Petrologic Evolution of Large Failed Rifts in the Eastern Pacific: Petrology of Volcanic and Plutonic Rocks from the Mathematician Ridge Area and the Guadalupe Trough

The Mathematician failed rift was a fast spreading oceanic ridgeprior to abandonment about 6.5 Ma. but presently has the morphologyof a slow spreading ridge crest. Dredges of its inner rift valleywalls indicate that it also has the petrologic characteristicsof a slow-spreading ridge: the dredges recovered serpentinizedperidotite, cumulate and isotropic gabbro, diabase and mid-oceanridge basalt (MORB) lava. The gabbros include fresh troctolites,cpx gabbros and two pyroxene gabbros, all of which have relativelyiron-rich mafic minerals and sodic plagioclase. Other gabbrosamples are sheared and metamorphosed and presently have lowergreenschist to lower amphibolite mineral assemblages. Peak metamorphictemperatures calculated from coexisting amphibole and plagioclaseare about 600 ?C at 1-2 kb pressure but many of the assemblagesreflect much lower temperatures. The MORB lavas from the riftvalley walls are relatively fresh and very primitive (100 ?Mg/(Mg?Fe^2?) = 72–74). This suggests that they were eruptedin the last stages of rift abandonment after the disappearanceof the shallow crustal magma chamber that may have been presentwhen the rift was a fast-spreading one. One dredge of the Guadalupefailed rift also recovered MORB lava, but it is more evolvedthan that at the Mathematicians. Dredges of failed transformfaults in the Mathematicians recovered fresh alkali basalt.This post-abandonment alkali basalt may be analogous to alkaliclavas erupted off-ridge near active spreading ridges like theEast Pacific Rise. Dredges of an E-W bathymetric linear featurenear the Mathematician ridge south of the Clarion fracture zonealso recovered very fresh alkalic lavas. These are distinctfrom the post-abandonment alkalic rocks in the Mathematiciansand include hawaiites and trachytes, some of which were ‘popping’when first brought to the surface. We conclude that in somecases of fast-spreading rift abandonment, the rift first slowsdown and ceases to have a steady state magma chamber in thecrust. This is followed by the eruption of alkali basalts atfailed transforms as the lithosphere thickens in place.     

Erosion and deposition of fine-grained sediments from the Bay of Fundy

Simulations of the erosion, transport and deposition of fine-grained sediment, such as that of Greenberg & Amos and the Hydraulics Research Station, have illustrated a general lack of reliable field data. Consequently, some standard equations and constants used in modelling the sedimentation character of fine-grained cohesive sediment were evaluated based on data from two field studies and a flume experiment with undisturbed sediment from the Bay of Fundy. Initial results showed that the resistance to erosion of intertidal fine-grained sediment is controlled largely by the degree of subaerial exposure and the consequent dehydration and compaction. The sediment shear strength was high (4 kPa), but generally decreased seawards across the intertidal zone. The resistance of intertidal mud to erosion can be 80 times greater than sub-tidal counterparts. The rate of sediment erosion varied as a complex function of the applied bottom shear stress. At stresses immediately above the critical, the erosion rate decreased asymptotically with time. At higher excess stresses, the erosion rate was linear with respect to time. Thus sediment erosion cannot be represented by a single coefficient. The Krone method of computing sedimentation rates of suspended material was shown, by comparisons with direct measurement, to overpredict by 29%. All variables used in his method were measured in the evaluation with the exception of the critical deposition stress (τ_d). The closest comparisons were obtained when τ_d was assigned a value of 0.1 N m^?2 following Creutzberg & Postma. The in situ still-water particle settling rate (V_o) was constant with respect to time (2.1 × 10^?3 m s^?1). However, the settling tube measures of settling rate, compared to in situ results, underpredicted particle settling by an order of magnitude (2.7 × 10^?4 m s^?1). The reason for this discrepancy is not apparent from our results.     

Good, bad or 'necessary evil'? Reinterpreting the colonial burning experiments in the savanna landscapes of West Africa

A simple ecological model underlies contemporary fire policy in many West African countries. The model holds that the timing (or seasonality) of annual savanna fires is a principal determinant of vegetation cover. The model's origin can be traced to the ideas held by influential colonial scientists who viewed anthropogenic fire as a prime force of regional environmental degradation. The main evidence in support of the model derives from the results of a series of long-term burning experiments carried out during last century. The experimental results have been repeatedly mapped onto fire policy often taking the form of a three-tiered model in which fire exclusion is considered the ultimate management objective, late dry-season fire is discouraged and early dry-season fire is allowed but only under specific, often state-controlled circumstances. This paper provides a critique of contemporary fire policy in the region and the fire ecology model on which it is based. Through an analysis of burn scars for the 2002–3 fire season generated from ETM+ imagery, the study documents the spatiotemporal pattern of burning for an area in southern Mali. It argues that current policy, which is informed by an a-spatial model, cannot adequately account for the critical pattern of burning that is characteristic of the region. A reinterpretation of the burning experiments is presented in light of four factors: empirical data; recent developments in patch-mosaic theory; historical evidence on the effects of fire suppression; and data on indigenous burning strategies, all of which suggest a need to reconsider current fire policy.     

Separated by common ground? Bringing (post)development and (post)colonialism together

The relationship between mainstream development policy (and perhaps also development studies) and postcolonial theorists has often been characterized as a dialogue of the deaf. Rather like in the old 'debates' between adherents of modernization and neo-Marxist theories, the protagonists are often thought to be talking at or past one another, rather than with each other. This paper reassesses some firmly held views on both sides of the schism. On the one hand, many official development agencies appear to promote business as usual (often quite literally, as a recent War on Want report attests in the case of the UK's DFID using its aid budget to promote profitable opportunities for British corporations). On the other hand, some postcolonial purists rely on surprisingly modernist, totalizing discursive techniques while claiming post-structural credentials, or baulk at the prospects of practical engagement. Discrepancies between theory, discourse, policy and practice are not the preserve of one side. However, the middle ground is firmer and better trodden than most believe. Considerable progress has been made and the paper assesses examples of productive engagement and concludes with suggestions for carrying forward the challenges.     

Lithofacies and origin of late Quaternary mass transport deposits in submarine canyons, central Scotian Slope, Canada

Mass transport deposits, up to 3·9 m thick, have been identified from piston cores collected from canyon floors and inter-canyon ridges on the central Scotian Slope. These deposits are characterized by four distinct mass-transport facies – folded mud, dipping stratified mud, various types of mud-clast conglomerate, and diamicton. Commonly, the folded and stratified mud facies are overlain by mud-clast conglomerate, followed by diamicton and then by turbidity current deposits of well-sorted sand. Stratified and folded mud facies were sourced from canyon walls. Overconsolidation in clasts in some mud-clast conglomerates indicates that the source sediment was buried 12–33 m, much deeper than the present cored depth, implying a source in canyon heads and canyon walls. The known stratigraphic framework for the region and new radiocarbon dating suggests that there were four or five episodes of sediment failure within the past 17 ka, most of which are found in more than one canyon system. The most likely mechanism for triggering occasional, synchronous failures in separate canyons is seismic ground shaking. The facies sequence is interpreted as resulting from local slides being overlain by mud-clast conglomerate deposits derived from failures farther upslope and finally by coarser-grained deposits resulting from retrogressive failure re-mobilizing upper slope sediments to form debrisflows and turbidity currents.     

HYDROLOGY AND GEOMORPHIC EFFECTS OF A HIGH-MAGNITUDE FLOOD IN AN ALPINE RIVER

The catchment of the River Partnach, a torrent situated in a glacial valley in the Northern Calcareous Alps of Bavaria/Germany, was affected by a high‐magnitude flood on 22/23 August 2005 with a peak discharge of more than 16 m³s^‐1 at the spring and about 50 m³s^‐1 at the catchment outlet. This flood was caused by a long period of intense rainfall with a maximum intensity of 230 mm per day. During this event, a landslide dam, which previously held a small lake, failed. The flood wave originating from the dam breach transported a large volume of sediment (more than 50 000 m³) derived from bank erosion and the massive undercutting of a talus cone. This caused a fundamental transformation of the downstream channel system including the redistribution of large woody debris and channel switching. Using terrestrial survey and aerial photography, erosional and depositional consequences of the event were mapped, pre‐ and post‐event surfaces were compared and the sediment budget of the event calculated for ten consecutive channel reaches downstream of the former lake. According to the calculations more than 100 000 tonnes of sediment were eroded, 75% of which was redeposited within the channel and the proximal floodplain. A previous large flood which occurred a few weeks prior to the August 2005 event had a significant effect on controlling the impact of this event.