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Геология России

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THE 1996 AD DELTA COLLAPSE AND LARGE TURBIDITE IN LAKE BRIENZ
(2007) Girardclos S.; Anselmetti F.S.; Schmidt O.T.; Sturm M.; Ariztegui D.; Pugin A.
In spring 1996 AD, the occurrence of a large mass-transport was detected by a series of events, which happened in Lake Brienz, Switzerland: turbidity increase and oxygen depletion in deep waters, release of an old corpse into surface waters and occurrence of a small tsunami-like wave. This mass-transport generated a large turbidite deposit, which is studied here by combining high-resolution seismic and sedimentary cores. This turbidite deposit correlates to a prominent onlapping unit in the seismic record. Attaining a maximum of 90 cm in thickness, it is longitudinally graded and thins out towards the end of the lake basin. Thickness distribution map shows that the turbidite extends over ∼ 8.5 km2 and has a total volume of 2.72 ⁎ 106 m3, which amounts to ∼ 8.7 yr of the lake's annual sediment input. It consists of normally graded sand to silt-sized sediment containing clasts of hemipelagic sediments, topped by a thin, white, clay-sized layer. The source area, the exact dating and the possible trigger of this turbidite deposit, as well as its flow mechanism and ecological impact are presented along with environmental data (river inflow, wind and lake-level measurements). The combined results indicate that no particular cause (i.e., earthquake, explosion, flood, wind storm, lake-level change, seiche or sediment dredging) triggered the 1996 AD large turbidite deposit. Instead, it was due to a delta slope failure most likely caused by normal sediment accumulation.
MAURITANIA SLIDE COMPLEX: MORPHOLOGY, SEISMIC CHARACTERISATION AND PROCESSES OF FORMATION
(2007) Antobreh A.A.; Krastel S.
Recently acquired Parasound and high resolution multi-channel seismic reflection data have afforded a more detailed investigation of the Mauritania Slide Complex. The slide is more complex than previously reported, and has affected an area in the order of 34,000km2 between ∼600 and >3,500m water depths. The ovate-shaped slide displays a long run-out distance >300km. Slide formation was pre-conditioned mainly by uninterrupted deposition of upwelling-induced organic-rich sediment in an open slope environment which gave rise to rapid accumulation of poorly consolidated bedded sediment intercalated with thin weak layers. The stages of slide development were characterised by multiple failure events probably occurring mainly as retrogressive sliding which exploited widespread weak layers as glide planes. The study suggests excess pore pressures as being the most important trigger mechanism for slide formation. Earthquakes associated with nearby Cape Verde Islands may have played a mostly complementary or, at one time, a leading role in triggering sediment failures. Diapiric growths have locally triggered minor instability events which resulted in remobilizing of pre-existing debris flows as well as translational sliding. The combined activities of all these triggering factors are the most like cause of the complex morphology of the Mauritania Slide Complex.
GEOLOGICAL AND HISTORICAL RECORDS OF TSUNAMI IN AUSTRALIA
(2007) Dominey-Howes D.
The Indian Ocean tsunami (IOT) of 2004 has resulted in significant interest within Australia about the record of tsunami for the continent because an understanding of tsunami hazard begins with a catalogue of past events. Here, a preliminary catalogue of tsunami affecting Australia is presented. The catalogue contains entries for 57 tsunami events. The oldest event is dated at 3.47 Ga, the most recent is the July 17th 2006. Forty-four tsunami were recorded on the New South Wales coast although the NW coast of Western Australia records a significant number of events. Forty-seven events have affected Australia since AD1858. Maximum run-up for an historic event is + 6 m asl whilst the maximum run-up for a palaeotsunami event is reported at an elevation of at least + 100 m asl. Twenty-three percent of historic Australian tsunami were generated by unknown causes and Papua New Guinea, the Solomon Islands and Indonesia collectively represent the most important source area of historic tsunami for Australia. Geological records for palaeo and historic tsunami are identified and summarised. The geological record of tsunami represents a potentially important source of information for Australian tsunami. However, at the present time, the geological record is both limited and controversial and future research should seek to re-examine proposed geological evidence of tsunami. From an analysis of this preliminary catalogue of Australian tsunami, a series of key research priorities have been identified to guide future research in the region.
REMELTING OF REFRACTORY INCLUSIONS IN THE CHONDRULE-FORMING REGIONS: EVIDENCE FROM CHONDRULE-BEARING TYPE C CALCIUM-ALUMINUM-RICH INCLUSIONS FROM ALLENDE
(2007) Krot A.N.; Yurimoto H.; Hutcheon I.D.; Chaussidon M.; MacPherson G.J.; Paque J.
We describe the mineralogy, petrology, oxygen, and magnesium isotope compositions of three coarse‐grained, igneous, anorthite‐rich (type C) Ca‐Al‐rich inclusions (CAIs) (ABC, TS26, and 93) that are associated with ferromagnesian chondrule‐like silicate materials from the CV carbonaceous chondrite Allende. The CAIs consist of lath‐shaped anorthite (An99), Cr‐bearing Al‐Ti‐diopside (Al and Ti contents are highly variable), spinel, and highly åkermanitic and Na‐rich melilite (Åk63–74, 0.4–0.6 wt% Na2O). TS26 and 93 lack Wark‐Lovering rim layers; ABC is a CAI fragment missing the outermost part. The peripheral portions of TS26 and ABC are enriched in SiO2 and depleted in TiO2 and Al2O3 compared to their cores and contain relict ferromagnesian chondrule fragments composed of forsteritic olivine (Fa6–8) and low‐Ca pyroxene/pigeonite (Fs1Wo1–9). The relict grains are corroded by Al‐Ti‐diopside of the host CAIs and surrounded by haloes of augite (Fs0.5Wo30–42). The outer portion of CAI 93 enriched in spinel is overgrown by coarse‐grained pigeonite (Fs0.5–2Wo5–17), augite (Fs0.5Wo38–42), and anorthitic plagioclase (An84). Relict olivine and low‐Ca pyroxene/pigeonite in ABC and TS26, and the pigeonite‐augite rim around 93 are ¹⁶O‐poor (Δ17O ∼ −1‰ to −8‰). Spinel and Al‐Ti‐diopside in cores of CAIs ABC, TS26, and 93 are ¹⁶O‐enriched (Δ17O down to −20‰), whereas Al‐Ti‐diopside in the outer zones, as well as melilite and anorthite, are ¹⁶O‐depleted to various degrees (Δ17O = −11‰ to 2‰). In contrast to typical Allende CAIs that have the canonical initial ²⁶Al/²⁷Al ratio of ∼5 × 10⁻⁵ ABC, 93, and TS26 are ²⁶Al‐poor with (²⁶Al/²⁷Al)0 ratios of (4.7 ± 1.4) × 10⁻⁶ (1.5 ± 1.8) × 10⁻⁶ <1.2 × 10⁻⁶ respectively. We conclude that ABC, TS26, and 93 experienced remelting with addition of ferromagnesian chondrule silicates and incomplete oxygen isotopic exchange in an ¹⁶O‐poor gaseous reservoir, probably in the chondrule‐forming region. This melting episode could have reset the ²⁶Al‐²⁶Mg systematics of the host CAIs, suggesting it occurred ∼2 Myr after formation of most CAIs. These observations and the common presence of relict CAIs inside chondrules suggest that CAIs predated formation of chondrules.
THE CHIMAKURTI, ERRAKONDA, AND UPPALAPADU PLUTONS, EASTERN GHATS BELT, INDIA: AN UNUSUAL ASSOCIATION OF THOLEIITIC AND ALKALINE MAGMATISM
(2007) Kumar K.V.; Frost C.D.; Frost B.R.; Chamberlain K.R.
Mesoproterozoic tholeiitic and alkalic magmatism is widespread in the southern segment of the Eastern Ghats Belt, India, and is particularly well recorded in three cospatial plutons: Chimakurti, a pluton consisting of clinopyroxenite, anorthosite, and gabbro; Errakonda, a ferrosyenite pluton; and the Uppalapadu nepheline syenite. U–Pb zircon ages of ferrosyenite and nepheline syenite suggest that these cospatial plutons were intruded at approximately 1352 Ma, and Nd and Sr isotope compositions suggest that each pluton experienced varying degrees of crustal assimilation. Petrography, mineral chemistry, geochemistry and isotopic compositions suggest that the clinopyroxenite, anorthosite, and gabbro of the Chimakurti suite were derived from a high-Al tholeiitic parental magma. The ferrosyenite exhibits A-type characteristics and are interpreted as a differentiate or partial melt of a tholeiitic source. The Uppalapadu nepheline syenites were derived from an alkaline (basanitic) parental magma. These three plutons illustrate the spectrum of basaltic magmas that may be involved in producing intraplate intrusive complexes. Tholeiitic magmas emplaced at depth in the crust may differentiate to produce massif anorthosites and related A-type differentiates, represented in the Eastern Ghats Belt by the Chimakurti and Errakonda plutons. More alkalic basalts, such as basanites, will evolve to nepheline syenites like those of the Uppalapadu pluton. The 1352 Ma magmatism in the Eastern Ghats is unusual in that it documents a spatial and temporal association of both tholeiitic and alkaline endmembers of continental rift magmatism.