Overturned cliff-top mega-boulders at Little Beecroft Head, Jervis Bay, NSW, Australia: a mega-tsunami or aboveground bolide impact about 20ka BP?

The largest of several detached boulders on top of the 33 m high cliffs at Little Beecroft Head measures 3.5 x 2.1 x 1.6 m and weighs ~31 tonnes. Sedimentary structures (cross‐bedding) show that this boulder has overturned. The boulder and the nearby cliff are pebbly quartz sandstone of the Permian Snapper Point Formation in the southern Sydney Basin. A comparison with the local stratigraphy indicates that the boulder was detached from the nearby north‐east facing cliff face, in which a bed of identical lithology crops are at a lower elevation. A Holocene tsunami has been invoked by other authors for emplacement of the boulder onto the cliff top. Using the isotopes 10Be and 26Al, we determined the cosmogenic exposure ages of (a) the currently exposed upper surface of the mega‐boulder; (b) the lower surface which is in partial contact with the cliff‐top platform upon which the mega‐boulder rests; and (c) the cliff‐top platform surface itself, ~3 meters landward of the boulder. 10Be and 26Al exposure ages for all three samples are consistent. Simple age modelling (zero‐erosion case) shows that today’s lower surface of the boulder was exposed for ~63 ka prior to the overturning event and that today’s upper surface has been exposed for ~19 ka, which represents the detachment age or the elapsed time since the overturning event exposed the top surface of the boulder. The adjacent platform surface has a zero‐erosion exposure age of 84 ka. At the time of global Last Glacial Maximum, 20–22 ka, sea level was 120–130 m lower than at present and the land-ocean boundary lay some 20 km east of the site. This precludes boulder emplacement and overturning by a storm wave or even by a ‘normal’ tsunami wave. We suggest that the event which dislodged the boulder from the vertical cliff face, lifted and overturned it and emplaced it ~2 meters further inland could be (1) a mega‐tsunami resulting from a large bolide impact in the distant ocean; or (2) a mid‐air cometary explosion similar to that which is thought to have occurred at Tunguska, Russia, in 1908.Geological Society of Australi

The petrogenesis of sodic island arc magmas at Savo volcano, Solomon Islands

Savo, Solomon Islands, is a historically active volcano dominated by sodic, alkaline lavas, and pyroclastic rocks with up to 7.5 wt% Na2O, and high Sr, arc-like trace element chemistry. The suite is dominated by mugearites (plagioclase–clinopyroxene–magnetite ± amphibole ± olivine) and trachytes (plagioclase–amphibole–magnetite ± biotite). The presence of hydrous minerals (amphibole, biotite) indicates relatively wet magmas. In such melts, plagioclase is relatively unstable relative to iron oxides and ferromagnesian silicates; it is the latter minerals (particularly hornblende) that dominate cumulate nodules at Savo and drive the chemical differentiation of the suite, with a limited role for plagioclase. This is potentially occurring in a crustal “hot zone”, with major chemical differentiation occurring at depth. Batches of magma ascend periodically, where they are subject to decompression, water saturation and further cooling, resulting in closed-system crystallisation of plagioclase, and ultimately the production of sodic, crystal and feldspar-rich, high-Sr rocks. The sodic and hydrous nature of the parental magmas is interpreted to be the result of partial melting of metasomatised mantle, but radiogenic isotope data (Pb, Sr, Nd) cannot uniquely identify the source of the metasomatic agent. Electronic supplementary material The online version of this article (doi:10.1007/s00410-009-0410-9) contains supplementary material, which is available to authorized users

Complex morphology of subducted lithosphere in the mantle beneath the Tonga trench

At the Tonga trench, old Pacific sea floor subducts at a rapid rate below the Indo-Australia plate, generating most of the world's deep earthquakes and producing a deep slab of former oceanic lithosphere