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USGS1985

Catastrophic eruptions of the directed-blast type at Mount St. Helens, Bezymianny and Shiveluch volcanoes

This paper describes catastrophic eruptions of Mount St. Helens (1980), Bezymianny (1955–1956), and Shiveluch (1964) volcanoes. A detailed description of eruption stages and their products, as well as the quantitative characteristics of the eruptive process are given. The eruptions under study belong to the directed-blast type. This type is characterized by the catastrophic character of the climatic stage during which a directed blast, accompanied by edifice destruction, the profound ejection of juvenile pyroclastics and the formation of pyroclastic flows, occur. The climatic stage of all three eruptions has similar characteristics, such as duration, kinetic energy of blast (10 17 −10 18 J), the initial velocity of debris ejection, morphology and size of newly-formed craters. But there are also certain differences. At Mount St. Helens the directed blast was preceeded by failure of the edifice and these events produced separable deposits, namely debris avalanche and directed blast deposits which are composed of different materials and have different volumes, thickness and distribution. At Bezymianny, failure did not precede the blast and the whole mass of debris of the old edifice was outburst only by blast. The resulting deposits, represented by the directed blast agglomerate and sand facies, have characteristics of both the debris avalanche and the blast deposit at Mount St. Helens. At Shiveluch directed-blast deposits are represented only by the directed-blast agglomerate; the directed-blast sand facies, or blast proper, seen at Mount St. Helens is absent. During the period of Plinian activity, the total volumes of juvenile material erupted at Mount St. Helens and at Besymianny were roughly comparable and exceeded the volume of juvenile material erupted at Shiveluch, However, the volume of pyroclastic-flow deposits erupted at Mount St. Helens was much less. The heat energy of all three eruptions is comparable: 1.3 × 10 18 , 3.8−4.8 × 10 18 and 1 × 10 17 J for Shiveluch, Bezymianny, and Mount St. Helens, respectively.

Journal of Geodynamics↗

USGS1985

Forecasts and predictions of eruptive activity at Mount St. Helens, USA: 1975-1984

Public statements about volcanic activity at Mount St. Helens include factual statements, forecasts, and predictions. A factual statement describes current conditions but does not anticipate future events. A forecast is a comparatively imprecise statement of the time, place, and nature of expected activity. A prediction is a comparatively precise statement of the time, place, and ideally, the nature and size of impending activity. A prediction usually covers a shorter time period than a forecast and is generally based dominantly on interpretations and measurements of ongoing processes and secondarily on a projection of past history. The three types of statements grade from one to another, and distinctions are sometimes arbitrary. Forecasts and predictions at Mount St. Helens became increasingly precise from 1975 to 1982. Stratigraphic studies led to a long-range forecast in 1975 of renewed eruptive activity at Mount St. Helens, possibly before the end of the century. On the basis of seismic, geodetic and geologic data, general forecasts for a landslide and eruption were issued in April 1980, before the catastrophic blast and landslide on 18 May 1980. All extrusions except two from June 1980 to the end of 1984 were predicted on the basis of integrated geophysical, geochemical, and geologic monitoring. The two extrusions that were not predicted were preceded by explosions that removed a substantial part of the dome, reducing confining pressure and essentially short-circuiting the normal precursors.

Journal of Geodynamics↗

USGS1985

Volcano hazards program in the United States

Volcano monitoring and volcanic-hazards studies have received greatly increased attention in the United States in the past few years. Before 1980, the Volcanic Hazards Program was primarily focused on the active volcanoes of Kilauea and Mauna Loa, Hawaii, which have been monitored continuously since 1912 by the Hawaiian Volcano Observatory. After the reawakening and catastrophic eruption of Mount St. Helens in 1980, the program was substantially expanded as the government and general public became aware of the potential for eruptions and associated hazards within the conterminous United States. Integrated components of the expanded program include: volcanic-hazards assessment; volcano monitoring; fundamental research; and, in concert with federal, state, and local authorities, emergency-response planning. In 1980 the David A. Johnston Cascades Volcano Observatory was established in Vancouver, Washington, to systematically monitor the continuing activity of Mount St. Helens, and to acquire baseline data for monitoring the other, presently quiescent, but potentially dangerous Cascade volcanoes in the Pacific Northwest. Since June 1980, all of the eruptions of Mount St. Helens have been predicted successfully on the basis of seismic and geodetic monitoring. The largest volcanic eruptions, but the least probable statistically, that pose a threat to western conterminous United States are those from the large Pleistocene-Holocene volcanic systems, such as Long Valley caldera (California) and Yellowstone caldera (Wyoming), which are underlain by large magma chambers still potentially capable of producing catastrophic caldera-forming eruptions. In order to become better prepared for possible future hazards associated with such historically unpecedented events, detailed studies of these, and similar, large volcanic systems should be intensified to gain better insight into caldera-forming processes and to recognize, if possible, the precursors of caldera-forming eruptions.

Journal of Geodynamics↗

USGS2004

Tectonics and metallogenesis of Proterozoic rocks of the Reading Prong

Detailed geologic mapping, petrography, and major and trace-element analyses of Proterozoic rocks from the Greenwood Lake Quadrangle, New York are compared with chemical analyses and stratigraphic information compiled for the entire Reading Prong. A persistent regional stratigraphy is evident in the mapped area whose geochemistry indicates protoliths consistent with a back-arc marginal basin sequence. The proposed marginal basin may have been floored by an older sialic basement and overlain by a basin-fill sequence consisting of a basal tholeiitic basalt, basic to intermediate volcanic or volcaniclastic rocks and carbonate sediments, a bimodal calc-alkaline volcanic sequence, and finally volcaniclastic, marine, and continental sediments. The presence of high-chlorine biotite and scapolite may indicate circulation of brine fluids or the presence of evaporite layers in the sequence. Abundant, stratabound magnetite deposits with a geologic setting very unlike that of cratonic, Proterozoic banded-iron formations are found throughout the proposed basin sequence. Associated with many of the magnetite deposits is unusual uranium and rare-earth element mineralization. It is proposed here that these deposits formed in an exhalative, volcanogenic, depositional environment within an extensional back-arc marginal basin. Such a tectonic setting is consistent with interpretations of protoliths in other portions of the Reading Prong, the Central Metasedimentary Belt of the Canadian Grenville Province, and recent interpretation of the origin of the Franklin lead-zinc deposits, suggesting a more cohesive evolving arc/back-arc tectonic model for the entire Proterozoic margin of the north-eastern portion of the North American craton. Published by Elsevier Ltd.

Journal of Geodynamics↗

USGS2004

Vestiges of an Iapetan rift basin in the New Jersey Highlands: Implfications for the Neoproterozoic Laurentian margin

Thin, discontinuous remnants of Neoproterozoic intracratonic rift-basin deposits of the Chestnut Hill Formation occur in the western New Jersey Highlands. These deposits form an important link between well-documented Iapetan rift-basins in both the northern and southern Appalachians. The close spatial relations of Chestnut Hill rocks to Paleozoic sedimentary rocks open the possibility that additional Iapetan rift-basins could be concealed beneath the rocks of the Valley and Ridge Province to the west indicating a much broader zone of rifting than has been previously proposed. The Chestnut Hill Formation is intermittently exposed along a 100 km-long band that extends northeast from Pennsylvania nearly to New York State. The lower part of the Chestnut Hill Formation is composed of interbedded lithic pebble- to boulder-conglomerate and feldspathic sandstone grading upward into interbedded phyllite, feldspathic and quartz sandstone, local paleosaprolite, quartz-pebble conglomerate, thin limestone lenses, volcanic, and volcaniclasic rocks, abundant bedded ironstone (hematite ore), and ultimately into diamictites that are interpreted as possible tilloids and containing rounded intra and extrabasinal clasts of the other lithologies. Extensive soft-sediment deformation, cross bedding, and clastic dikes are common in all but the lowest and upper facies. Banded hematite layers occur preferentially in fine-grained tuffs and tuffaceous sediments, but hematitization has affected most lithologies. Volcanic rocks consist of altered rhyolitic tuffs and lapilli tuffs that are interbedded with sediments. The Chestnut Hill Formation is interpreted to have been deposited in early alluvial, and later a complex of fluvial, lacustrine and deltaic environments. Provenance studies based upon petrographic and geochemical analysis of clastic rocks indicate that the sediments are predominantly immature and reflect derivation from local uplifted felsic basement sources in a rifted-margin tectonic setting. Low to moderate weathering of the source rocks is indicated by the geochemistry of most samples, as is the locally intense effect of hydrothermal alteration. Most occurrences of the Chestnut Hill Formation are associated with major faults that exhibit normal movement of apparent Neoproterozoic age. Rocks from the Morgan Hill fault near Easton, Pennsylvania display consistent normal shear sense and vary from low temperature S-C mylonites to breccia that contains deformed pieces of Chestnut Hill Formation. ?? 2004 Elsevier Ltd. All rights reserved.

Journal of Geodynamics↗

USGS2004

Geophysical modeling of the northern Appalachian Brompton-Cameron, Central Maine, and Avalon terranes under the New Jersey Coastal Plain

A regional terrane map of the New Jersey Coastal Plain basement was constructed using seismic, drilling, gravity and magnetic data. The Brompton-Cameron and Central Maine terranes were coalesced as one volcanic island arc terrane before obducting onto Laurentian, Grenville age, continental crust in the Taconian orogeny [Rankin, D.W., 1994. Continental margin of the eastern United States: past and present. In: Speed, R.C., (Ed.), Phanerozoic Evolution of North American Continent-Ocean Transitions. DNAG Continent-Ocean Transect Volume. Geological Society of America, Boulder, Colorado, pp. 129-218]. Volcanic island-arc rocks of the Avalon terrane are in contact with Central Maine terrane rocks in southern Connecticut where the latter are overthrust onto the Brompton-Cameron terrane, which is thrust over Laurentian basement. Similarities of these allochthonous island arc terranes (Brompton-Cameron, Central Maine, Avalon) in lithology, fauna and age suggest that they are faulted segments of the margin of one major late Precambrian to early Paleozoic, high latitude peri-Gondwana island arc designated as "Avalonia", which collided with Laurentia in the early to middle Paleozoic. The Brompton Cameron, Central Maine, and Avalon terranes are projected as the basement under the eastern New Jersey Coastal Plain based on drill core samples of metamorphic rocks of active margin/magmatic arc origin. A seismic reflection profile across the New York Bight traces the gentle dipping (approximately 20 degrees) Cameron's Line Taconian suture southeast beneath allochthonous Avalon and other terranes to a 4 sec TWTT depth (approximately 9 km) where the Avalonian rocks are over Laurentian crust. Gentle up-plunge (approximately 5 degrees) projections to the southwest bring the Laurentian Grenville age basement and the drift-stage early Paleozoic cover rocks to windows in Burlington Co. at approximately 1 km depth and Cape May Co. at approximately 2 km depths. The antiformal Shellburne Falls and Chester domes and Chain Lakes-Pelham dome-Bronson Hill structural trends, and the synformal Connecticut Valley-Gaspe structural trend can be traced southwest into the New Jersey Coastal Plain basement. A Mesozoic rift basin, the "Sandy Hook basin", and associated eastern boundary fault is identified, based upon gravity modeling, in the vicinity of Sandy Hook, New Jersey. The thickness of the rift-basin sedimentary rocks contained within the "Sandy Hook basin" is approximately 4.7 km, with the basin extending offshore to the east of the New Jersey coast. Gravity modeling indicates a deep rift basin and the magnetic data indicates a shallow magnetic basement caused by magnetic diabase sills and/or basalt flows contained within the rift-basin sedimentary rocks. The igneous sills and/or flows may be the eastward continuation of the Watchung and Palisades bodies. ?? 2004 Elsevier Ltd. All rights reserved.

Journal of Geodynamics↗

USGS2007

Two lithospheric profiles across southern California derived from gravity and seismic data

We present two detailed 2-D density transects for the crust and uppermost mantle across southern California using a linear gravity inversion technique. This technique parameterizes the crust and upper mantle as a set of blocks that are based on published geologic and seismic models. Each block can have a range of densities that are constrained where possible by borehole measurements, seismic velocities, and petrologic data. To further constrain the models, it is assumed that the lithosphere is close to isostatic equilibrium at both ends of the profiles, in the deep ocean and east of the Mojave Desert. We calculate the lithostatic pressure variations field for the whole cross section to rule out the geophysically insignificant solutions. In the linear equation, ρ = a + bV ( V , seismic P-wave velocity; ρ , density), which approximates the mantle density–velocity ( ρ – V ) relationship, different coefficients for b were evaluated. Lower coefficients ( b < 0.2) correspond to an almost purely thermally perturbed mantle, while higher coefficients ( b > 0.3) imply that other effects, such as composition and/or metamorphic changes, play an important role in the mantle. Density models were constructed with the coefficient b ranging from 0 to 0.6. The results indicate that a high b value in the mantle ρ – V relationship is associated with less dense crust in the Mojave block and more dense crust in the Catalina schist block. In the less dense Mojave block, the average density of the whole crust is ∼2.75 g/cm 3 , while that of the lower crust is ∼2.72 g/cm 3 . These densities imply a high silica content in the crust, and a minor fraction of basic rock in the lower crust, or perhaps the absence of a basaltic layer altogether. By comparison, the average density of a typical continental stable platform is ∼2.85 g/cm 3 . Models with higher b coefficients (0.5–0.6) are characterized by a large isostatic imbalance. On the other hand, lower b values (0–0.2) require a consolidated whole crust density in the Mojave Desert of ∼2.78 g/cm 3 , and a lower crust density of ∼2.89 g/cm 3 with mostly basaltic composition. This contradicts the observed, lower V p / V s -ratio in the Mojave Desert associated with mostly felsic and low-density crust. Models with lower b coefficients (0.1–0.2) are characterized by an absence of local Airy compensation beneath the San Gabriel Mountains at the LARSE-1 profile. These, and other non-gravity arguments, suggest optimal solutions to the mantle ρ – V relation of b ∼ 0.2–0.4. This, in turn, means that both thermal and petrological effects occur inside the downwelling of the uppermost mantle high velocity body located beneath the Transverse Ranges. During the development of this mantle downwelling, the basaltic layer of the Mojave block was likely eroded and pulled down into the high velocity body. Those basaltic fragments may have been transformed into eclogites, and this metamorphic change implies a higher b -coefficient density–velocity relationship than would be expected for a purely thermal process.

California↗

USGS2009

Evidence for an Alleghanian (Early Carboniferous to Late Permian) tectonothermal event in the New Jersey Coastal Plain basement from 40 Ar/ 39 Ar biotite data, geochemistry and gravity modeling

40 Ar/ 39 Ar dating of biotite from felsic orthogneiss recovered from the -3890-foot level of the Island Beach State Park (IBSP) well beneath the outer New Jersey Coastal Plain was accomplished using CO 2 laser incremental-heating techniques. Over 75% of the Ar released from the incremental-heating experiment form a well-behaved plateau with a calculated age of 243.98 ?? 0.10 Ma. The new 244 Ma biotite age reported here is a cooling age younger than the metamorphic event that crystallized or reheated the biotite. We consider reheating of older biotite to be unlikely because the concordant 40 Ar/ 39 Ar spectrum upon repeated incremental laser heating showed a well-developed plateau. Thus, biotites from the IBSP gneiss are interpreted as having crystallized during a single thermal event, followed by cooling to below 300 ??C. The IBSP well falls on a structural and geophysical anomaly trend that is along strike with rocks of the Bronson Hill anticlinorium to the north of the IBSP gneiss. Locally graphitic metasedimentary schists and gneisses recovered from New Jersey wells inboard of the IBSP well gneiss correlate to similar lithologies of the Connecticut Valley synclinorium west of the Hartford basin. Our reinterpretation of the IBSP gneiss as metamorphosed dacite or dacitic tuff is consistent with a correlation to some rocks of the Bronson Hill magmatic arc east of the Hartford basin. If correct, this would imply a Late Ordovician age for the protolith of the IBSP gneiss. Reported 40 Ar/ 39 Ar biotite ages of 235-253 Ma from southwestern Rhode Island, and of 238-247 Ma from southeastern Connecticut, are interpreted as cooling ages following a tectonothermal event associated with the Alleghanian orogeny (Early Carboniferous to Late Permian). Cooling ages of Alleghanian age (Early Carboniferous to Late Permian) are not recognized west of the Bronson Hill volcanic arc in either central Connecticut or in Massachusetts. Therefore, the 244 Ma cooling age presented here, and the geochemical affinity of the IBSP gneiss to some orthogneisses of the Bronson Hill arc, support an interpretation of the IBSP well as representing the southern continuation of the Bronson Hill arc into New Jersey. Moreover, it documents the presence of rocks beneath the outer New Jersey Coastal Plain that experienced a Permian Alleghanian metamorphism. ?? 2009 Elsevier Ltd. All rights reserved.

Journal of Geodynamics↗

USGS2020

Detrital record of the late Oligocene – Early Miocene mafic volcanic arc in the southern Patagonian Andes (~51 °S) from single-clast geochronology and trace element geochemistry

Retroarc foreland basins are important archives of continental arc magmatism and upper plate deformational processes that control the evolution of continental lithosphere. However, resolving source areas in foreland basin infill dominated from mixed mafic and recycled sediment using conventional methods such as detrital zircon geochronology poses a challenge to thorough analysis due to lower zircon fertility and the higher susceptibility to weathering of mafic lithologies. Here, we integrate whole rock 40 Ar/ 39 Ar geochronology and major and trace element geochemistry data from volcanic clasts from the lower Miocene infill of the Magallanes-Austral Basin and local Sierra Baguales intrusive rocks to understand the distribution of mafic sources and Neogene changes in arc magmatism in between multiple ridge subduction events in the southern Patagonian Andes. Potential source areas for the coarse-grained mafic detritus include the Eocene plateau lavas, the Late Jurassic-Miocene Southern Patagonian batholith, and the Late Jurassic Sarmiento Ophiolitic Complex. Published detrital zircon U-Pb age spectra suggest that all three sources are viable contributors to the basin, though the paucity of Jurassic and Eocene zircons preclude these as major sources. Here, new 40 Ar/ 39 Ar dating of the volcanic clasts from the early Miocene Río Guillermo Formation reveals latest Oligocene to early Miocene eruptive ages (∼25-22 Ma), indicating syndepositional eruption with the ancestral Río Guillermo fluvial sedimentation. A single dated clast yields a Late Cretaceous age (∼102 Ma). The clasts are dominantly basaltic andesite with Ba/Ta ∼500-1000, La/Ta >20, and Ba/La >15, indicating an arc-derived melt source. We propose that the clasts record a Miocene mafic continental arc source area in the Patagonian Andes, which has since been removed by erosion and is thus sparsely represented in the batholith. Furthermore, we suggest that this early Miocene phase of arc volcanism, which postdates Eocene and Oligocene backarc magmatism and pre-dates middle Miocene to recent Chile Ridge backarc magmatism, reflects a return to normal arc volcanism along the Patagonian margin following a cessation due to ridge subduction and subsequent slab window migration. New geochemistry and 40 Ar/ 39 Ar data from a basaltic dike in the Sierra Baguales, which crosscuts the Cenozoic stratigraphic section, records plateau magmatism ∼16 Ma associated with incipient Chile Ridge slab window volcanism.

Patagonian Andes↗

USGS2002

Magnetic fields over active tectonic zones in ocean

The aim of our work is to estimate the electromagnetic effects that can be detected in the submarine zones with hydrothermal activity. It is known that meso-scale flows appear in the regions over underwater volcanoes or hot rocks. Their origin is connected with heat flux and hot jets released from underwater volcanoes or faults in a sea bottom. Values of mean velocities and turbulent velocities in plumes were estimated. Quasiconstant magnetic fields induced by a hot jet and a vortex over a plume top are about 1-40 nT. Variable magnetic fields are about 0.1-1 nT. These magnetic disturbances in the sea medium create an additional natural electromagnetic background that must be considered when making detailed magnetic surveys. ?? 2002 Elsevier Science Ltd. All rights reserved.

Journal of Geodynamics↗

USGS1999

Geologic processes of accretion in the Cascadia subduction zone west of Washington State

The continental margin west of Oregon and Washington undergoes a northward transition in morphology, from a relatively narrow, steep slope west of Oregon to a broad, midslope terrace off Washington. Multichannel seismic (MCS) reflection data collected over the accretionary complex show that the morphologic transition is accompanied by significant change in accretionary style: West of Oregon the direction of thrust vergence in the wedge toe flip-flops between landward and seaward, whereas off Washington, thrust faults in the toe verge consistently landward, except near the mouth of the Columbia River where detachment folding of accreted sediment is evident. Furthermore, rocks under the broad midslope terrace west of Washington appear to be intruded by diapirs. The combination of detachment folding, diapirs, and landward-vergent thrust faults all suggest that nearly as far landward as the shelf break, coupling along the interplate decollement is, or has been, low, as suggested by other lines of evidence.

Journal of Geodynamics↗

USGS1999

Thermo-hydraulics of the Peruvian accretionary complex at 12°S

Coupled heat and fluid transport at the Peruvian convergent margin at 12°S wasstudied with finite element modelling. Structural information was available from two seismicreflection lines. Heat production in the oceanic plate, the metamorphic basement, and sedimentswas estimated from literature. Porosity, permeability, and thermal conductivity for the modelswere partly available from Ocean Drilling Program (ODP) Leg 112; otherwise we used empiricalrelations. Our models accounted for a possible permeability anisotropy. The decollement was bestmodelled as a highly permeable zone (10 −13 m 2 ). Permeabilities of thePeruvian accretionary wedge adopted from the model calculations fall within the range of 2 to7×10 −16 m 2 at the ocean bottom to a few 10 −18 m 2 at the base and need to be anisotropic. Fluid expulsion at the sea floor decreases graduallywith distance from the deformation front and is structure controlled. Small scale variations of heatflux reflected by fluctuations of BSR depths across major faults could be modelled assuming highpermeability in the faults which allow for efficient advective transport along those faults. The models were constrained by the thermal gradient obtained from the depth of bottomsimulating reflectors (BSRs) at the lower slope and some conventional measurements. We foundthat significant frictional heating is required to explain the observed strong landward increase ofheat flux. This is consistent with results from sandbox modelling which predict strong basalfriction at this margin. A significantly higher heat source is needed to match the observed thermalgradient in the southern line.

Journal of Geodynamics↗

USGS1999

Grenville age of basement rocks in Cape May NJ well: New evidence for Laurentian crust in U.S. Atlantic Coastal Plain basement Chesapeake terrane

The Chesapeake terrane of the U.S. mid-Atlantic Coastal Plain basement is bounded on the northwest by the Salisbury positive gravity and magnetic anomaly and extends to the southeast as far as the Atlantic coast. It underlies the Coastal Plain of Virginia, Maryland, Delaware and southern New Jersey. Rubidium/Strontium dating of the Chesapeake terrane basement yields an age of 1.025 ?? 0.036 Ga. This age is typical of Grenville province rocks of the Middle to Late Proterozoic Laurentian continent. The basement lithologies are similar to some exposed Grenville-age rocks of the Appalachians. The TiO2 and Zr/P2O5 composition of the metagabbro from the Chesapeake terrane basement is overlapped by those of the Proterozoic mafic dikes in the New Jersey Highlands. These new findings support the interpretation that Laurentian basement extends southeast as far as the continental shelf in the U.S. mid-Atlantic region. The subcrop of Laurentian crust under the mid-Atlantic Coastal Plain implies unroofing by erosion of the younger Carolina (Avalon) supracrustal terrane. Dextral-transpression fault duplexes may have caused excessive uplift in the Salisbury Embayment area during the Alleghanian orogeny. This extra uplift in the Salisbury area may have caused the subsequent greater subsidence of the Coastal Plain basement in the embayment.

Journal of Geodynamics↗

USGS2000

Active, capable, and potentially active faults - a paleoseismic perspective

Maps of faults (geologically defined source zones) may portray seismic hazards in a wide range of completeness depending on which types of faults are shown. Three fault terms - active, capable, and potential - are used in a variety of ways for different reasons or applications. Nevertheless, to be useful for seismic-hazards analysis, fault maps should encompass a time interval that includes several earthquake cycles. For example, if the common recurrence in an area is 20,000-50,000 years, then maps should include faults that are 50,000-100,000 years old (two to five typical earthquake cycles), thus allowing for temporal variability in slip rate and recurrence intervals. Conversely, in more active areas such as plate boundaries, maps showing faults that are <10,000 years old should include those with at least 2 to as many as 20 paleoearthquakes. For the International Lithosphere Programs' Task Group II-2 Project on Major Active Faults of the World our maps and database will show five age categories and four slip rate categories that allow one to select differing time spans and activity rates for seismic-hazard analysis depending on tectonic regime. The maps are accompanied by a database that describes evidence for Quaternary faulting, geomorphic expression, and paleoseismic parameters (slip rate, recurrence interval and time of most recent surface faulting). These maps and databases provide an inventory of faults that would be defined as active, capable, and potentially active for seismic-hazard assessments.

Journal of Geodynamics↗

USGS2008

Late Cretaceous to Miocene sea-level estimates from the New Jersey and Delaware coastal plain coreholes: An error analysis

Sea level has been estimated for the last 108 million years through backstripping of corehole data from the New Jersey and Delaware Coastal Plains. Inherent errors due to this method of calculating sea level are discussed, including uncertainties in ages, depth of deposition and the model used for tectonic subsidence. Problems arising from the two-dimensional aspects of subsidence and response to sediment loads are also addressed. The rates and magnitudes of sea-level change are consistent with at least ephemeral ice sheets throughout the studied interval. Million-year sea-level cycles are, for the most part, consistent within the study area suggesting that they may be eustatic in origin. This conclusion is corroborated by correlation between sequence boundaries and unconformities in New Zealand. The resulting long-term curve suggests that sea level ranged from about 75-110 m in the Late Cretaceous, reached a maximum of about 150 m in the Early Eocene and fell to zero in the Miocene. The Late Cretaceous long-term (107 years) magnitude is about 100-150 m less than sea level predicted from ocean volume. This discrepancy can be reconciled by assuming that dynamic topography in New Jersey was driven by North America overriding the subducted Farallon plate. However, geodynamic models of this effect do not resolve the problem in that they require Eocene sea level to be significantly higher in the New Jersey region than the global average. ?? 2008 The Authors. Journal compilation ?? 2008 Blackwell Publishing.

Basin Research↗

USGS2019

Four major Holocene earthquakes on the Reelfoot fault recorded by sackungen in the New Madrid seismic zone, USA

Three sequences of well-documented, major ~M7+ earthquakes (1811-1812 CE, ~1450 CE, and ~900 CE) in the New Madrid seismic zone, USA, contribute significantly to seismic hazard in the region. However, it is unknown whether this <550 yr recurrence interval has been constant throughout the Holocene given limited geomorphic evidence of prior earthquakes. We extend the record of paleoearthquakes along the Reelfoot fault via investigation of ridge-top gravitational failure features, interpreted as sackungen. The sackungen occur in bluffs along the eastern margin of the Mississippi River floodplain and are concentrated near (<15 km) the southwest-dipping Reelfoot reverse fault. A paleoseismic trench excavated across sackungen at the Paw Paw site exposed four packages of colluvial sediment that postdate 30-11 ka Peoria loess. We interpret the colluvial packages to have been deposited following episodic failure of the sackungen as a result of strong ground motions from the following sequence of earthquakes: event 4, 1640 ± 1730 BCE; event 3, 340 ± 670 CE; event 2, 1430 ± 380 CE; and event 1, 1810 ± 50 CE (2-sigma). Event timing corresponds to previously documented earthquakes and represents the longest archive of paleoearthquakes on the Reelfoot fault. If the trenched sackungen record all major Reelfoot fault earthquakes, our observations in combination with prior investigations indicate a period of quiescence from at least 11 – 4.7 ka, followed by four major seismic events culminating in the 1811-1812 CE sequence. This clustered earthquake recurrence helps place bounds on seismic-hazard and geodynamic models in the New Madrid seismic zone.

Arkansas, Missouri↗

USGS2024

Reexamining the Honolulu Volcanics: Hawai‘i's classic case of rejuvenation volcanism

Rejuvenated volcanism is a worldwide phenomenon occurring on many oceanic islands in all of the major ocean basins. This plume-related volcanism follows the main edifice-building stage after a hiatus of variable duration (e.g. 0.6–2 Myrs in Hawai'i). The Honolulu Volcanics (HV), the classic case of rejuvenated volcanism, involved monogenetic eruptions from at least 48 vent areas. Previous studies inferred these vents were aligned along 3 to 11 rifts oriented orthogonal to the propagation direction of the Hawaiian plume. HV basalts are known for having high MgO contents (greater than 10 wt %) and upper mantle xenoliths. Thus, HV magmas are assumed to be relatively primitive and to have ascended rapidly (less than 1 day) through the crust. However, new analyses of olivine cores in basalts from 24 HV vents are mostly too low in forsterite content (74–86 mol %) to be in equilibrium with mantle melts. Olivine and clinopyroxene in HV basalts commonly show reverse zoning indicating magma mixing prior to eruption. These results are inconsistent with the rapid ascent of HV magmas directly from their mantle source. Many of the HV magmas underwent storage (probably in the lower crust or uppermost mantle), crystal fractionation and magma mixing prior to eruption. New 40 Ar/ 39 Ar dates were determined for 11 HV lavas to evaluate their eruptive history. These ages, 80 to 685 ka, combined with our previous and other 40 Ar/ 39 Ar ages for HV lavas reveal long gaps (greater than 50 kyr) between some eruptions. Our comprehensive, whole-rock major and trace element database (63 XRF analyses, 57 ICPMS analyses) of basalts from 37 vents show remarkable compositional diversity with no obvious spatial pattern or temporal trends. The two most recent eruptive sequences have the greatest diversity (basanite and melilitite compositions). HV basanites show systematic trace element trends that may reflect mixing of multiple source components. The nephelinites and melilitites require a complex source history that may have involved residual accessory minerals during mantle melting and a metasomatic component that was not carbonatitic. The new ages and geochemical data show eruptions along most of the previously proposed rift systems were unrelated (except for the Koko Rift). Therefore, geodynamic models that relate HV volcanism to these rift systems are invalid. Lava volumes for two HV eruptions were estimated at 0.11 and 0.23 km 3 using surface mapping and water well data. Similar size, recent monogenetic eruptions in Auckland, New Zealand, were inferred to have lasted several months. Thus, if another HV eruption were to occur, which is possible given the long hiatus between eruptions, it would be extremely disruptive for the nearly 1 million residents of Honolulu. None of the existing geodynamic models fully explain the age duration, volumes and the locations of Hawai'i's rejuvenated volcanism. Thus, the cause of this secondary volcanism remains enigmatic.

Hawaii↗

USGS2000

Responses of stable bay-margin and barrier-island systems to Holocene sea-level highstands, western Gulf of Mexico

The microtidal, wave-dominated coast of the western Gulf of Mexico displays a variety of Holocene geomorphic features indicating higher-than-present water levels that were previously attributed to storm processes while geoidal sea level was at its present position. Field and aerial-photograph examinations of bay margins, barrier islands, and beach-ridge plains following major hurricanes show that the elevated features are inundated periodically by high storm surge. Despite their inundation, these highstand features are not modified by modern storm processes. Instead, storm-related erosion and deposition are always seaward of and lower than the highstand features and are always limited to the extant shorezone, where elevations typically are less than 1.5 m above present sea level. Bay-margin and lagoonal highstand indicators include raised marshes and subtidal flats, wave-cut benches, abandoned wave-cut scarps with fringing marshes and/or beach ridges, and accretionary islands and recurved spits. Other emergent marine features include abandoned compound flood-tidal delta and washover fan complexes attached to barrier islands and anomalously high beach ridges within both the barrier-island complexes and beach-ridge plains. The highest beach ridges, raised marshes and flats, and erosional scarps and benches are manifestations of one or more rising phases and highstands in sea level, whereas the lower marshes and accretionary topography are mainly products of the falling phases and shoreface adjustment to present sea level. Different elevations of beach-ridge sets, discordant truncation of beach ridges, and elevated marine- and brackish-water faunal assemblages preserved in beach ridges, raised marshes and flats, and natural levees are compelling evidence of sea-level fluctuations of ±1 to 1.5 m from about 5500 to 1200 cal yr BP. Independent evidence from studies of geodynamic, climatic, and glacio-eustatic processes can explain the mid-Holocene highstands and late Holocene lowering of sea level that is observed in tectonically stable coastal regions far from former centers of glaciation.

Journal of Sedimentary Research↗