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461.
Volcán Aucanquilcha, northern Chile, has produced ∼37 km3 of dacite (63–66 wt% silica), mainly as lavas with ubiquitous magmatic inclusions (59–62 wt% silica) over the last ∼1 million
years. A pyroclastic flow deposit related to dome collapse occurs on the western side of the edifice and a debris avalanche
deposit occurs on the eastern side. The >6,000-m high edifice defines a 9-km E–W ridge and lies at the center of a cluster
of more than 15 volcanoes, the Aucanquilcha Volcanic Cluster, that has been active for at least the past 11 million years.
The E–W alignment of vents is nearly orthogonal to the arc axis. A majority of Volcán Aucanquilcha was constructed during
the first 200,000 years of eruption, whereas the last 800,000 years have added little additional volume. The peak eruptive
rate during the edifice-building phases was ∼0.16 km3/ka and the later eruptive rate was ∼0.02 km3/ka. Comparable dacite volcanoes elsewhere show a similar pattern of high volcanic productivity during the early stages and
punctuated rather than continuous activity. Volcán Aucanquilcha lavas are dominated by phenocrysts of plagioclase, accompanied
by two populations of amphibole, biotite, clinopyroxene, Fe–Ti oxides and (or) orthopyroxene. Accessory phases include zircon,
apatite and rare quartz and sanidine. One amphibole population is pargasite and the other is hornblende. The homogeneity of
dacite lava from Volcán Aucanquilcha contrasts with the heterogeneity (52–66 wt% silica) at nearby Volcán Ollagüe, which has
been active over roughly the same period of time. We attribute this homogeneity at Aucanquilcha to the thermal development
of the crust underneath the volcano resulting from protracted magmatism there, whereas Volcán Ollagüe lacks this magmatic
legacy. 相似文献
462.
463.
Ambient noise tomography is a powerful tool that has found increasing application in reservoir analysis and imaging. The Cachar fold belt in lower Assam, northeast India encompasses several wells under active hydrocarbon production, along with several dry wells. To overcome the lack of active seismic data over the entire fold belt, a passive seismic study was carried out to image the concealed three-dimensional sub-surface structures. The data were recorded from February to November 2011 by a network of 65 wideband seismometers spanning an area of about 40 × 60 km2. The data are crosscorrelated in the 2–5 s band, followed by phase-weighted stacking to estimate noise correlation functions with surface wave signatures. The traveltimes picked from the frequency-time analysis are utilized in a tomographic inversion for Rayleigh wave group velocities. The group velocity anomalies have a lateral resolution of ~ 3.5 × 5.5 km2 and variations of up to for each period. The group velocities are in turn inverted for S-wave velocity distribution as a function of depth. The three-dimensional S-wave velocity tomograms reveal the tight anticlines and broad synclines, with high- and low-velocity zones corresponding to structural highs and lows, respectively. The structural interpretation is supported for the part of the region with producing wells and covered by active seismic data, wherein the post-stack time migrated seismic section shows anticlinal and synclinal features similar to those obtained from ambient noise tomography. The structures revealed by ambient noise tomography can help identify zones of interest to be targeted by active seismic surveys in the Cachar fold belt. 相似文献