Paleomagnetism of Lower-Middle Devonian and Upper Proterozoic-Cambrian(?) Rocks from Mejeria (Mauritania, West Africa)

The paleomagnetism of two sedimentary rock units from the foreland of the Mauritanides of West Africa, in the Taganet region of Mauritania (Taoudeni basin) was studied to provide constraints on the paleocontinental positions of the southern continents in the Paleozoic. Thermal demagnetization of samples from the lower to middle Devonian Gneiguira supergroup isolated a predominantly single polarity characteristic magnetization (D = 135.7°, I = 27.3°, alpha 95 = 5.3° for N = 10 sites/44 samples) which gives a south paleopole position at Lat = 35.2°S, Long = 43.6°E (dp, dm = 3.0°, 5.6°). The only other direction sometimes present is one aligned near to the present dipole field axis, notably as a high temperature component of reversed polarity in 7 samples (D = 177.9°, I = −26.9°) obtained from 2 sites in weathered outcrop. The Upper Proterozoic to Cambrian (?) Mejeria red sandstone unit, equivalent to the Adrar CO10, although apparently unweathered has multicomponent magnetization. Most common is an intermediate temperature (300° to 550°C) direction (D = 137.2°, 1 = 14.4°, alpha95 = 13.2° for N = 4 sites/ 17 samples) similar to the characteristic direction of the Gneiguira. A high temperature component can be isolated in 11 samples but the directions are randomly distributed. Comparison of the Gneiguira paleopole with other middle to late Paleozoic poles from Africa and Australia suggests that either it represents a Carboniferous remagnetization or that the south paleomagnetic pole for Gondwana already was off southern Africa by the Devonian. The paleogeographic and tectonic consequences of these possibilities differ considerably for the Atlantic bordering continents

Moravian missions.

Letter from Killinek; Letter from Killinek;Moravian missions

The AuScope Far North Queensland survey

[Extract] The Australian Government's National Collaborative Research Infrastructure Strategy (NCRIS) intiative awarded AuScope (http://www.auscope.org.au) $42.8 million to support geoscience. AuScope will establish world-class infrastructure to characterise the structure and evolution of the Australian continent in a global context, from surface to core in space and time; and provide better understanding of the implications for natural resources, hazards and environment. The Earth Imagine and Structure (ANSIR) component of AuScope is focused on providing 3D databases of geologically important regions, which will be achieved through the collection of GeoTransects. AuScope will collaborate with, and use the services of ANSIR to achieve this.\ud \ud In August 2007, the Far North Queensland (FNQ) Tasman Line project became the first AuScope Tranverse to be acquired. This survey links with the Ga/GSQ Isa-Georgetown-Charters Towers survey and together, provide an exceptional opportunity to image this important region of Australia crust in three-dimensions. FNQ best preserves the Tasman Line, which is the boundary between the Precambrian craton of Australia, and the Phanerozoic Tasmanides to the east.\ud \ud The correlation between the lithospheric-scale structures evident in the seismic tomography images with mapped surface structures from observed geology suggests that this region is ideal for investigating the relationship between major upper crustal province boundaries and major features observed in geophysical images.\ud \ud The FNQ AuScope Reflection Traverse will address important questions regarding the nature of continental growth in eastern Australia. The raw field stack indicates a significant change in Moho depth on either side of the Tasman Line, considerable coherent reflectivity within the middle crust and evidence of shallow mid-crustal structures. However, the seismic imaging does not indicate teh presence of a single major structure that corresponds with the Tasman Line. The data now requires processing to enhance the seismic image within the upper crust

New constraints on the seismic structure of West Australia: Evidence for terrane stablization prior to the assembly of an ancient continent?

We present a new, near-comprehensive survey of the variations in seismic structure across the West Australian craton at the scale of the main terrane groups. Analyzing data from distant earthquakes recorded at temporary and permanent stations located across the region, we found the best-fitting structure by modeling the conversions from P- to S-wave motion (the receiver function) that take place as the seismic energy travels upward through the lithosphere. Such methods can be used to delineate the extent of cratonic and orogenic terranes in regions where geological exposure of the surface is limited, and they provide an effective alternative to active-source seismic techniques for deep crustal targets. The seismic structure is consistent within several of the individual Archean terranes, most notably the Pilbara, Murchison, and Southern Cross. These terranes are underlain by lower crust of low seismic velocity and show a sharp seismic Moho. The structure shows significant contrasts between neighboring terranes; thus, major tectonic units have a velocity profile that is a signature of that terrane or terrane group. We infer that the seismic structure of the Archean crust and upper mantle was fixed before craton assembly and preserved through the subsequent collision and accretion of the tectonic units that formed the West Australian craton