Younger Sheets Research Articles

Unusually large tsunamis frequent a currently creeping part of the Aleutian megathrust

AbstractCurrent models used to assess earthquake and tsunami hazards are inadequate where creep dominates a subduction megathrust. Here we report geological evidence for large tsunamis, occurring on average every 300–340 years, near the source areas of the 1946 and 1957 Aleutian tsunamis. These areas bookend a postulated seismic gap over 200 km long where modern geodetic measurements indicate that the megathrust is currently creeping. At Sedanka Island, evidence for large tsunamis includes six sand sheets that blanket a lowland facing the Pacific Ocean, rise to 15 m above mean sea level, contain marine diatoms, cap terraces, adjoin evidence for scour, and date from the past 1700 years. The youngest sheet and modern drift logs found as far as 800 m inland and >18 m elevation likely record the 1957 tsunami. Previously unrecognized tsunami sources coexist with a presently creeping megathrust along this part of the Aleutian Subduction Zone.

Fractionated Ilmenite-series Granites in Southwest Japan: Source Magma for REE-Sn-W Mineralizations

Abstract. Various leucocratic biotite granites, low-temperature I-type, from the middle zone of the Sanyo ilmenite-series granitic terrane were studied chemically. These granites are locally associated with REE-Sn-W mineralizations, and were compared with unmineralized granites and batholithic Ryoke granites in three areas of the Chubu, Kinki and Chugoku Districts. They are unique in the region because they have extremely low ferromagnesian components but high Rb/Sr and 10000Ga/Al ratios. These granites are divided petrographically into the main phase, finer-grained marginal phase and younger sheets and dikelets. These rocks have increasing of HREE+Y and Nb+Ta contents in this order, which is also followed by decreasing zircon saturation temperature from 780 to 725d̀C. Together with the mode of occurrence of these granites, the leucogranitic magmas are considered to have formed by in-situ fractionation of the host granitic magmas near the top of the magma chambers. The concentration of HREE, Y, Nb and Ta in these Sanyo Belt leucogranites is principally controlled by magmatic fractionation.

Probable low-pressure intrusion of gabbro into serpentinized peridotite, northern California

The Castle Lake gabbroic body intrudes the Trinity ultramafic sheet in northern California. The extent of concentric reaction zones of talc, tremolite, and chloritic blackwall on included blocks of peridotite correlates with increasing proportions of igneous hornblende in the gabbroic host; consequently, the reaction zones probably formed during initial cooling of the gabbro. The principal source of water in the gabbroic magma and in the reaction zones was probably a hydrothermal system which existed during and after crystallization of the magma. Blocks of pyroxenite in a succession of intrusive sheets of gabbroic rock indicate repeated intrusion of magma into its previously solidified margins. An increasing proportion of hornblende in the younger sheets correlates with a marked increase in the grain size. Variable distribution of dissolved and exsolved water and transitory minor decompression events can explain the grain-size layering in the younger sheets. Vugs as much as 5 mm in diameter in the pegmatitic hornblende gabbro are interpreted to be igneous in origin and, together with the patchiness of the metamorphism, suggest that the gabbro crystallized at a pressure of ∼2 kbar or less. Igneous hornblende (near solidus) and rare garnet are consistent with such a low pressure. The lack of sodium-rich plagioclase in the patchily metamorphosed gabbros suggests that the hydrothermal (metamorphic) fluids were poor in sodium (nonmarine or distilled marine). Because of the correlation between these igneous and metamorphic features, it is likely that the Castle Lake gabbro intruded its own hydrothermal envelope of serpentinized peridotite.