurassic mafic lavas, fossiliferous limestone, and a volcaniclastic unit that collectively we informally refer to as the Honolulu Pass formation. The uppermost 75 m of the Honolulu Pass formation represent a condensed stratigraphic interval that records limited sedimentation over a period of up to ca. 25 m.y. during Early Jurassic time. The contact between the Honolulu Pass formation and the overlying Upper Jurassic-Lower Cretaceous clastic marine strata of the Kahiltna assemblage represents a ca. 20 m.y. depositional hiatus that spans the Middle Jurassic and part of Late Jurassic time. The Kahiltna assemblage may to be up to 3000 m thick and contains detrital zircons that have a robust U-Pb peak probability age of 119.2 Ma (i.e., minimum crystallization age/maximum depositional age). These data suggest that the upper age of the Kahiltna assemblage may be a minimum of 10-15 m.y. younger than the previously reported upper

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A sinuous zone of gently southeast-dipping low-angle Tertiary normal faults is exposed for 100 km along the eastern margins of the Anaconda and Flint Creek ranges in southwest Montana. Faults in the zone variously place Mesoproterozoic through Paleozoic sedimentary rocks on younger Tertiary granitic rocks or on sedimentary rocks older than the overlying detached rocks. Lower plate rocks are lineated and mylonitic at the main fault and, below the mylonitic front, are cut by mylonitic mesoscopic to microscopic shear zones. The upper plate consists of an imbricate stack of younger-on-older sedimentary rocks that are locally mylonitic at the main, lowermost detachment fault but are characteristically strongly brecciated or broken. Kinematic indicators in the lineated mylonite indicate tectonic transport to the east-southeast. Syntectonic sedimentary breccia and coarse conglomerate derived solely from upper plate rocks were deposited locally on top of hanging-wall rocks in low-lying areas between fault blocks and breccia zones. Muscovite occurs locally as mica fish in mylonitic quartzites at or near the main detachment. The 40Ar/39Ar age spectrum obtained from muscovite in one mylonitic quartzite yielded an age of 47.2 + 0.14 Ma, interpreted to be the age of mylonitization. The fault zone is interpreted as a detachment fault that bounds a metamorphic core complex, here termed the Anaconda metamorphic core complex, similar in age and character to the Bitterroot mylonite that bounds the Bitterroot metamorphic core complex along the Idaho-Montana state line 100 km to the west. The Bitterroot and Anaconda core complexes are likely components of a continuous, tectonically integrated system. Recognition of this core complex expands the region of known early Tertiary brittle-ductile crustal extension eastward into areas of profound Late Cretaceous contractile deformation characterized by complex structural interactions between the overthrust belt and Laramide basement uplifts, overprinted by late Tertiary Basin and Range faulting.

Volcanic rocks (Cenozoic) MESOPROTEROZOIC SEDIMENTARY ROCKS Belt Supergroup (Mesoproterozoic)-Area containing major bodies patterned Yellowjacket Formation and associated metasedimentary rocks (Mesoproterozoic)-Area containing major bodies patterned TRANS-HUDSON OROGEN Volcanic-plutonic arc complexes (Paleoproterozoic)-Includes Archean gneisses TRANS-MONTANA OROGEN (ACCRETED TERRANES) Wallace terrane (Paleoproterozoic)-Covered Biotite-quartz-feldspar gneiss and amphibolite of Wallace terrane (Paleoproterozoic)-Exposed in Bitterroot Range Granite, diorite, and gneiss (Paleoproterozoic)-Crops out in Little Belt Mountains. U-Pb zircon age of 1,860-1,880 Ma (Mueller and others, 2002) Medicine Hat block (Archean)-Covered CONTINENTAL-MARGIN ASSEMBLAGE (FOLD-AND-THRUST BELT) Imbricately intercalated rocks (Paleoproterozoic and Archean)-Covered Meta-sandstone, shale, iron-formation, and graphitic shale intruded by gabbro dikes (Paleoproterozoic)-Crops out in Gravelly Range. Interpreted as foreland basin deposit Marble, quartzite, and schist (Paleoproterozoic)-Inferred to have formed on a rifted, passive margin (about 2.0 Ga). Formerly regarded as Archean Massive to weakly foliated granite to granodiorite (Late Archean) Foliated and gneissic granitoid rocks (Late Archean)-About 2.74-2.79 Ga (Wooden and others, 1988) Mafic to ultramafic rocks in Spanish Peaks area and Tobacco Root Mountains (Archean) WYOMING PROVINCE (CRATON) Stillwater Complex (Late Archean)-2.7 Ga (Wooden and others, 1988). Exposed Foliated and gneissic granitoid rocks (Late Archean)-About 2.74-2.79 Ga (Wooden and others, 1988). Exposed Granitic rocks, undivided (Archean)-Magmatic domain. Covered Gneissic rocks, undivided (Archean)-Gneiss domain. Covered