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At least 361 records · Page 20Linked to original sources
USGS1992

A revised configuration of the southern California uplift

Recently recovered archival leveling data from southwestern Arizona and adjacent parts of California compel major revisions in the configuration and interpretation of the southern California uplift, both at its culmination and following its partial collapse. Re-examination of the older vertical-control record indicates that similar modifications may be equally appropriate in any reconsideration of the early-20th-century analogue of the southern California uplift. The impact of these revisions is limited to the southeastern part of the uplift, which is now believed to have projected well into northern Mexico. The chief changes in our earlier reconstructions appear as a sharply diminished isobase gradient south of Cottonwood Pass, dramatically reduced tectonic subsidence centering on the Salton Sea, and short-lived uplift of at least 0.3 m at the latitude of El Centro. A newly refined qualitative interpretation of the data implies contraction and decoupling at the base of the seismogenic zone, in conjunction with right-stepping movement and extension between an en echelon transform pair through the Salton Basin, combining to produce both the regional uplift and concurrently developed and relatively localized differential subsidence within the Salton Trough.

California↗
USGS1993

Seismic reflection profiling across Tertiary extensional structures in the eastern Amargosa Desert, southern Nevada, Basin and Range province

Outcrops, shallow well control, and coincident geophysical surveys are used to interpret a seismic reflection profile in the Amargosa Desert, within the Basin and Range province, of southern Nevada. The east-west-trending, 27-km-long seismic line crosses all or parts of three Tertiary subbasins, revealing that basin growth occurred by progressive shifts of basin-bounding faults. The reflection line images Tertiary strata that is rotated by steeply dipping listric faults and that noses into normal faults. A shallow (less than 100 to 200 m deep), laterally continuous, flat-lying, low-frequency reflector, interpreted as a Tertiary basalt flow, suggests that little vertical deformation has occurred within the easternmost of the small Tertiary basins since the eruption of the flow about 10 million years ago. Moderately dipping reflections within the pre-Tertiary bedrock may image Mesozoic thrust faults. The reflection data indicate that, whereas the top of the reflective lower crust shallows to the west, possibly in the direction of increasing crustal extension, the Moho is relatively flat between 30 and 33 km deep. Apparent bright-spot reflections from the lower crust are interpreted as evidence for ductile shearing of the lower crust, not for active magma chambers. Doming of the lower crust resembles that observed elsewhere in the Basin and Range province and is consistent with ductile flow in the lower crust.

Geological Society of America Bulletin↗
USGS1993

Analysis of the seismic origin of landslides: Examples from the New Madrid seismic zone

By analyzing two landslides in the New Madrid seismic zone, we develop an approach for judging if a landslide or group of landslides of unknown origin was more likely to have formed as a result of earthquake shaking or in aseismic conditions. The two landslides analyzed are representative of two groups of land-slides that previous research on the geomorphology and regional distribution of landslides in this region indicates may have been triggered by the 1811-1812 New Madrid earthquakes. Slope-stability models of aseismic conditions show that neither landslide is likely to have formed aseismically even in unrealistically high ground-water conditions. Dynamic stability analysis using Newmark's method shows that both slides probably would have experienced large inertial displacements during earthquake shaking similar to that which occurred in 1811-1812; these displacements are large enough that catastrophic failure is highly probable. Thus, the stability analyses are consistent with other lines of evidence that these landslides formed as a result of strong earthquake shaking during the 1811-1812 earthquakes. Our analysis yields a general relationship between Newmark landslide displacement, earthquake shaking intensity, and the critical acceleration of a landslide. Using this relationship, we estimate the minimum shaking intensities required to trigger the types of landslides studied: an m b = 5.8 or M = 5.9 earthquake is the lower bound threshold at zero epicentral distance that could trigger catastrophic movement of typical block slides in the New Madrid seismic zone; for earth flows, m b = 5.4 or M = 5.3 is the threshold earthquake.

Geological Society of America Bulletin↗
USGS1993

Epiguruk: A late Quaternary environmental record from northwestern Alaska

Epiguruk, a prominent bluff along the Kobuk River in northwestern Alaska, exposes a rich depositional record of Quaternary eolian and fluvial sand, with associated loess, paleosols, and periglacial features. Three major complexes of alluvial and eolian deposits are separated by two conspicuous organic-rich paleosols which formed during cool-moist interstadial intervals. Sediments between the two paleosols include eolian, channel, and flood-plain deposits that formed during alluviation of the Kobuk River to a height of about 12 m above the present level. The youngest depositional complex, which overlies the upper paleosol, is divisible into late Wisconsinan and Holocene components and into fluvial-channel, flood-plain, eolian-dune, sand-sheet, loess, and pond facies. Eolian sand from the active Kobuk sand sea overloaded the river during late Wisconsinan time, causing it to alluviate to about 13 m above its modern level. The Holocene record reflects erosion and deposition by a small southern tributary to the Kobuk River, downcutting by the Kobuk River toward its modern level, and subsequent erosion across a meander belt nearly 8 km wide. Sixty-six radiocarbon ages, many from rooted shrubs, provide a firm chronology for the past 35 k.y. at Epiguruk. The Kobuk River incised to near its present level by about 35 ka, and the upper paleosol began forming near that level about 33 ka, coincident with the Fox termal event of central Alaska. Although higher-standing sandy facies of this paleosol began forming earlier, thick peat beds did not accumulate until about 33 ka at those sites. Late Wisconsinan loess influx and alluviation began about 24 ka, synchronous with the Itkillik II glaciation of the Brooks Range. The river overlapped progressively higher surfaces of the upper paleosol, attaining its maximum height of about 13 m above modern river level between 20 and 19 ka. It downcut about 18.5 ka and subsequently migrated northward. Deposits at the north end of the bluff show that the river had begun building its modern flood plain by 8.6 ka, and it has varied little in elevation since that time.

Alaska↗
USGS1993

Deposition of Franciscan Complex cherts along the paleoequator and accretion to the American margin at tropical paleolatitudes

Red radiolarian cherts from three localities within the Franciscan subduction complex of northern California contain three components of remanent magnetization which are best isolated by progressive thermal demagnetization. The first component, usually removed by 300 °C, has an in situ direction similar to the present axial-dipole field and is probably a recently acquired thermoviscous overprint. A second component, generally removed between 300 and 630 °C, has constant (normal) polarity and direction within each section and is interpreted to have been acquired by low-temperature chemical alteration during subduction and accretion at the continental margin. The third component, isolated between ∼560 and 680 °C, has both normal and reversed polarities, passes a fold test, and is inferred to have been acquired during or soon after deposition. The available paleomagnetic, biostratigraphic, and geochemical data indicate deposition of these cherts along the paleoequator (0°-2°N or S paleolatitude) between Pliensbachian and Oxfordian time as the oceanic plate moved eastward, relative to North America, beneath the equatorial zone of high biologic productivity. Between Bathonian and Cenomanian time, the chert sequences apparently moved progressively away from the paleoequator (2°-15°N or S), and were soon after accreted to the American continental margin. Plate reconstruction models for the Farallon plate corroborate low-paleolatitude trajectories from ridge crest to subduction zone (for example, from 3°S to 11°N), and they imply subsequent northward translation of the Franciscan Complex (> 4,000 km) by strike-slip faulting related to relative motions between the Farallon, Kula, Pacific, and North American plates.

Geological Society of America Bulletin↗
USGS1993

The Mount Evans batholith in the Colorado Front Range: Revision of its age and reinterpretation of its structure

The Mount Evans batholith, in the central Front Range of Colorado, is composed of a main phase of massive to conspicuously foliated monzogranite and granodiorite and undeformed aplite and pegmatite. The Mount Evans batholith was previously considered to be part of the 1.7 Ga Routt Plutonic Suite. New U-Pb zircon ages on four samples (granodiorite, monzogranite, and granite), however, indicate that the batholith was emplaced at 1,442 ± 2 Ma and belongs to the Berthoud Plutonic Suite. Most of the batholith has igneous textures and structures, except in the vicinity of the Idaho Springs-Ralston shear zone where those features are tectonically recrystallized and foliated. Foliation elsewhere in the batholith is a flow structure. Zircons in two granodiorite samples, collected near the shear zone (just south of the Colorado Mineral Belt), are reversely discordant by about 0.8%-2.1%, with a considerable spread in 207 Pb/ 206 Pb ages. Many of the zircons from these samples contain apatite, K-feldspar, and quartz inclusions that appear to replace zircon along cracks and imperfections from rim to core. We suggest that these inclusions formed during a Laramide ore-forming event and incorporated Early and Middle Proterozoic radiogenic lead scavenged from the country rock. The excess radiogenic lead caused the scatter and reverse discordance in the data. The Mount Evans batholith is anomalous in composition and structure compared to most other 1.4 Ga plutons of the southwestern United States. The differences probably reflect different sources of partial melting; the specific tectonic setting where rocks of such disparate origin are temporally and spatially juxtaposed is not understood.

Colorado↗
USGS1993

Late Cenozoic structure and correlations to seismicity along the Olympic-Wallowa Lineament, northwest United States

Late Cenozoic fault geometry, structure, paleoseismicity, and patterns of recent seismicity at two seismic zones along the Olympic-Wallowa lineament (OWL) of western Idaho, northeast Oregon, and southeast Washington indicate limited right-oblique slip displacement along multiple northwest-striking faults that constitute the lineament. The southern end of the OWL originates in the Long Valley fault system and western Snake River Plain in western Idaho, where it accommodates basin-and-range extension. Several kilometers west of the Long Valley fault system, post-middle Miocene displacement along the OWL has resulted in adjacent areas of uplift and subsidence. The OWL in northeast Oregon consists of a wide zone of northwest-striking faults and is associated with several large, inferred, pull-apart basins. The OWL then emerges from the Blue Mountain uplift as a much narrower zone of faults in the Columbia Plateau known as the Wallula fault zone (WFZ). Structural relationships in the WFZ strongly suggest that it is a right-slip extensional duplex. An ash horizon that correlates with the Mount St. Helens "J" composition (10.7 Ka) is displaced vertically 5 m by a normal fault along a segment of the WFZ. The intensity center for a damaging M = 6.1 earthquake in 1936 is located in the WFZ only a few kilometers northeast of this Holocene fault. Other youthful-appearing, strike-slip and thrust faults are located farther northwest along the OWL and may pose a seismic hazard to adjacent population centers and critical facilities.

Geological Society of America Bulletin↗
USGS1993

A late Pliocene to middle Pleistocene pluvial lake in Fish Lake Valley, Nevada and California

The question of whether a pluvial lake existed in Fish Lake Valley, Nevada and California, has been debated for more than 100 yr. New stratigraphic evidence indicates that a lake did exist in this valley at intervals during late Pliocene to middle Pleistocene time. This lake may have drained northward, or it may have been periodically contiguous with a pluvial lake to the north in Columbus Salt Marsh. Proof of the existence of this lake, informally named Pluvial Lake Rennie, is derived from three principal outcrops of shallow-water deposits, two outcrops of deep-water deposits, and several drilling logs. The deposits contain beds of silicic tephra, which provide age control. On the basis of thickness, grain size, major-oxide chemistry of glass shards, and paleomagnetism, three of the shallow-water deposits, including deltaic(?), beach, and siliceous hot-spring sediments, consist mainly of Bishop ash derived from the 0.77 Ma eruption of the Long Valley caldera. A fourth shallow-water deposit(?) is associated with ∼1 Ma Glass Mountain tephra beds. The exposed deep-water deposits consist of green claystone, siltstone, and fine-grained sandstone containing tephra derived from the eruptions of the ∼2.1 Ma tuff of Taylor Canyon and the ∼2.0 Ma Huckleberry Ridge Tuff. The drilling logs record numerous thick beds of clay and sandy clay inferred to be deep-water lacustrine deposits. Pluvial Lake Rennie fluctuated in size and depth beginning prior to 2 Ma and continuing until sometime alter 0.77 Ma. At about 0.77 Ma, the lake had a highstand at an elevation of ∼1,460 m, covered an area of 400-500 km 2 , and had a maximum depth of ∼250 m. The lake level dropped just after the eruption of the Bishop ash, but the lake may have persisted at a lower level until ∼0.5 Ma. No large, long-lived lake existed in Fish Lake Valley in late Pleistocene time, probably due to the increasing rain-shadow effect caused by the relative uplift of the White Mountains and Sierra Nevada in the Pleistocene. These results indicate that the late middle to late Pleistocene history of Pluvial Lake Rennie is similar to that of Lake Tecopa but is quite different from those of Lake Lahontan and Searles Lake.

California, Nevada↗
USGS1993

Relict colluvial boulder deposits as paleoclimatic indicators in the Yucca Mountain region, southern Nevada

Early to middle Pleistocene boulder deposits are common features on southern Nevada hillslopes. These darkly varnished, ancient colluvial deposits stand but in stark contrast to the underlying light-colored bedrock of volcanic tuffs, and they serve as minor divides between drainage channels on modern hillslopes. To demonstrate the antiquity of these stable hillslope features, six colluvial boulder deposits from Yucca Mountain, Nye County, Nevada, were dated by cation- ratio dating of rock varnish accreted on boulder surfaces. Estimated minimum ages of these boulder deposits range from 760 to 170 ka. Five additional older deposits on nearby Skull and Little Skull Mountains and Buckboard Mesa yielded cation-ratio minimum-age estimates of 1.38 Ma to 800 ka. An independent cosmogenic chlorine-36 surface exposure date was obtained on one deposit, which confirms an estimated early to middle Quaternary age. These deposits have provided the oldest age estimates for unconsolidated hillslope deposits in the southwestern United States. We suggest that the colluvial boulder deposits were produced during early and middle Pleistocene glacial/pluvial episodes and were stabilized during the transition to drier interglacial climates. By comparison to modern periglacial environments, winter minimum monthly temperatures of -3 to -5 °C were necessary to initiate freeze-thaw conditions of such vigor to physically weather relatively large volumes of large boulders from the upper hillslopes of the Yucca Mountain area. These conditions imply that early and middle Pleistocene glacial winter temperatures were at least 1 to 3 °C colder than existed during the last Pleistocene glacial episode and 7 to 9 °C colder than present. We conclude that at least several early and middle Pleistocene glacial episodes were colder, and perhaps wetter, than glacial episodes of the late Pleistocene in the southern Great Basin. Geomorphic processes necessary to form these colluvial boulder deposits are not active on modern hillslopes in the southern Great Basin. In addition, the lack of young, relatively unvarnished colluvial boulder deposits on these hillslopes suggests that boulder-forming conditions did not exist during the late Pleistocene in this region. Modern semiarid hillslope processes primarily erode colluvium during infrequent high-intensity storms. The preservation of old, thin hillslope deposits and the less-than-2-m incision by hillslope runoff adjacent to these deposits, however, indicate that extremely low denudation rates have occurred on resistant volcanic hillslopes in the southern Great Basin during Quaternary time.

Nevada↗
USGS1993

Aspects of three-dimensional strain at the margin of the extensional orogen, Virgin River depression area, Nevada, Utah, and Arizona

The Virgin River depression and surrounding mountains are Neogene features that are partly contiguous with the little-strained rocks of the structural transition to the Colorado Plateau province. This contiguity makes the area ideally suited for evaluating the sense, magnitude, and kinematics of Neogene deformation. Analysis along the strain boundary shows that, compared to the adjacent little-strained area, large-magnitude vertical deformation (both uplift and depression, locally exceeding 10 km) greatly exceeds extensional deformation and that significant amounts of lateral displacement approximately parallel the province boundary. Extensional kinematics, the directions of lateral displacements, and the position and orientation of uplifts and one major depression are such that isostatic rebound following tectonic denudation is an unlikely direct cause of the strong vertical structural relief adjacent to the strain boundary. Instead, the observed structures are first-order features defining a three-dimensional strain field produced by approximately east-west extension, vertical structural attenuation, and extension-normal shortening. All major structural elements of the strain-boundary strain field are also found in the adjacent Basin and Range. Especially evident is a discontinuous and inhomogeneous distribution of vertical structural relief resulting from uplift and tilting of the footwalls on convex-upward normal faults, combined with formation of extension-parallel folds. Deformation resulted in a bewilderingly complex mixture of coeval folds, normal faults, strike-slip faults, and, in some areas, reverse faults, thrust faults, and thrust faults reactivated as extensional detachment faults. These complexly mixed structures exhibit a tectonically important aspect of scale independence, ranging from micro- structures to mountain-range scale. Many intensely extended domains appear to reflect a process of surface-relief minimization above uplifted and tilted blocks. Some intensely extended zones are depth-limited by relatively undeformed rocks above, and intact tilted blocks below, and are best viewed as subhorizontal zones of accommodation for the uplift and tilt of subjacent blocks. Whether or not the intense deformation extends to the surface, its depth distribution commonly reflects lithologic or paleostructural controls. Whatever the cause for the concentration of extensional strain, many gently dipping zones are more accurately viewed as local features recording small to moderate extension-parallel translations than a regionally interconnected detachment faults recording enormous extension-parallel translations. Although the total Neogene strain field in the Nevada-Utah-Arizona tricorner area has unique aspects, other parts of the Basin and Range contain generally similar structures that presumably formed from similar processes. These processes include lateral mass transfer, probably by viscoelastic flow, of the material needed to inflate areas of strong uplift and to translate blocks laterally by tectonic rafting. Directions of mass transfer vary but are commonly both parallel and normal to the direction of extension or parallel to the boundaries of extended areas. Much of the strong vertical structural relief probably reflects boundary-condition or localized perturbations (in the form of restraints or releases) of the lateral flow system. These conclusions are consistent with modern concepts of the flexural strength and rheological stratification of the crust in regions undergoing extension.

Geological Society of America Bulletin↗
USGS1993

Upper-crustal structure beneath the Columbia River Basalt Group, Washington: Gravity interpretation controlled by borehole and seismic studies

A three-dimensional gravity model based on deep-borehole stratigraphy, densities from borehole gravimetry, and crustal structure from deep seismic and electrical surveys puts limits on sub-basalt crustal structure in the northwest quadrant of the Columbia Plateau, Washington. The gravity model indicates that the Miocene Columbia River Basalt Group (CRBG) overlies a broad basin filled with sedimentary rocks that reach thicknesses in excess of 5 km; thickest sub-basalt sedimentary rocks are beneath the late Cenozoic Yakima basin. Analysis of residual gravity shows that the Eocene Chiwaukum graben does not continue beneath the CRBG and that sub-basalt sedimentary rocks are not thrust into the cores of the basalt anticlines in the Yakima fold belt. A gravity high that trends north-south in the center of the CRBG is probably caused by a broad basement ridge marked by thinner sedimentary rocks beneath the Columbia River basalts; the western edge of the north-south basement high is traced at the surface by the Hog Ranch-Naneum Ridge. A major crustal feature, the Olympic- Wallowa lineament, is expressed in the gravity field; the lineament crosses the north-south basement high but does not offset it, suggesting that any strike- slip motion on the lineament preceded development of the north-south basement high. An alternative interpretation of the north-south gravity high, based on a previous seismic study, relates it to a high-density structure in the crystalline basement, perhaps associated with a failed continental rift.

Idaho, Oregon, Washington↗
USGS1993

Tilting, burial, and uplift of the Guadalupe Igneous Complex, Sierra Nevada, California

It is often incorrectly assumed that plutons have a relatively uneventful structural history after emplacement. The 151 Ma Guadalupe Igneous Complex (GIC) in the Foothills Terrane, California, was involved in three post-emplacement events: (1) ∼30° of southwestside-up tilting during ductile regional faulting and contraction, (2) burial of the pluton from ∼4 to 12 km during crustal thickening of the wall rocks, and (3) uplift with only minor tilting in the Late Cretaceous. Tilting of the pluton is indicated by (1) southwest to northeast gradational changes from layered gabbros and diorites to granites and granophyres; (2) northeastward dips of layering in gabbro, internal contacts, and bedding of overlying coeval(?) volcanic rocks; (3) northeastward decrease in wall-rock metamorphic grade; and (4) paleomagnetic data from 14 localities across the pluton. We argue that tilting occurred between 146-135 Ma during southwest-northeast-directed regional contraction. This contraction is indicated by widespread folds and cleavages and by reverse motion on the Bear Mountains fault zone (BMFZ), a large northeast-dipping shear zone that bounds the GIC on its southwest side. Burial of the GIC, which overlapped in time but outlasted tilting, is suggested by (1) post-emplacement contractional faulting, folding, and cleavage development; (2) analyses of strains associated with widespread cleavage that indicate vertical thickening of ∼100% and (3) microstructural and mineral assemblage data that indicate shallow emplacement of the GIC, in contrast to mineral assemblage and limited geobarometric data from adjacent 120-110 Ma plutons that indicate moderate emplacement levels. Late Cretaceous uplift is indicated by 95-75 Ma sedimentary rocks that unconformably overlie the 120-110 Ma plutons. This geologic history is interesting for several reasons. First, although the GIC participated in extensive post-emplacement deformation, it lacks internal structural evidence of these events, except locally along the Bear Mountains fault zone. Second, the agreement between paleomagnetic and structural evidence for tilting suggests that no large latitudinal displacement of the GIC is required. Third, the paleomagnetic data also help to define the geometry of the magma chamber now represented by the GIC. Lack of streaking of paleomagnetic site-mean directions demonstrates that the pluton acted as a single unit after cooling through the blocking temperature (450-560 °C) of low-titanium titanomagnetite; however, variations in the dip of internal layering and contacts, from 70° at the base to 30° near the top of the pluton, indicate that not all of these features were horizontal and planar when they formed. We propose that this variation in dip of layering is most consistent with sidewall crystallization of magma resulting in drape of layering along the walls of the intrusion. Therefore, internal layering within this pluton does not record paleohorizontal.

California↗
USGS1994

Explosive tephra emissions of Mount St. Helens, 1989-1991: the violent escape of magmatic gas following storms?

From 24 August 1989 until 18 June 1991, Mount St. Helens produced at least 28 shallow, explosion-like seismic events with signatures similar to those produced by gas explosions on the dome during the mid 1980s. At least six were accompanied by violent emission of non-juvenile tephra, ejection of blocks of rock nearly 1 km from the vent, and avalanching of debris off the north side of the dome. All six confirmed emissions and most (although not all) other seismic events took place hours to days after storms. The short delay between storms and emissions suggests that the events that follow storms originate at very shallow depth, probably within the dome itself. Although the exact causal mechanism is not known, it is speculated that slope instability or accelerated growth of cooling fractures following storms may have released gas trapped within or at the base of the dome. -from Author

Geological Society of America Bulletin↗
USGS1994

Late Quaternary normal faulting of the Hat Creek basalt, northern California

The Hat Creek fault is a major, young, north-striking, normal fault along the western boundary of extensional Basin and Range deformation in the Lessen region of northeastern California. Volcanic rocks of Quaternary and late Pliocene age are displaced a total of >500 m down to the west along west-facing, en echelon scarps now retreated to ∼35° slopes. Fresh, young scarps as much as 30 m high cut the Hat Creek Basalt (erupted between 15 and ∼40 ka) a few tens of meters west of the retreated scarps. Prior to the late 1980s, these young scarps were interpreted as lava slump scarps formed as the Hat Creek Basalt ponded against the older fault scarps and then drained away to the northwest. Numerous pieces of geologic evidence, however, show that the young scarps formed after the Hat Creek Basalt solidified and cooled and are true fault features formed by the youngest displacements of the Hat Creek fault. Structural details are remarkably well preserved along the series of left-stepping scarps cutting the Hat Creek Basalt. Near the central parts of individual segments, the fault is displayed as a single, vertical scarp. Near the ends of the segments, the scarp decreases in height and becomes a monodinal flexure on which the recent dip separation has been taken up by small-scale offset along columnar cooling joints in the basalt These monodinal flexures commonly rotate into east-west monoclines that join adjacent north-south segments. Displacement of outwash gravel overlying the Hat Creek Basalt shows that vertical separation on the Hat Creek fault has averaged ∼1.3 mm yr −1 for the past 15,000 yr. The Hat Creek fault thus represents a potential earthquake hazard, despite the low level and diffuse nature of modern seismidty in the region.

Geological Society of America Bulletin↗
USGS1994

Late Mesozoic and possible early Tertiary accretion in western Washington State: The Helena-Haystack mélange and the Darrington-Devils Mountain fault zone

The Helena-Haystack mélange (HH mélange) and coincident Darrington-Devils Mountain fault zone (DDMFZ) in northwestern Washington separate two terranes, the Northwest Cascade System (NWCS) and the western and eastern mélange belts (WEMB). The two terranes of Paleozoic and Mesozoic rocks superficially resemble each other but record considerable differences in structural and metamorphic history. The HH mélange is a serpentinite-matrix mélange containing blocks of adjacent terranes but also exotic blocks of schistose metavolcanic rocks and Jurassic tonalite and associated amphibolite. The HH mélange must have formed between early Cretaceous and late middle Eocene time, because it contains tectonic clasts of early Cretaceous Shuksan Greenschist and is overlain by late middle Eocene sedimentary and volcanic rocks. Less certain constraints on its age are a tectonic clast of metarhyolite that yields 90 Ma metamorphic ages and the presumption that the mélange was emplaced before the outboard Olympic terrane arrived at about 50 Ma. The apparent continuity of the HH mélange and the Decatur terrane of the San Juan Islands suggests that the mélange is the strongly tectonized equivalent of the Fidalgo ophiolite. The out-crop pattern suggests that the HH mélange overlies rocks of the NWCS and it may have formed when the WEMB terranes were thrust over rocks of the NWCS. Much of the exposed belt of the HH mélange is overlain by late middle Eocene feldspathic sandstone and volcanic rocks of the Barlow Pass Volcanics of Vance (1957a), which are cut by numerous faults of the DDMFZ paralleling the mélange. The Barlow Pass Volcanics appear to overlie the Straight Creek fault without large offset, but a displaced exotic block of amphibolite with attached early or early middle Eocene(?) sandstone in the mélange suggests that strike-slip movement along the DDMFZ was synchronous with movement on the Straight Creek fault, and stretched cobbles in the conglomerates of the Barlow Pass Volcanics suggest post-Straight Creek movement. The possible continuation of the DDMFZ to the northwest as the San Juan and the West Coast faults on Vancouver Island suggests That the structure has had a major role in the emplacement of all the westernmost terranes in the Pacific Northwest. This major suture is strongly bowed to the northeast opposite the great oroclinal bend of the Olympic terrane, suggesting that the emplacement of that terrane may have deformed a once straighter strike-slip zone.

Washington↗
USGS1994

Emplacement and inflation of pahoehoe sheet flows: observations and measurements of active lava flows on Kilauea volcano, Hawaii

Inflated pahoehoe sheet flows have a distinctive horizontal upper surface, which can be several hundred meters across, and are bounded to steep monoclinal uplifts. The inflated sheet flows studied ranged from 1 to 5 m in thickness, but initially propagated as thin sheets of fluid pahoehoe lava, generally 20-30 cm thick. The morphology of the lava as flow advanced is described. Inflated sheet flows from Kilauea and Mauna Loa are morphologically similar to some thick Icelandic and submarine sheet flows, suggesting a similar mechanism of emplacement. -from Authors

Geological Society of America Bulletin↗
USGS1994

The northern Nevada rift: Regional tectono-magmatic relations and Middle Miocene stress direction

As defined by the most recent aeromagnetic surveys, the north-northwest-trending northern Nevada rift zone extends for at least 500 km from southern Nevada to the Oregon Nevada border. At several places along the rift, the magnetic anomaly is clearly related to north-northwest-trending dikes and flows that, based on new radiometric dating, erupted between 17 and 14 Ma and probably during an even shorter time interval. The tectonic significance of the rift is dramatized by its length, its coincidence in time and space (at its northern terminus) with the oldest silicic caldera complex along the Yellowstone hot-spot trend, and its parallelism with the subduction zone along the North American coast prior to the establishment of the San Andreas fault.

Geological Society of America Bulletin↗
USGS1994

Submarine canyon initiation by downslope-eroding sediment flows: Evidence in late Cenozoic strata on the New Jersey continental slope

Multibeam bathymetry and seismic reflection profiles of the New Jersey continental slope reveal a series of abandoned and now-buried submarine canyons that have apparently influenced the development of modern canyons. The buried canyons are infilled along nine slope-wide unconformities separating upper-middle Miocene to Pleistocene sediments that thin downslope. Canyons infilled during the Miocene occur in the southwest part of the study area where Miocene sediments are thickest. Other canyons, infilled during the Pleistocene, occur in the northeast part of the study area where Pleistocene sediments are thickest. When followed downslope, each of the buried canyons arrives at a confluence with a modern canyon, usually where the downslope-tapering sediment cover has failed to smooth over the buried canyon, leaving a sea-floor trough. Sea-ward of the confluences, the modern canyons have exhumed the buried canyons and use the older valleys to reach the base of the slope. Re-use of the lower slope reach of the buried canyons appears to have begun when the sea-floor troughs over the buried canyons captured sediment flows initiated along the upper slope and shelf break and confined them to follow the former path of the buried canyons to the base of the slope. The downslope erosion caused by the sediment.flows is proposed to have initiated the modern canyons, which eventually excavated and deepened the former routes of the buried canyons seaward of the sites of sediment flow capture. The occurrence of buried canyons where strata thickens alongslope suggests that infilling of the buried canyons occurred seaward of shelf-edge depocenters. The heightened sediment input to the slope in these regions may have also led to the initiation and growth of modern-day canyons. The temporal relation between modern canyon formation, sediment supply, and sea level, however, remains to be established.

New Jersey↗