LA RESISTIVITE APPARENTE COMME TRANSFORMATION LINEAIRE DU POTENTIEL ET LA TRANSFORMATION INVERSE*

The apparent resistivity is considered as a linear transformation of the potential by an operator in the form of an infinite matrix A. The inverse transformation expresses the potential as a function of the apparent resistivity or of the difference of potentials. It is found by calculation of the inverse matrix^-1A. The relation between apparent resistivities for different arrays is expressed as a product of transformations.     

Melt Extraction during Formation of K-Feldspar + Sillimanite Migmatites, West of Revelstoke, British Columbia

An exposure of sillimanite-rich, strongly deformed, stromatic,K-feldspar-bearing migmatites in the Monashee Terrane west ofRevelstoke, British Columbia, has been examined to determinethe process of migmatization and to evaluate whether the systemwas open or closed during leucosome formation. An anatecticorigin for the migmatites is supported by: (1) the minimum meltcomposition of the leucosomes; (2) textures suggesting a fluidbehavior of the leucosomes and local pegmatitic textures; and(3) P–T estimates (720–820C; 75–9 kbar)above vaporabsent melting conditions of muscovitt + quartz. To establish whether melt was extracted or added during migmatization,measured volume percents of leucosome were compared with estimatesof melt production modeled by muscovite + quartz dehydrationmelting. Quantitative estimates of volume percent of leucosomeat present in the outcrop are between 20 and 30%. The amountof melt produced from the model muscovite dehydration meltingreaction is constrained by measured modal percent of sillimanite(15–25%) in the outcrop and is dependent on modal proportionof muscovite in the unmelted protolith and the melt water contentUsing a muscovite-rich protolith and a melt water content of4 wt%, complete dehydration melting of muscovite results ina production of 54 vol % melt and 25 vol % sillmanite, indicatinga melt loss of 29 vol %. A melt water content of 6 wt% resultsin production of 41 vol % melt and 23 vol % sillimanite, indicatinga melt loss of 16 vol %. Melt loss may have occurred by meltmovement along foliation planes during flattening, during formationof shear bands or locally along subvertical fractures. Spatialproximity of the outcrop to the Monashee dcollement suggeststhat thrusting was localized to zones of high melt production,which in turn facilitated melt migration. KEY WORDS: migmatites; British Columbia; Monashee Tarrane; anatexis; melt extraction ^*Corresponding author. Present address: Department of Earth and Planetary Sciences, The Univenity of New Mexico, Albuquerque, NM 817131, USA     

Timing of the last deglaciation in Lithuania

Boulders from the Grūda Moraine, which is associated with the maximum extent of the Scandinavian Ice Sheet (SIS) during the last glaciation, and the Baltija (also referred to as the South Lithuanian), the Middle and North Lithuanian moraines, which are associated with recessional stages of the SIS, were sampled for surface exposure dating using ¹⁰Be. By combining these data with existing radiocarbon ages, we developed a chronology for the retreat of the SIS margin in Lithuania. Our new ¹⁰Be ages suggest that the SIS margin began to retreat from its maximum extent at 18.3 ± 0.8 ¹⁰Be kyr. Based on a probable correlation of the Baltija Moraine with the Pomeranian Moraine in Poland, we infer that the Baltija Moraine was formed following a re-advance of the SIS margin. The ice margin retreated from the Baltija position at 14.0 ± 0.4 ¹⁰Be kyr. The SIS-margin retreat paused at least two more times to form the Middle Lithuanian Moraine at 13.5 ± 0.6 ¹⁰Be kyr and the North Lithuanian Moraine (tentatively correlated to the Pajūris Moraine) at 13.3 ± 0.7 ¹⁰Be kyr. Subsequent ice-margin retreat from the North Lithuanian Moraine represented the final deglaciation of Lithuania. Direct dating of these moraines better constrains the relation of ice-margin positions in Lithuania to those in adjacent countries as well as the SIS response to climate change.     

Inter- and supratidal sedimentology of a fjord-head estuary, south-western British Columbia

Detailed examination of inter- and supratidal delta and floodplain sediments exposed in eroding bank sections at 52 locations along the Squamish River estuary provides the basis for recognizing seven distinct facies within the 5500 m-long estuary. Estuary sedimentation is initially driven by the development of sand bar complexes along the seaward edge of the intertidal delta. Sedimentation continues within interdistributary bay environments as intertidal sandflats and then tidal marshes develop, Aggradation of the delta within interdistributary bay environments results in a gradual transition from delta to alluvial plain. Of the seven facies identified, only the intertidal sands and tidal marsh deposits provide evidence of their tidal origin. Examination of deposits throughout the riverine estuary reveals a number of gradual yet distinct changes of sediment size, structure, and sequence architecture. These trends record the changing nature of tidal and riverine control on sedimentation along the tidal gradient. Generally, with increasing distance up-estuary, sediment grain-size increases, the thickness of fine-grained overbank deposits decreases, and bedding changes from fine parallel bedding to higher energy bedforms. In addition, fining-upward successions become capped by coarser sands, facies contacts change from gradational to abrupt and occasionally erosional, and facies successions become increasingly complex and less predictable. Squamish River estuary has been divided into four zones based on sedimentological and stratigraphical evidence, each zone reflecting changes in the relative influence of tidal and riverine control on sedimentation. Each zone contains distinctly different facies sequences, although zone boundaries generally are gradational.     

INTERPRÉTATION DIRECTE DES COURBES DE SONDAGE ÉLECTRIQUE ET LE PROBLÈME DE DIFFÉRENTS DISPOSITIFS DE MESURE *

Direct interpretation methods of resistivity curves are discussed, which use the kernel function of the apparent resistivity. This function results from the consideration of the problem of diverse electrodes configurations. Several expressions for the determination of the kernel function of the potential from the kernel function of the apparent resistivity are given.     

Timing of the last deglaciation in Belarus

We measured ¹⁰Be concentrations in boulders collected from the Orsha and Braslav moraines, associated with the Last Glacial Maximum extent and a recessional stage of the Scandinavian Ice Sheet (SIS), respectively, providing a direct dating of the southeastern sector of the ice-sheet margin in Belarus. By combining these data with selected existing radiocarbon ages, we developed a chronology for the last deglaciation of Belarus. The northeastern part of the country remained ice free until at least 19.2±0.2 cal. kyr BP, whereas the northwestern part of the country was ice free until 22.3±1.5 cal. kyr BP. A lobate ice margin subsequently advanced to its maximum extent and deposited the Orsha Moraine. The ice margin retreated from this moraine at 17.7±2.0 ¹⁰Be kyr to a position in the northern part of the country, where it deposited the Braslav Moraine. Subsequent ice-margin retreat from that moraine at 13.1±0.5 ¹⁰Be kyr represented the final deglaciation of Belarus. Direct dating of these moraines better constrains the relation of ice-margin positions in Belarus to those in adjacent countries as well as the SIS response to climate change.     

The surface geometry of the Last Glacial Maximum ice sheet in the Andøya-Skånland region, northern Norway, constrained by surface exposure dating and clay mineralogy

Blockfields, weathering boundaries and marginal moraines have been mapped along a longitudinal transect from northern Andøya to Skånland in northern Norway. The degree of rock-surface weathering above and below glacial trimlines, clay-mineral assemblages and surface exposure dating based on in situ cosmogenic ¹⁰Be have been used to reconstruct the vertical dimensions and timing of the Last Glacial Maximum (LGM) of the Scandinavian Ice Sheet in this region. The cosmogenic exposure dates suggest that the lower blockfield boundary/trimline along the Andøya-Skånland transect represents the upper limit of the Late Weichselian ice sheet, with an average surface gradient of c . 9.5 m/km. The surface exposure dates from Andøya pre-date the LGM, suggesting that the LGM ice sheet did not reach mountain plateaux at northwest Andøya. The results thus support evidence from lake sediment records that the northern tip of Andøya was not covered by the Scandinavian Ice Sheet during the LGM.