The San Jacinto right-lateral strike-slip fault zone is crucial for understanding plate-boundary dynamics, regional slip partitioning, and seismic hazards within the San Andreas fault system of southern California, yet its age of initiation and long-term average slip rate are controversial. This synthesis of prior and new detailed studies in the western Salton Trough documents initiation of structural segments of the San Jacinto fault zone at or slightly before the 1.07-Ma base of the Jaramillo subchron. The dextral faults changed again after ca. 0.5–0.6 Ma with creation of new fault segments and folds. There were major and widespread basinal changes in the early Pleistocene when these new faults cut across the older West Salton detachment fault. We mapped and analyzed the complex fault mesh, identified structural segment boundaries along the Clark, Coyote Creek, and San Felipe fault zones, documented linkages between the major dextral faults, identified previously unknown active strands of the Coyote Creek fault 5 and 8 km NE and SW of its central strands, and showed that prior analyses of these fault zones oversimplify their complexity. The Clark fault is a zone of widely distributed faulting and folding SE of the Santa Rosa Mountains and unequivocally continues 20–25 km SE of its previously inferred termination point to the San Felipe Hills. There the Clark fault zone has been deforming basinal deposits at an average dextral slip rate of ≥10.2 +6.9/−3.3 mm/yr for ~0.5–0.6 m.y. Five new estimates of displacement are developed here using offset successions of crystalline rocks, distinctive marker beds in the late Cenozoic basin fill, analysis of strike-slip–related fault-bend folds, quantification of strain in folds at the tips of dextral faults, and gravity, magnetic, and geomorphic data sets. Together these show far greater right slip across the Clark fault than across either the San Felipe or Coyote Creek faults, despite the Clark fault becoming “hidden” in basinal deposits at its SE end as strain disperses onto a myriad of smaller faults, strike-slip ramps and flats, transrotational systems of cross faults with strongly domain patterns, and a variety of fault-fold sets. Together the Clark and Buck Ridge–Santa Rosa faults accumulated ~16.8 +3.7/−6.0 km of right separation in their lifetime near Clark Lake. The Coyote Ridge segment of the Coyote Creek fault accumulated ~3.5 ± 1.3 km since roughly 0.8–0.9 Ma. The San Felipe fault accumulated between 4 and 12.4 km (~6.5 km preferred) of right slip on its central strands in the past 1.1–1.3 Ma at Yaqui and Pinyon ridges. Combining the estimates of displacement with ages of fault initiation indicates a lifetime geologic slip rate of 20.1 +6.4/−9.8 mm/yr across the San Jacinto fault zone (sum of Clark, Buck Ridge, and Coyote Creek faults) and about ~5.4 +5.9/−1.4 mm/yr across the San Felipe fault zone at Yaqui and Pinyon ridges. The NW Coyote Creek fault has a lifetime slip rate of ~4.1 +1.9/−2.1 mm/yr, which is a quarter of that across the Clark fault (16.0 +4.5/−9.8 mm/yr) nearby. The San Felipe fault zone is not generally regarded as an active fault in the region, yet its lifetime slip rate exceeds those of the central and southern Elsinore and the Coyote Creek fault zones. The apparent lower slip rates across the San Felipe fault in the Holocene may reflect the transfer of strain to adjacent faults in order to bypass a contractional bend and step at Yaqui Ridge. The San Felipe, Coyote Creek, and Clark faults all show evidence of major structural adjustments after ca. 0.6–0.5 Ma, and redistribution of strain onto new right- and left-lateral faults and folds far removed from the older central fault strands. Active faults shifted their locus and main central strands by as much as 13 km in the middle Pleistocene. These changes modify the entire upper crust and were not localized in the thin sedimentary basin fill, which is only a few kilometers thick in most of the western Salton Trough. Steep microseismic alignments are well developed beneath most of the larger active faults and penetrate basement to the base of the seismogenic crust at 10–14 km. We hypothesize that the major structural and kinematic adjustments at ca. 0.5–0.6 Ma resulted in major changes in slip rate within the San Jacinto and San Felipe fault zones that are likely to explain the inconsistent slip rates determined from geologic (1–0.5 m.y.; this study), paleoseismic, and geodetic studies over different time intervals. The natural evolution of complex fault zones, cross faults, block rotation, and interactions within their broad damage zones might explain all the documented and implied temporal and spatial variation in slip rates. Co-variation of slip rates among the San Jacinto, San Felipe, and San Andreas faults, while possible, is not required by the available data. Together the San Jacinto and San Felipe fault zones have accommodated ~25.5 mm/yr since their inception in early Pleistocene time, and were therefore slightly faster than the southern San Andreas fault during the same time interval. If the westward transfer of plate motion continues in southern California, the southern San Andreas fault in the Salton Trough may change from being the main plate boundary fault to defining the eastern margin of the growing Sierra Nevada microplate, as implied by other workers.

ABSTRACT The geomorphology, soils, and climate of Columbia Basin vineyards are the result of a complex and dynamic geologic history that includes the Earth's youngest flood basalts, an active fold belt, and repeated cataclysmic flooding. Miocene basalt of the Columbia River Basalt Group forms the bedrock for most vineyards. The basalt has been folded by north-south compression, creating the Yakima fold belt, a series of relatively tight anticlines separated by broad synclines. Topography related to these structures has strongly influenced the boundaries of many of the Columbia Basin's American Viticultural Areas (AVAs). Water gaps in the anticlinal ridges of the Yakima fold belt restrict cold air drainage from the broad synclinal basins where many vineyards are located, enhancing the development of temperature inversions and locally increasing diurnal temperature variations. Vineyards planted on the southern limbs of Yakima fold belt anticlines benefit from enhanced solar radiation and cold air drainage. Most Columbia Basin vineyards are planted in soils formed in eolian sediment that is primarily derived from the deposits of Pleistocene glacial outburst floods. The mineralogy of the eolian sediment differs substantially from the underlying basalt. Vineyard soil chemistry is thus more complex in areas where eolian sediment is comparatively thin and basalt regolith lies within the rooting zone. The components of physical terroir that broadly characterize the Columbia Basin, such as those described above, vary substantially both between and within its AVAs. The vineyards visited on this field trip are representative of both their AVAs and the variability of terroir within the Columbia Basin.

Abstract A prevailing hypothesis for the central Cascade Range of Washington State is that it underwent regional extension or transtension during the Eocene. This hypothesis is based on the idea that kilometers-thick, clastic, Eocene formations were deposited syntectonically in local basins. Our mapping and structural analysis indicate that these formations are preserved in fault-bounded, regional synclines, not in separate depositional basins. Thus, the type area for the hypothesis, the so-called Chiwaukum graben, is here renamed the Chiwaukum Structural Low. The Eocene arkosic Chum-stick Formation, which was thought to have been syntectonically deposited in the graben, is the proximal equivalent of the Roslyn Formation 25 km southwest of the graben. Because the name “Roslyn Formation” has precedence, the name “Chumstick Formation” should be abandoned. Additionally, several areas previously mapped as Chumstick Formation in the Chiwaukum Structural Low probably are parts of the older Swauk Formation and younger Wenatchee Formation. The southwestern boundary of the Chiwaukum Structural Low includes the Leav-enworth fault zone, which consists of postdepositional, northwest-striking reverse faults with adjacent northwest-striking folds. The reverse faults place the regionally extensive early-Eocene, arkosic Swauk Formation over the mid-Eocene, arkosic Chumstick Formation. A diamictite, which previously was placed in the Chumstick Formation and inferred to have been syntectonically derived from the Leavenworth fault zone, is part of the older Swauk Formation. We mapped a 0.6–1-km-thick conglomerate-bearing sandstone as a robust marker unit in the Chumstick Formation; instead of being spatially related to the bounding faults, this unit has a >30 km strike length around the limbs of folds in the structural low. The northwest-striking reverse faults and fold hinges of the structural low are cut by north-striking strike-slip faults, which likely are late Eocene to Oligocene; these north-south faults partially bound the structural low. The Eocene folds and faults were reactivated by deformation of the Miocene Columbia River Basalt Group; this younger folding largely defines the regional map pattern, including the structural low. A model to account for the above characteristics is that all of the Eocene formations, not just the Roslyn Formation, are kilometers thick and are remnants of regional unconformity-bounded sequences that were deposited on the Eocene margin of this part of North America. Their present distribution is governed by younger faults, folds, and erosion. Thus, the Eocene to Recent history of the central Cascade region is characterized not by crustal extension, but by episodes of folding (with related reverse faults) and strike-slip faulting.

Abstract Newberry Volcano is located in central Oregon at the intersection of the Cascade Range and the High Lava Plains. Its lavas range in age from ca. 0.5 Ma to late Holocene. Erupted products range in composition from basalt through rhyolite and cover ~3000 km 2 . The most recent caldera-forming eruption occurred ~80,000 years ago. This trip will highlight a revised understanding of the volcano's history based on new detailed geologic work. Stops will also focus on evidence for ice and flooding on the volcano, as well as new studies of Holocene mafic eruptions. Newberry is one of the most accessible U.S. volcanoes, and this trip will visit a range of lava types and compositions including tholeiitic and calc-alkaline basalt flows, cinder cones, and rhyolitic domes and tuffs. Stops will include early distal basalts as well as the youngest intracaldera obsidian flow.

ABSTRACT Newberry Volcano in central Oregon is dry over much of its vast area, except for the lakes in the caldera and the single creek that drains them. Despite the lack of obvious glacial striations and well-formed glacial moraines, evidence indicates that Newberry was glaciated. Meter-sized foreign blocks, commonly with smoothed shapes, are found on cinder cones as far as 7 km from the caldera rim. These cones also show evidence of shaping by flowing ice. In addition, multiple dry channels likely cut by glacial meltwater are common features of the eastern and western flanks of the volcano. On the older eastern flank of the volcano, a complex depositional and erosional history is recorded by lava flows, some of which flowed down channels, and interbedded sediments of probable glacial origin. Postglacial lava flows have subsequently filled some of the channels cut into the sediments. The evidence suggests that Newberry Volcano has been subjected to multiple glaciations.

Abstract The northwest rift zone (NWRZ) eruption took place at Newberry Volcano ~7000 years ago after the volcano was mantled by tephra from the catastrophic eruption that destroyed Mount Mazama and produced the Crater Lake caldera. The NWRZ eruption produced multiple lava flows from a variety of vents including cinder cones, spatter vents, and fissures, possibly in more than one episode. Eruptive behaviors ranged from energetic Strombolian, which produced significant tephra plumes, to low-energy Hawaiian-style. This paper summarizes and in part reinterprets what is known about the eruption and presents information from new and ongoing studies. Total distance spanned by the eruption is 32 km north-south. The northernmost flow of the NWRZ blocked the Deschutes River upstream from the city of Bend, Oregon, and changed the course of the river. Renewed mafic activity in the region, particularly eruptions such as the NWRZ with tephra plumes and multiple lava flows from many vents, would have significant impacts for the residents of Bend and other central Oregon communities.

Abstract The 1980 eruption of Mount St. Helens caused instantaneous landscape disturbance on a grand scale. On 18 May 1980, an ensemble of volcanic processes, including a debris avalanche, a directed pyroclastic density current, voluminous lahars, and widespread tephra fall, abruptly altered landscape hydrology and geomorphology, and created distinctive disturbance zones having varying impacts on regional biota. Response to the geological and ecological disturbances has been varied and complex. In general, eruption-induced alterations in landscape hydrology and geomorphology led to enhanced stormflow discharge and sediment transport. Although the hydrolog-ical response to landscape perturbation has diminished, enhanced sediment transport persists in some basins. In the nearly 30 years since the eruption, 350 million (metric) tons of suspended sediment has been delivered from the Toutle River watershed to the Cowlitz River (roughly 40 times the average annual preeruption suspended-sediment discharge of the Columbia River). Such prodigious sediment loading has wreaked considerable socioeconomic havoc, causing significant channel aggradation and loss of flood conveyance capacity. significant and ongoing engineering efforts have been required to mitigate these problems. The overall biological evolution of the eruption-impacted landscape can be viewed in terms of a framework of survivor legacies. Despite appearances to the contrary, a surprising number of species survived the eruption, even in the most heavily devastated areas. With time, survivor “hotspots” have coalesced into larger patches, and have served as stepping stones for immigrant colonization. The importance of biological legacies will diminish with time, but the intertwined trajectories of geophysical and biological successions will influence the geological and biological responses to the 1980 eruption for decades to come.

Abstract The John Day Basin of central Oregon contains a remarkably detailed and well-dated Early Eocene–Late Miocene sedimentary sequence, known for its superb fossils. This field trip examines plant fossil assemblages from throughout the sequence in the context of their geological and taphonomic setting and regional and global significance. The Early to Late Eocene (>54–39.7 Ma) Clarno Formation contains fossil plants and animals that occupied an active volcanic landscape near sea level, interspersed with meandering rivers and lakes. Clarno assemblages, including the ca. 44 Ma Nut Beds flora, record near-tropical “Boreotropical” rainforest, which was replaced during late Clarno time by more open and seasonal subtropical forest. The overlying John Day Formation (39.7–18.2 Ma) was deposited in a backarc landscape of low hills dotted with lakes and showered by ashfalls from the Western Cascades. Fossils and paleosols record the advent of the “Icehouse” Earth during the earliest Oligocene, with decreasing winter temperature and more seasonal rainfall that supported open deciduous and coniferous forest, much like that of the southern Chinese highlands today. Sixteen and a half million years ago the Picture Gorge flood basalt covered the region. Animals and plants fossilized in the overlying (ca. 16 to >12 Ma) Mascall Formation occupied a relatively flat landscape during a warm and moist period known as the Middle Miocene Climatic Optimum. In total this sequence preserves a detailed series of time slices illustrating regional biotic and landscape evolution during the Cenozoic that is highly relevant for deciphering regional and global biotic, climatic, and geological trends.

ABSTRACT This field guide describes stops in the Oregon Klamath Mountains that visit near-complete ophiolite sections, pre- and post-accretion arc plutons, greenschist- to amphibolite-grade metamorphosed wallrocks, arc volcanic rocks, and interbedded chert, argillite, and olistostromal deposits. Structural features at these stops include local- and regional-scale folds and faults, as well as penetrative metamorphic fabrics such as slaty cleavage, gneissic layering, and mineral lineations. The geologic history here reveals a period of Late Triassic and Jurassic ophiolite and oceanic-arc formation followed by Middle Jurassic terrane accretion, tectonic mélange formation, and continued oceanic arc magmatism. Rifting from ca. 165 to 160 Ma produced the Rogue-Chetco arc, Josephine ophiolite, and remnant arc comprised of older Klamath Mountains terranes. Deformation and magmatism during the Late Jurassic Neva-dan orogeny accreted this active arc–inter-arc basin–remnant arc triad to western North America, producing the lithotectonic belts observed today. The Oregon Klam-ath Mountains therefore provide an exceptional opportunity to examine the deep to shallow levels of multi-phase oceanic lithosphere and deformational features related to the accretion of these terranes to the continental margin.

Abstract This field trip guide describes a two-day excursion through Mesozoic accreted terranes of the Blue Mountains Province in northeastern Oregon. Day 1 is focused on sedimentary rocks of the Izee terrane. These deposits are divided into two unconformity-bounded megasequences, MS-1 and MS-2, that record two stages of syntectonic basin formation. MS-1 (Late Triassic to Early Jurassic) accumulated in fault-bounded marine sub-basins on the flank of an inferred growing Baker terrane thrust belt. MS-1 sandstones, derived from the Baker terrane, contain abundant Paleozoic, Late Paleoproterozoic, and Late Archean detrital-zircon grains. These observations suggest affinity of the Baker terrane and MS-1 in the Izee area to portions of the Klamath and Sierra Nevada terranes that contain similar detrital-zircon age distributions. MS-2 (Early to early-Late Jurassic) accumulated in a large marine basin that received input from low-grade metavolcanic rocks to the east (modern coordinates). Detrital zircons are dominated by Mesozoic, Neoproterozoic, and Mesoproterozoic grains. Two possible interpretations for MS-2 are: (1) the Jurassic Izee basin was fed directly by the large Mesozoic trans-cratonal sediment-dispersal system, or (2) trans-cratonal sediment was deposited in a Triassic backarc basin in Nevada and was later recycled into the Jurassic Izee basin during Cordilleran orogenesis. Day 2 of the field trip is focused on Jurassic–Cretaceous magmatism in the Baker terrane. Late Middle Jurassic to Early Cretaceous igneous rocks in the Blue Mountains Province record three distinct pulses of plutonism that are characterized by distinctive spatial and geochemical signatures. These episodes consist of: (1) late Middle to Late Jurassic small gabbro to quartz diorite plutons (ca. 162–154 Ma; low Sr/Y); (2) Late Jurassic to Early Cretaceous plutons and batholiths (ca. 148 and 137 Ma; includes spatially distinct belts of low and high Sr/Y at 147–145 Ma); and (3) Early Cretaceous small plutons of tonalitic and trondhjemitic composition (ca. 124–111 Ma). Temporal transitions in geochemical characteristics between these suites raise fundamental questions regarding the origins of plutonism in the Baker terrane. In particular, the transition from low Sr/Y (group 1) to high Sr/Y (group 2) magmatism in the Greenhorn subterrane occurred ~ 7 Ma after regional contraction, and may record partial melting of thickened crust as a direct result of Late Jurassic orogenesis.

ABSTRACT The field trip guide describes nine stops that examine the mechanisms and timing of some of the abundant and often gigantic landslides that occur along the Winter Ridge–Slide Mountain escarpment in south-central Oregon. Subsidence of Summer Lake basin, situated in the northwestern Basin and Range province, has exposed a kilometer-thick Neogene sequence of dense volcanic flow rocks overlying very weak tuffaceous sedimentary rocks in the bounding escarpment. Subsidence is accommodated on the 58-km-long Winter Rim fault system, a normal fault which is capable of producing M w ≈ 7 earthquakes with near-field, maximum horizontal acceleration approaching 1 g on the bedrock footwall. Gigantic rock slides cubic kilometers in volume scallop the southwestern portion of the escarpment, and their deposits run out as rock avalanches several kilometers onto the basin floor. Limit-equilibrium slope stability analyses support observations that these gigantic bedrock landslides initiate within the weak tuffaceous sedimentary rocks along shallow, east-dipping, planar failure surfaces one to two kilometers in length; are insensitive to groundwater fluctuations; and, are stable under static conditions. Strong ground motions appear requisite to trigger landsliding and are necessary to replicate the long, shallow failure surfaces. Landslide, colluvial, and lacustrine deposits on the hanging wall have undergone widespread post-emplacement deformation, which may involve large-scale seismogenic lateral spreading and flow sliding controlled by the saturated, fine-grained basin fill.

Abstract Late Holocene dome-building eruptions at Mount Hood during the Timberline and Old Maid eruptive periods resulted in numerous dome-collapse pyroclastic flows and lahars that moved large volumes of volcaniclastic sediment into temporary storage in headwater canyons of the Sandy River. During each eruptive period, accelerated sediment loading to the river through erosion and remobilization of volcanic fragmental debris resulted in very high sediment-transport rates in the Sandy River during rain- and snowmelt-induced floods. Large sediment loads in excess of the river's transport capacity led to channel aggradation, channel widening, and change to a braided channel form in the lowermost reach of the river, between 61 and 87 km downstream from the volcano. The post-eruption sediment load moved as a broad bed-material wave, which in the case of the Old Maid eruption took ~2 decades to crest 83 km downstream. Maximum post-eruption aggradation levels of at least 28 and 23 m were achieved in response to Timberline and Old Maid eruptions. In each case, downstream aggradation cycles were initiated by lahars, but the bulk of the aggradation was achieved by fluvial sediment transport and deposition. When the high rates of sediment supply began to diminish, the river degraded, incising the channel fills and forming progressively lower sets of degradational terraces. A variety of debris-flow, hyperconcentrated-flow, and fluvial (upper and lower flow regime) deposits record the downstream passage of the sediment waves that were initiated by these eruptions. The deposits also presage a hazard that may be faced by communities along the Sandy River when volcanic activity at Mount Hood resumes.

Abstract Landslides and floods of lava and water tremendously affected the Columbia River during its long history of transecting the Cascade Volcanic Arc. This field trip touches on aspects of the resulting geology of the scenic Columbia River Gorge, including the river-blocking Bonneville landslide of ~550 years ago and the great late-Pleistocene Missoula floods. Not only did these events create great landscapes, but they inspired great geologists. Mid-nineteenth century observations of the Columbia River and Pacific Northwest by James Dwight Dana and John Strong Newberry helped germinate the “school of fluvial” erosion later expanded upon by the southwestern United States topographic and geologic surveys. Later work on features related to the Missoula floods framed the career of J Harlen Bretz in one of the great geologic controversies of the twentieth century.

Abstract More than 80 small volcanoes are scattered throughout the Portland-Vancouver metropolitan area of northwestern Oregon and southwestern Washington. These volcanoes constitute the Boring Volcanic Field, which is centered in the Neogene Portland Basin and merges to the east with coeval volcanic centers of the High Cascade volcanic arc. Although the character of volcanic activity is typical of many monogenetic volcanic fields, its tectonic setting is not, being located in the forearc of the Cascadia subduction system well trenchward of the volcanic-arc axis. The history and petrology of this anomalous volcanic field have been elucidated by a comprehensive program of geologic mapping, geochemistry, 40 Ar/ 39 Ar geochronology, and paleomag-netic studies. Volcanism began at 2.6 Ma with eruption of low-K tholeiite and related lavas in the southern part of the Portland Basin. At 1.6 Ma, following a hiatus of ~0.8 m.y., similar lavas erupted a few kilometers to the north, after which volcanism became widely dispersed, compositionally variable, and more or less continuous, with an average recurrence interval of 15,000 yr. The youngest centers, 50-130 ka, are found in the northern part of the field. Boring centers are generally monogenetic and mafic but a few larger edifices, ranging from basalt to low-SiO 2 andesite, were also constructed. Low-K to high-K calc-alkaline compositions similar to those of the nearby volcanic arc dominate the field, but many centers erupted magmas that exhibit little influence of fluids derived from the subducting slab. The timing and compositional characteristics of Boring volcanism suggest a genetic relationship with late Neogene intra-arc rifting.

ABSTRACT The field trip examines coupled hydrologic and landscape response after the cataclysmic eruption of Mount Mazama to form Crater Lake in the Cascade volcanic arc at ~7627 ± 150 cal. yr B.P. The Williamson River basin, east of Crater Lake and in the rain shadow of the Cascade Range, was buried beneath thick pumice and pyroclastic-flow deposits. The distinctive physical properties of pumice and volcanic ash affect the movement and retention of water and the ongoing evolution of the landscape. Three themes will be explored: (1) post-eruption transition from perched streams to losing streams along the eastern flank of the Cascade Range; (2) filling and catastrophic draining of a lake trapped behind a dam of pyroclastic flow deposits in the Williamson River canyon; and (3) post-eruption faulting and the hydrology of Klamath Marsh.

ABSTRACT Steens Mountain, a fault-block in the northern Basin and Range Province, rises 1.7 km above flanking basins and drives hydrologic systems that include hot springs, fresh-water streams, and cold artesian wells in the Alvord Valley. It also feeds freshwater streams, desert wetlands, and shallow fresh-water and alkali lakes in the Harney Basin. Steens Mountain melt water from the winter snow pack partitions to surface-water and groundwater systems. How the composition of these fluids evolve along the various flow paths as a result of differences in the geology, interaction with geother-mal aquifers, surface storage time, degree of evaporation, and biology will be examined. Deep-seated flow paths feed Alvord Valley hot springs, which discharge to the east, in the rain shadow of Steens Mountain. The largest of these hot spring systems— Borax Lake—along with features at Mickey Hot Springs, offer ample opportunity to investigate how biosignatures form and become preserved in hydrothermally precipitated sinter deposits. Surface water moving off the westward-dipping slope of Steens Mountain passes through wetland environments to Malheur Lake in Harney Basin. This key point along the Pacific flyway provides wonderful wildlife viewing and the chance to ponder the impacts of biology on lake chemistry. Finally, we will visit the saline-alkaline Harney Lake, the terminal sump for the water moving through Malheur Lake and all of the nearly 40,000 km 2 Harney Basin. At this locale, the focus will be on the influence of evaporative processes on water composition.

ABSTRACT Regional-scale processes of tectonism, late Quaternary marine transgression, and patterns of aeolian deposition and erosion largely control the geoarchaeological character of the Oregon coast. Dramatic changes to the landscape of the Oregon coast since the Last Glacial Maximum drove the evolution of terrestrial and marine environmental processes which in turn conditioned the location and nature of prehistoric human activities. Due to the geologic complexities of Oregon's coast, archaeological investigations must address a broad range of geological factors that worked to greatly modify the ancient coastal landscape. In many ways, the modern Oregon coastline bears little resemblance to that associated with prehistoric coastal peoples prior to 3000 years ago, requiring geoscientific perspectives to reconstruct the late Quaternary environmental context. Through the integration of geologic concepts and information, geoarchaeology offers an effective means of finding early sites in the modern coastal landscape and in the now-submerged paleocoastal landscape.

Abstract The dynamic landscape on the north flank of Mount St. Helens includes the largest debris avalanche deposit to accumulate within human history, covering more than 45 km 2 on the upper North Fork of the Toutle River on 18 May 1980. Most land-forms on the debris avalanche are now relatively stable and only affected significantly by geomorphic processes exceeding certain energy thresholds. Following the debris avalanche, the most significant landscape-forming event has been the mudflow of 19 March 1982. This mudflow overtopped the rim of the largest explosion pit, formed and deepened channels, and largely formed the present landscape of the debris avalanche surface. Now that the power of geomorphic processes has diminished, only finer sediment is being moved. Channels are armored with coarser clasts, and valleys are plugging with sediment. Hikers can observe the new landscape from two selected overlooks. Johnston Ridge Observatory is the staging area for a recommended roundtrip hike of 13.6 km (8.4 mi).

ABSTRACT The Salmon River suture zone in west-central Idaho is a steep ocean-continent plate boundary separating Paleozoic-Mesozoic island-arc terranes and the ancestral western Laurentian margin that characterizes much of the central North American Cordillera. In the Riggins region, the most complete record of arc-continent collision and subsequent modification of the accretionary boundary is exposed because of the lower abundance of Cretaceous plutonism as compared to exposures of the boundary regionally along strike, and the deep degree of erosion along the Salmon River Canyon. Using recent mapping, microtectonic analysis, geochronological data, and structural models, this field trip explores the time-transgressive structures currently exposed across the Salmon River suture zone from the eastern foothills of the Seven Devils Mountains into the Salmon River Canyon. The Salmon River suture zone contains a Late Jurassic to Early Cretaceous, west-vergent thrust belt that is overprinted along its eastern extent by the Late Cretaceous, transpressional western Idaho shear zone and Late Cretaceous(?) and Cenozoic ductile-brittle extensional structures. A distinct amalgamation of metavol-canogenic and metasedimentary rocks characterizes the thrust belt and includes the (1) northeastern Wallowa terrane, (2) western Salmon River belt, formally grouped with the Wallowa terrane, and (3) eastern Salmon River belt, known locally as the Riggins Group and Pollock Mountain Amphibolite. The western Idaho shear zone overprints the easternmost rocks and structures associated with the eastern Salmon River belt. It also contains heterogeneous units of tonalite, trondhjemite, and grano-diorite orthogneiss, as well as individual tonalite, granodiorite, and granite plutons that display a gradation and partitioning of deformation and strain internally within the shear zone. East of the magmatic injection zone located along the arc-continent boundary, Laurentian continental metasedimentary rocks and tonalite and granodio-rite plutons occupy the eastern portions of both the shear zone and larger suture zone. Geochronologic data, obtained largely from metaplutonic rocks in the McCall region south of the Riggins region, provide the temporal resolution to constrain current tectonic models proposed for Salmon River suture zone evolution.