Non-transparent uppermost mantle in the island-arc region of Japan

2006 ◽  
Vol 420 (1-2) ◽  
pp. 189-204 ◽  
Author(s):  
Takashi Iidaka ◽  
Takaya Iwasaki ◽  
Kazuo Yoshimoto
Keyword(s):  
Island Arc ◽  
Uppermost Mantle

Earthquake locations and seismic wave propagation in the upper 250 km of the Tonga island arc

1969 ◽  
Vol 59 (3) ◽  
pp. 1115-1135
Author(s):  
Walter Mitronovas ◽  
Bryan Isacks ◽  
Leonardo Seeber
Keyword(s):  
Seismic Waves ◽  
Seismic Wave Propagation ◽  
Island Arc ◽  
Seismic Zone ◽  
Magma Chambers ◽  
Uppermost Mantle ◽  
Short Term ◽  
High Attenuation ◽  
Upper Limit ◽  
Anomalous Propagation

abstract This paper reports results of a series of short-term seismic observations made on nearly all the major island groups of the Tonga island arc. These data extend the mapping of anomalous propagation of seismic waves from earthquakes beneath the island arc reported by Oliver and Isacks (1967), and significantly increase the number of accurately located earthquakes beneath the Tonga Islands. The data of this paper, combined with those of Sykes et al (1969), indicate that the inclined seismic zone of the Tonga island arc is very thin even at shallow depths. Most of the hypocenters are confined to a slab of about 25 km thickness. This thickness probably still reflects the precision of locations and may only be a measure of the upper limit of the thickness of the seismic zone. The inclined seismic zone, which has a dip of about 45° at intermediate depths in the mantle, appears to decrease in dip or bend over at depths less than about 50 km and appears to intersect the surface at or near the axis of the trench. The wedge-shaped region above the inclined seismic zone appears to be relatively aseismic, although some minor seismic activity has been located at shallow depths beneath the line of active volcanoes. P and S phases with anomalously large amplitudes and frequencies from deep earthquakes (> 400 km) were observed at all sites occupied in Tonga, including sites on two volcanoes. These observations and observations from shallower events include ray paths that emerge nearly vertically beneath the volcanoes. Thus, any region of high attenuation (low Q) below the volcanoes, such as magma chambers, must be limited in extent. These data, in conjunction with regional observations of Sn, indicate that a broad zone of low Q material in the uppermost mantle exists west of the volcanoes on the concave side of the arc. However, the P and S phases recorded on the volcanic ridge exhibit an emergent and drawn-out character that contrasts with the sharp and impulsive character of phases recorded on the non-volcanic ridge. These observations are interpreted to indicate considerable heterogeneity beneath the volcanoes.

The uppermost mantle section below a remnant proto-Philippine Sea island arc: Insights from the peridotite fragments from the Daito Ridge

2018 ◽  
Vol 103 (7) ◽  
pp. 1151-1160 ◽  
Author(s):  
Tomoaki Morishita ◽  
Ken-Ichiro Tani ◽  
Yusuke Soda ◽  
Akihiro Tamura ◽  
Tomoyuki Mizukami ◽  
...  
Keyword(s):  
Island Arc ◽  
Uppermost Mantle ◽  
Philippine Sea ◽  
Mantle Section

Triassic sponges (Sphinctozoa) from Hells Canyon, Oregon

1988 ◽  
Vol 62 (03) ◽  
pp. 419-423 ◽  
Author(s):  
Baba Senowbari-Daryan ◽  
George D. Stanley
Keyword(s):  
Endemic Species ◽  
Upper Triassic ◽  
Island Arc ◽  
Tropical Island ◽  
The Family ◽  
Wallowa Terrane ◽  
Unique Condition ◽  
Hells Canyon

Two Upper Triassic sphinctozoan sponges of the family Sebargasiidae were recovered from silicified residues collected in Hells Canyon, Oregon. These sponges areAmblysiphonellacf.A. steinmanni(Haas), known from the Tethys region, andColospongia whalenin. sp., an endemic species. The latter sponge was placed in the superfamily Porata by Seilacher (1962). The presence of well-preserved cribrate plates in this sponge, in addition to pores of the chamber walls, is a unique condition never before reported in any porate sphinctozoans. Aporate counterparts known primarily from the Triassic Alps have similar cribrate plates but lack the pores in the chamber walls. The sponges from Hells Canyon are associated with abundant bivalves and corals of marked Tethyan affinities and come from a displaced terrane known as the Wallowa Terrane. It was a tropical island arc, suspected to have paleogeographic relationships with Wrangellia; however, these sponges have not yet been found in any other Cordilleran terrane.

Volcanoes of the southwestern extension of the active Mariana island arc: New swath-mapping and geochemical studies

Island Arc ◽  
1998 ◽  
Vol 7 (3) ◽  
pp. 596-607
Author(s):  
F RYER ◽  
S UJIMOTO ◽  
S EKINE ◽  
J OHNSON ◽  
K ASAHARA ◽  
...  
Keyword(s):  
Island Arc ◽  
Swath Mapping ◽  
Geochemical Studies ◽  
Mariana Island Arc
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