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Research about Chugach Mountains

Source-linked reports with geographic coverage including Chugach Mountains.

4 recordsLinked to original sources
USGS2012

Crustal structure of the Chugach Mountains, southern Alaska: A study of peg‐leg multiples from a low‐velocity zone

A seismic refraction profile recorded along the geologic strike of the Chugach Mountains in southern Alaska shows three upper crustal high‐velocity layers (6.9, 7.2, and 7.6 km/s) and a unique pattern of strongly focussed echelon arrivals to a distance of 225 km. The group velocity of the ensemble of echelon arrivals is 6.4 km/s. Modeling of this profile with the reflectivity method reveals that the echelon pattern is due to peg‐leg multiples generated from within a low‐velocity zone between the second and third upper crustal high‐velocity layers. The third high‐velocity layer (7.6 km/s) is underlain at 18 km depth by a pronounced low‐velocity zone that produces a seismic shadow zone wherein peg‐leg multiples are seen as echelon arrivals. The interpretation of these echelon arrivals as multiples supersedes an earlier interpretation which attributed them to successive primary reflections arising from alternating high‐ and low‐velocity layers. Synthetic seismogram modeling indicates that a low‐velocity zone with transitional upper and lower boundaries generates peg‐leg multiples as effectively as one with sharp boundaries. No PmP or Pn arrivals from the subducting oceanic Moho at 30 km depth beneath the western part of the line are observed on the long‐offset (90–225 km) data. This may be due to a lower crustal waveguide whose top is the high‐velocity (7.6 km/s) layer and whose base is the Moho. A deep (∼54 km) reflector is not affected by the waveguide and has been identified in the data. Although peg‐leg multiples have been interpreted on some long‐range refraction profiles that sound to upper mantle depths, the Chugach Mountains profile is one of the few crustal refraction profiles where peg‐leg multiples are clearly observed. This study indicates that multiple and converted phases may be more important in seismic refraction/wide‐angle reflection profiles than previously recognized.

Alaska↗
USGS2012

Surface Mass Balance of the Columbia Glacier, Alaska, 1978 and 2010 Balance Years

Although Columbia Glacier is one of the largest sources of glacier mass loss in Alaska, surface mass balance measurements are sparse, with only a single data set available from 1978. The dearth of surface mass-balance data prohibits partitioning of the total mass losses between dynamics and surface forcing; however, the accurate inclusion of calving glaciers into predictive models requires both dynamic and climatic forcing of total mass balance. During 2010, the U.S. Geological Survey collected surface balance data at several locations distributed over the surface of Columbia Glacier to estimate the glacier-wide annual balance for balance year 2010 using the 2007 area-altitude distribution. This report also summarizes data collected in 1978, calculates the 1978 annual surface balance, and uses these observations to constrain the 2010 values, particularly the shape of the balance profile. Both years exhibit balances indicative of near-equilibrium surface mass-balance conditions, and demonstrate the importance of dynamic processes during the rapid retreat.

Alaska↗
USGS1972

Reconnaissance bedrock geologic map of the Chugach Mountains near Anchorage, Alaska

The area between Knik and Turnagain Arms east of Anchorage is underlain mostly by rocks that are part of an extensive arcuate belt of thick Mesozoic marine deposits that extend through the Chugach-Kenai-Kodiak Mountains. The two main units in this belt are the Jurassic (?) and Cretaceous Valdez (?) Group composed of flysch deposits and the McHugh Complex composed of oceanic metavolcanic sequences tectonically mixed with metaclastic rocks derived from a continental magmatic arc. Deformation of the McHugh Complex is characterized by pervasive, closely spaced shear fractures and is melange-like in some areas. Deformation of the Valdez (?) Group is characterized by tight folding that was initiated before the sediments were completely lithified. The folds have steeply dipping axial surfaces that are overturned to the northwest in some areas and to the southeast in other areas. The Jurassic and(or) Cretaceous McHugh Complex is separated from the Valdez (?) Group by the Eagle River thrust fault. The Valdez (?) Group was probably deposited primarily on oceanic crust that was collapsed against the continental margin in latest Cretaceous to early Tertiary time. An accurate belt of upper Paleozoic to lower Mesozoic rocks to the north and west had been deformed, accreted to the continental margin, and intruded by plutons prior to the deposition of the Jurassic (?) and Cretaceous sediments. Rocks of the continental terrane that underlie the Wrangell and Talkeetna Mountains and the Alaska Range are locally exposed in the map area north and west of the Knik fault zone. Near the village of Eklutna, the junction between the two terranes is marked by an ophiolitic assemblage.

Alaska↗