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579 records · Page 33Linked to original sources
USGS2006

Piggyback tectonics: Long-term growth of Kilauea on the south flank of Mauna Loa

Compositional and age data from offshore pillow lavas and volcaniclastic sediments, along with on-land geologic, seismic, and deformation data, provide broad perspectives on the early growth of Kilauea Volcano and the long-term geometric evolution of its rift zones. Sulfur-rich glass rinds on pillow lavas and volcaniclastic sediments derived from them document early underwater growth of a large compositionally diverse alkalic edifice. The alkalic rocks yield 40 Ar/ 39 Ar ages as old as about 275 ka; transitional-composition lavas, which mark beginning of the shield stage while most or all the edifice remained below sea level, probably first erupted after about 150 ka, and tholeiitic lavas of present-day type are probably younger than 100 ka. Breccia clasts from Papau Seamount and along the lower southwest corner of the Hilina bench are derived from subaerial Mauna Loa, requiring that Mauna Loa's flank underlies western parts of Kilauea at shallow depth. The volume of the Kilauea edifice is therefore smaller (∼10,000 km 3 ) than previous estimates (15–40,000 km 3 ); lava-thickness accumulation rates appear to have remained nearly constant during edifice growth, as effusion rates increased from ∼25×10 6 m 3 /yr at end of the alkalic stage to the present-day tholeiite rate of ∼100×10 6 m 3 /yr. Seismic and gravity data show that the deep plumbing system for Kilauea's magma supply extends nearly vertically through the oceanic crust at least to mantle depths of 30–35 km, directly below its present-day caldera. Proximity of Kilauea's caldera to the surface boundary with Mauna Loa and the presence of Mauna Loa rocks at shallow depth beneath the south flank are difficult to reconcile with a submarine origin for early Kilauea alkalic lavas, unless geometric relations between the two volcanoes have changed substantially during growth of the Kilauea shield. Seismic and ground deformation data suggest seaward spreading of the entire south flank of Hawaii Island, independently of the boundary between Kilauea and Mauna Loa, along a landward-dipping detachment fault system near the basal contact of the composite volcanic edifices with underlying oceanic crust. Current steady-state horizontal displacements increase seaward, at rates of ∼1.5 cm/yr on the lower flank of Mauna Loa and reaching 5–8 cm/yr at the Kilauea coastline. Infrequent (∼100 yr?) large earthquakes generate similar geometries, but 10 2 larger displacements per event. Present-day Kilauea is the more dynamic edifice, but prior to inception of Kilauea and during its early growth, Mauna Loa is inferred to have undergone intense volcano spreading, involving the Kaoiki–Honuapo fault system (considered a geometric analog of the Hilina system on Kilauea). Cumulative deformation of Mauna Loa's south flank during growth of Kilauea since 200–300 ka is estimated to have involved > 10 km of seaward spreading, displacing the rift zones of Kīlauea while its deep plumbing system and summit magma reservoir remained nearly fixed in space. Kilauea's rift zones, rather than migrating southward with time solely due to dike emplacement preferentially on the mobile seaward side, alternatively are interpreted to have been transported passively southward, “piggyback” style, during shield-stage growth of Kilauea as a blister on the still-mobile south flank of Mauna Loa. Such an evolution of Kilauea accounts for the arcuate geometry of the present-day rift zones, proximity of the summit magma supply to the exposed flank of Mauna Loa, initial submarine growth of the ancestral edifice, and present-day location of Mauna Loa rocks at shallow depth beneath the south flank of Kilauea.

Hawai'i↗
USGS2012

The M2 tide on the Amazon Shelf

As part of A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS), moored and shipboard current measurements made over the Amazon shelf during 1990–1991 have been analyzed to determine the dominant semidiurnal tidal constituent, the M 2 . These results have been combined with coastal sea level data from within the Amazon and Para Rivers, the adjacent shelf, and with satellite‐derived tidal elevation data from off the shelf to provide a more complete description of the M 2 tide in this complex river/shelf system. Near the Amazon River mouth the M 2 tide propagates across the shelf and through the mouth as a damped progressive wave, with its amplitude decreasing and phase increasing upriver. Over the adjacent shelf north of Cabo Norte, the M 2 tide approaches a damped standing wave, with large amplitudes (greater than 1.5 m) near the coast due to near resonance within the coastal embayment formed by the Cabo Norte shoal to the south and Cabo Cassipore to the north. The observed M 2 tidal currents are nearly rectilinear and oriented primarily across the local isobaths. Comparisons between tidal observations in both the North Channel and the Cabo Norte‐Cabo Cassipore embayment and a simple variable‐width channel tidal model indicate that (1) most of the M 2 tidal energy dissipation occurs over the mid‐ and inner shelf (in water depths less than 20 m) and (2) fluid muds found there cause a significant reduction (of order 50%) in the effective bottom friction felt by the M 2 tide. The approximate resonant period of the Cabo Norte‐Cabo Cassipore embayment is 11.9 hours, and at resonance the average energy dissipation per forcing period is roughly 2.2 times the average mechanical energy in the embayment. This damping rate is large enough that the tidal amplification is rather insensitive to forcing frequency, so that the response of the embayment to forcing over the semidiurnal band should be essentially the same. The vertical structure of the M 2 tidal current is examined at one outer shelf site located in 65‐m water depth. The observed semimajor axis increases logarithmically with height above bottom within the lowest 1–2 m and reaches a maximum in excess of 0.5 m/s at approximately 11 m above bottom. The mean ellipticity is small (less than 0.1) and positive, indicating clockwise rotation of a nearly rectilinear current, and the semimajor axis is oriented within 10° of the local cross‐isobath direction. The M 2 phase increases with height above bottom, with flood at the bottom leading flood at the surface by about 1 hour. A simple, local homogeneous tidal model with time‐ and space‐dependent eddy viscosity simulates the observed near‐bottom velocity reasonably well, however, the model suggests that stratification above the lowest few meters may significantly affect the tidal boundary layer structure at this site. The M 2 energy flux onto the Amazon shelf and into the Amazon and Para Rivers has been estimated using current and surface elevation data and the best fit variable‐width channel model results. The net M 2 energy flux into the mouths of the Amazon and Para Rivers is 0.47×10 10 W and 0.19×10 10 W, respectively. A net M 2 energy flux of about 3.3×10 10 W occurs onto the shelf between the North Channel of the Amazon River and Cabo Cassipore. This stretch of the Amazon shelf accounts for about 1.3% of the global dissipation of the M 2 tide.

Journal of Geophysical Research C: Oceans↗
USGS2012

The nature of the crust in the Yukon-Koyukuk province as inferred from the chemical and isotopic composition of five Late Cretaceous to Early Tertiary volcanic fields in western Alaska

Late Cretaceous and early Tertiary volcanic and plutonic rocks in western Alaska comprise a vast magmatic province extending from the Alaska Range north to the Arctic Circle, south to Bristol Bay, and west to the Bering Sea Shelf. The chemical and isotopic composition of five of these Late Cretaceous to early Tertiary volcanic fields in the north central part of this province were studied to determine if Paleozoic or older continental crust underlies the Yukon-Koyukuk province. Three of the fields, the Blackburn Hills, Yukon River, and Kanuti, occur within the Yukon-Koyukuk province and two, the Sischu and Nowitna, overlie bordering Precambrian and Paleozoic metamorphic terranes to the southeast. High initial 87 Sr/ 86 Sr of 0.7075–0.7079 and moderate initial 143 Nd/ 144 Nd of 0.51244–0.51247 of rhyolite, dacite, and high-silica andesite of the Sischu volcanic field indicate that the magmas have interacted with the underlying Paleozoic or older continental crust. The relatively limited variation of isotopic (initial 87 Sr/ 86 Sr = 0.7044–0.7051; initial 143 Nd/ 144 Nd = 0.51256–0.51257) and elemental compositions of andesites from the Nowitna field can be accounted for by assimilation of small amounts of Paleozoic or older continental crust during crystal fractionation of andesite parent magmas at crustal levels. The Blackburn Hills field, which consists of medium-K basalt, andesite, and rhyolite intruded by a small granitic pluton, has a large range in initial 87 Sr/ 86 Sr and initial 143 Nd/ 144 Nd that plot in the field for 60 Ma mantle, from near mid-ocean ridge basalts to near “bulk-earth” compositions (initial 87 Sr/ 86 Sr = 0.7033–0.7052; initial 143 Nd/ 144 Nd = 0.51253–0.51290). Andesites and basalts from the Blackburn Hills are divided into two group on the basis of rare earth element (REE) and isotopic composition. Isotopic variation in the more primitive group 1 is best explained by assimilation of the lower crust of the Jurassic to Early Cretaceous Koyukuk terrane by mantle-derived basalts during crystal fractionation, though part of the isotopic variation may be due to metasomatism of an oceanic island basalt type mantle source by fluids derived from subducted sediments. Group 2 andesites from the Blackburn Hills have lower heavy REE abundances and more enriched isotopic compositions. These group 2 andesites and dacites from the Kanuti field, which have ( 87 Sr/ 86 Sr) i = 0.7043–0.7048 and ( 143 Nd/ 144 Nd) i = 0.51248–0.51267, appear to have formed by partial melting of the lower crust of the Koyukuk terrane. The Yukon River field consists of basalt, andesite, dacite, and rhyolite having ( 87 Sr/ 86 Sr) i = 0.7037–0.7051 and ( 143 Nd/ 144 Nd) i = 0.51266–0.51280; its isotopic composition does not require the presence of Paleozoic or older continental crust under the volcanic field and may have formed by interaction between mantle-derived melts and the oceanic Angayucham/Tozitna or island arc Koyukuk terrane. Most of the intrusive rocks and rhyolite domes from the Blackburn Hills volcanic field have ( 87 Sr/ 86 Sr) i = 0.7038–0.7041 and dacites from the Kanuti volcanic field have ( 87 Sr/ 86 Sr) i = 0.7043–0.7048. Thus little or no old continental crust was involved in the genesis of the Late Cretaceous and early Tertiary rocks and therefore probably does not extend beneath this part of the Yukon-Koyukuk province. However, the ultimate source of the small volumes of enriched shoshonitic andesite ( 87 Sr/ 86 Sr = 0.7075, 143 Nd/ 144 Nd = 0.5125) erupted at 118 Ma in the Yukon-Koyukuk province may be continental lithosphere, which may have been thrust under this part of the Yukon-Koyukuk province during arc-continent collision in the Early Cretaceous.

Journal of Geophysical Research Solid Earth↗