Adakitic lavas in the Central Luzon back‐arc region,Philippines: lower crust partial melting products?

Abstract Volcanism in the back-arc side region of Central Luzon, Philippines, with respect to the Manila Trench is characterized by fewer and smaller volume volcanic centers compared to the adjacent forearc side-main volcanic arc igneous rocks. The back-arc side volcanic rocks which include basalts, basaltic andesites, andesites and dacites also contain more hydrous minerals (ie, hornblende and biotite). Adakite-like geochemical characteristics of these back-arc lavas, including elevated Sr, depleted heavy rare earth elements and high Sr/Y ratios, are unlikely to have formed by slab melting, be related to incipient subduction, slab window magmatism or plagioclase accumulation. Field and geochemical evidence show that these adakitic lavas were most probably formed by the partial melting of a garnet-bearing amphibolitic lower crust. Adakitic lavas are not necessarily arc–trench gap region slab melts.     

Gravity variations along the Southeast Bohol Ophiolite Complex (SEBOC), Central Philippines: Implications on Ophiolite Emplacement

Abstract The basement complex of Bohol Island consists of the Southeast Bohol Ophiolite Complex (SEBOC), Cansiwang Melange and Alicia Schist. The SEBOC is a complete, but dismembered ophiolite with outcrops generally trending northeast– southwest and dipping north-west. The harzburgite units of the SEBOC are almost always observed to be thrusted onto the Cansiwang Melange, which in turn is thrusted onto the Alicia Schist. Bouguer gravity values on Bohol range from about +60 mGal in the west to +120 mGal in the east, in the region to the north-east of the SEBOC outcrops. Based on the present distribution of the SEBOC units and their thrust fault relationship with the Cansiwang Melange and Alicia Schist, it is proposed that the SEBOC was emplaced by onramping towards the south-eastward direction. However, the orientation of the Bouguer highs suggests that the thrusting direction of the ophiolite units is towards the south-west and not towards the south-east.     

Complex origin for the south-western Zamboanga metamorphic basement complex, Western Mindanao, Philippines

Abstract Amphibolites unconformably overlain by a metasedimentary sequence of quartz-muscovite-feldspar-kyanite schists, metagraywackes and epidote-bearing amphibolites occur in the northern portion of the south-western Zamboanga metamorphic basement complex, western Mindanao. These amphibolites (here identified as the Mount Dansalan amphibolites) display relict magmatic textures inherited from cumulate gabbro protoliths. Bulk-rock major and trace-element data are consistent with this hypothesis. Together with the chemistry of relict igneous clinopyroxenes, they indicate a magmatic arc-related signature for the gabbro protoliths. Geochemical data allow us to identify various sources for the associated metasediments: the gabbro themselves for the metagraywackes and a continental basement for the quartz-muscovite-feldspar-kyanite schists. Both sources contributed to the genesis of the epidote-amphibolite metasediments. The compositions of the metamorphic mineral assemblages suggest that the rocks have undergone metamorphism at temperatures ranging from 550°C to 700°C and pressures probably in the range of 5–9 kbar. ⁴⁰K–⁴⁰Ar isotopic study of amphibole separates from the Mount Dansalan samples document a metamorphic event dated at 24.6 ± 1.4, 22.2 ± 1.4 and 21.2 ± 1.2 Ma. Our results are in agreement with plate tectonic models which describe the south-western Zamboanga metamorphic basement as a continental terrane. However, its evolution was not as simple as it was usually considered. In particular the basement incorporated slivers of magmatic arc crust, which cannot be unambiguously related to any of the Tertiary arcs documented in the area.     

Historical Constraints on Slope Movement Age: a Case Study at Broadway, United Kingdom

The Broadway area of the Cotswolds has been extensively investigated in an attempt to delineate areas of slope instability, including'relict'landslides, which may have been initiated under periglacial climatic conditions. It is, therefore, useful both in terms of the geomorphology and of the understanding of the evolution of the area, to investigate the rates and timing of any reactivation and subsequent movements of landslides in this area. The remains of ridge and furrow cultivation can be seen extensively throughout the Cotswolds. This particular agricultural practice, which documentary evidence suggests dates back to Anglo-Saxon times (approximately 10th century AD), has been disturbed in many places by slope movements. A detailed study of Parish Records and other local his-torical sources has revealed that ridge and furrow cultivation ceased on Parliamentary Enclosure, which in this area, occurred in 1771. Therefore, it is possible to identify patterms of slope movement between these dates. Landslides have been identified by ground mapping and aerial photography, and can be divided into three categories. Firstly, active landslides which are those which have shown evidence of movement during the study period. Secondly, suspended landslides, which show evidence of movement, but have not been observed to move during the study period. In this context, this has been taken to mean landslides, which have shown evidence of movement since 1771. Thirdly, relict landslides which have shown no evidence of movement since 1771 and thus were probably active under a different climatic regime. By incorporating historical data with the geomorphological survey it has been possible to identify areas of potentially difficult ground for engineering geomorphological purposes.     

Availability of suitable land-fast ice and predation as factors limiting ringed seal populations, Phoca hispida, in Svalbard

We examined the stability of fast ice areas in western and northern Spitsbergen, the area north of Nordaustlandet, the bays and sounds of Hinlopen Stretet and the large area in the northern part of Storfjorden. NOAA satellite imagery from 1974 and 1988 and NOAA (AVHRR) imagery from 1980-87 were used to determine the dates of freeze-up and break-up. The number of days of fast ice present before the nominal birth date of ringed seal pups were computed for all major bays and fjords. Ice thickness was then computed from these data. Known prime breeding habitat in Svalbard is found in areas near glacier fronts in protected fjords and bays, where densities of birth lairs are 5.46 km⁻², corresponding to a ringed seal female density of 2.6 km⁻². Most of the ringed seal breeding habitat in Svalbard, however, consists of flat fjord ice where snow accumulation is rarely deep enough to permit birth lair construction. In these areas pups are often born in the open. Based on breathing hole densities, the density of adult females in the flat ice areas in the breeding period was estimated to 0.98 km⁻². A preliminary estimate is that approximately 19,500 pups could be born annually in the fast ice of Svalbard. Annual recruitment could be quite variable given the unpredictable nature of the fast ice areas and the high predation mortality on newborn pups. Discrepancies between our calculated ringed seal production and numbers of seals required to feed the large polar bear population in the area signal cause for management concern.     

Basin architecture and thermal maturation in the strike-slip Deer Lake Basin, Carboniferous of Newfoundland

Abstract The Deer Lake Basin is an entirely non-marine basin associated with the Cabot fault zone. Structural and stratigraphic evidence strongly suggest dextral strike-slip movements along the fault zone during Tournaisian-Visean time. Two elongated, end-on structural blocks (probable positive flower structures) contain fold axes and second-order faults oriented obliquely to fault traces bounding the blocks, in a manner implying dextral movements. In one part of the basin, the stratigraphic thickness of a long homoclinal section of later basin-fill sediment (Deer Lake Group) greatly exceeds the suggested depth to basement based on gravity measurements, a situation common to strike-slip basins. Formations representing basin fill can be arranged into megasequences (from oldest to youngest: Anguille Group, Wetstone Point and Wigwam Brook Formations, Deer Lake Group, Howley Formation) corresponding to lateral growth stages of the basin. Gravity, magnetic, and seismic data show that depths to basement on either side of the end-on flower structures are comparable, so that the youngest strata in the basin (Howley Formation) are not underlain by earlier basin fill. These geophysical data, therefore, corroborate the geological conclusion of onlapping stratigraphic relations. The geophysical data suggest participation of basement in Carboniferous gravity faulting and show the location of the subsurface extension of the Taylors Brook Fault in the western part of the Deer Lake Basin. Thermal maturation of the Anguille and Deer Lake Groups, as measured by vitrinite reflectance, clay mineral assemblages, illite crystallinity, and Rock-Eval pyrolysis, indicate a much higher level of maturation for the Anguille than for the Deer Lake Group. Palaeotemperatures for the Anguille and Deer Lake Groups are estimated to be around 200 and 100^oC, respectively, suggesting that Anguille Group rocks are overmature whereas Deer Lake Group strata are within the oil-generating window. Onlapping stratigraphic relations and areally homogeneous time/temperature effects, however, have created a situation in which the Deer Lake Group and Howley Formation have similar maturation levels.     

Decoupling as an alternate model for transpression during the initial opening of the Norwegian-Greenland Sea

Transpressive plate motions during the opening of the Norwegian-Greenland Sea were in some manner responsible for the development of Spitsbergen's Tertiary fold-and-thrust belt. A flower model has been proposed for the large-scale structural architecture of Tertiary deformation (Lowell 1972). An alternate model of decoupling, where convergent and transcurrent motions were accommodated in totally or partially separated, subparallel belts is suggested.