strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

extreme effusive eruptions palaeoflow data on an extensive felsic lava in the mesoproterozoic gawler range volcanics the gawler range volcanics are the eroded remnants of a mesoproterozoic dominantly felsic large igneous province the eucarro rhyolite is one of three voluminous 675 km3 units in the upper part of the succession eight stratigraphic sections through the eucarro rhyolite were sampled for anisotropy of magnetic susceptibility ams and petrofabric analysis in order to determine the source location the ams foliations kmax kint plane consistently strike northwest and most dip steeply at sites in the western and central parts of the unit there is a change from steep southwesterly dips near the base to vertical or northeasterly dips in the middle and near the top in the same area kmax axes plunge to the south or southwest in the lower parts and to the north in upper parts of the unit at sites in the eastern part of the unit the pattern is reversed ams foliations dip northerly near the base and southerly near the top and kmax axes plunge northerly near the base and southerly near the top such up section reversals in dip or plunge directions of ams parameters resemble the patterns shown by other lavas a dominance of very steep dips and an up section reversal in dip direction are also expected for flow bands in felsic lavas furthermore the ams pattern implies a northeasterly flow direction for the western and central parts of the unit and a southerly flow direction for the eastern part the opposing flow directions may indicate that the western and central parts of the unit versus the eastern part belong to different flow lobes or that they came from separate sources lineations defined by preferred alignment of elongate phenocryst intersections on gently dipping surfaces show a less consistent pattern and alone could not be used to define palaeoflow directions this study confirms the existence of felsic lavas that have dimensions comparable with the largest known mafic lavas

pliocene pleistocene climate change sea level and uplift history recorded by the horingbaai fan delta nw namibia its location on a tectonically relatively stable passive margin and its degree of interaction with the sea make the horingbaai fan delta nw namibia an exceptional record of coastal activity providing insights into the response of ephemeral fluvial systems to changes in climate sea level and continent scale uplift the fan comprises upper middle and lower segments the upper fan and middle fan are dominated by a braided river system only in the upper fan are fluvial sediments interleaved with hyperconcentrated flow deposits plio pleistocene sea level highstands have left marine terraces on the fan surface which enable correlation with the offshore marine record and provide timelines to constrain fan growth the bulk of fan delta progradation took place at 2 7 2 4 ma between the formation of a widespread erosional surface incising the middle pliocene karpfenkliff and kamberg formations but prior to the emplacement of the warm water fauna bearing late pliocene oyster terrace 2 4 2 2 ma an equivalent of the 30 mp marine terrace package in coastal southwestern africa major fan progradation is contemporaneous with widespread regional uplift 12 5 m ma and climate change in southwestern africa the latter associated with intensification of northern hemisphere glaciations younger fan growth phases are weaker and constrained by 10 m asl marine terrace bodies that yield mostly cold water fauna corresponding to the onset of strong glacial interglacial climatic fluctuations superimposed on a general aridification trend and the introduction of colder sea surface temperatures after 2 2 ma

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

post breakup evolution and palaeotopography of the north namibian margin during the meso cenozoic passive margins display various structural and morphological patterns resulting in a complex coupling between local effects we performed an integrated analysis from the ocean to the continent at the crustal scale of the post breakup evolution of the northern namibian margin in terms of vertical displacements and palaeotopography a main topographic scarp that disappears where the palaeo proterozoic damara belt crosses the coast separates the namibian margin and the south african plateau we analysed this transition using a synthetic crustal cross section based on a compilation of published data as well as new field data the thinned margin displays two domains separated by a deep sedimentary trough this trough separates a proximal part with landward tilted blocks from the distal part with seaward tilted blocks this structural pattern reveals a complex flowing pattern of the ductile crust controlled by magmatism onshore the basalts and cenozoic conglomerates recorded a progressive seaward tilting of the coastal zone post breakup vertical motions were estimated with subsidence curves in the offshore domain and from isostatic adjustments induced by the variations of cover thickness depth of magmatic intrusions and apatite fission tracks in onshore domain three domains are recognised a subsiding offshore domain a uplifting continental domain and a transitional domain located on both sides of the current shoreline with successive phases of uplift and subsidence the offshore subsidence displays variations along the margin controlled by the thinning crust generated by the continental breakup the onshore palaeotopogaphy is approximately assessed since the end of rifting by integrating the isostatic adjustment induced by denudation the mean elevation of the plateau has remained above 1000 m since continental breakup no significant additional uplift was pointed out during the late cretaceous or late neogene therefore we conclude that the south african plateau has been present in northern namibia since 130 ma at least

a monolith sampler for saltmarsh sediments a method is described for the collection of unconsolidated salt marsh sediment resulting in no compaction and minimal disturbance to the sample the monolith sampler consists of an acrylic casing of optional length with a wide flat base and short right angled sides that is inserted into a cleaned vertical face use of this inexpensive technique allows observation of any structures present through the sediment record and provides sufficient material for various high resolution geochemical and sedimentological analyses

mantle plumes and flood basalt stratigraphy in the parana south america geochemical studies of the parana continental flood basalts in brazil have led to the recognition of distinct magma types which have been used to infer the internal stratigraphy of the lava pile the overstepping of stratigraphic units toward the north and a similar compositional change in sills within the underlying parana basin sedimentary rocks imply that the site of magnatism migrated 750 km toward the north during this volcanic event this has important implications for recent plume related models that have argued whether fithospheric rifting must accompany the presence of a mantle plume in order to generate continental flood basalt magmatism it is difficult to ascribe the magnitude and direction of the shift in the locus of the parana magmatism to movement of the brazilian lithosphere relative to the underlying tristan mantle plume instead it is suggested that the observed lava distribution and internal structure of the parana etendeka flood basalt province were imposed by the intracacies of the rifting process during the initial opening of the south atlantic ocean

the etendeka igneous province magma types and their stratigraphic distribution with implications for the evolution of the parana etendeka flood basalt province detailed geochemical and field data for the volcanic sequence and intrusions of the etendeka igneous province are used to construct a stratigraphic framework for petrogenetic interpretation of the evolution of the etendeka parana continental flood volcanic event geochemical and petrographic characterization of over 1 000 analyzed samples allows 8 mafic and 17 silicic magma types to be recognized both silicic and mafic types can be grouped into high ti and low ti suites on the basis of elevated ti relative to other elements the mafic magmas are khumib high ti tafelberg kuidas horingbaai huab tafelkop albin and esmeralda all low ti amongst the silicic types the goboboseb springbok wereldsend grootberg and beacon low ti quartz latites and the nil desperandum high ti latite have been described previously in addition the hoas low ti nadas sechomib and hoarusib all high ti latites and the fria low ti sarusas ventura khoraseb naude and elliott all high ti quartz latites are described for the first time here there is a marked provinciality in the distribution of the high and low ti suites with the former concentrated in the northern etendeka region and the latter dominant in the southern etendeka stratigraphic distribution of magma types allows two new formations to be defined in the northern etendeka the khumib formation of basaltic flows and the skeleton coast formation dominated by silicic sheets the geochemical provinciality hampers precise correlations between northern and southern etendeka available evidence suggests that the lower part of the awahab formation in the southern etendeka is coeval with the lower part of the khumib formation and that the silicic units in the upper part of the tafelberg formation probably correlate with the skeleton coast formation the paucity of khumib dykes in relation to tafelberg dykes and their field relationships with regard to the volcanic sequence in the northern etendeka suggests that the main khumib eruptive centers lay further north consistent with southward thinning of the khumib basalts in the southern etendeka the doros complex is the eruptive center of the plume derived tafelkop basalts which probably built a shield volcano within the regional flood lava field this work indicates that the etendeka has a greater variety of both silicic and low ti mafic magma types than are known from the se parana detailed comparisons reveal that all the important silicic types in the parana have geochemical equivalents in the etendeka new correlations are santa maria fria ourinhos khoraseb guarapuava sarusas thus extending the previously recognized correlations between southern etendeka quartz latites and the caixas do sul and anita garibaldi rhyolites these correlations emphasize that very large volume silicic systems develop with pronounced lithospheric rifting in continental flood basalt provinces

petrology of the alkaline core of the messum igneous complex namibia evidence for the progressively decreasing effect of crustal contamination the messum complex of nw namibia a part of the parana introduction etendeka volcanic province consists of a dominantly felsic central the etendeka igneous province in the damaraland core surrounded by older gabbros the igneous rocks of the core region of northwest namibia fig 1 contains numerous can be divided in order of decreasing age into 1 a sub alkaline anorogenic igneous complexes of mesozoic age martin suite 2 an outer quartz syenite suite and 3 an inner silicaet al 1960 piranjo 1994 one of which is the messum undersaturated suite dominated by nepheline syenite compositional complex the range in chemical composition both within differences within the quartz syenite suite can be explained by and between the various complexes is remarkable and fractional crystallization but srand o isotope data indicate that includes both silica oversaturated and silica underthese rocks contain a significant crustal component the younger saturated mafic and felsic rocks including carbonatite central nepheline syenites trend towards the 0 1 gpa phonolite messum was classified as a mixed complex by harris minimum and appear to have evolved by closed system fractional 1995 as it contains substantial amounts of both silicacrystallization of a mantle derived magma whereas earlier nepheline undersaturated and silica oversaturated rock types syenites appear to be contaminated with quartz syenite the dating of several damaraland complexes yields ages decreasing effects of crustal contamination with time may reflect that lie between 137 and 124 ma siedner miller progressive armouring of the magmatic plumbing system by less 1968 milner et al 1995b renne et al 1996 the contaminated material basanite dykes that intrude the plutonic alignment of the complexes along two parallel ne sw rock of the core show trace element and ndand sr isotope trends suggests that they represent a hotspot trace caused characteristics that are similar to those of basanites from tristan by the motion of the african plate over a fixed mantle da cunha this suggests that the mantle derived component of the plume e g rhodes 1971 because there is no systematic age progression along the lineament this view has genmessum core rocks came from the tristan plume erally been discounted marsh 1973 recent ideas that large continental flood basalt cfb provinces such as the parana etendeka province are manifestations of mantle plume activity white mackenzie 1989 has refocused attention on the relationship between plumes and linear arrays of complexes in cfb provinces milner

the circular uneged uul structure east gobi basin mongolia geomorphic and structural evidence for meteorite impact into an unconsolidated coarse clastic target abstract the uneged uul structure is a 10 km circular complex multi ridged domal feature in the unegt subbasin of the east gobi basin southeastern mongolia as revealed by remote sensing and recent field reconnaissance the central part of the uneged uul structure comprises a complex central peak of outward radiating curved ridges composed of stratigraphically uplifted greenschist facies basement schists surrounded by an annular moat the most prominent feature of the structure is a central annular ridge 3 km in diameter composed of pebble boulder conglomerates and gravels of the upper jurassic sharilyn formation surrounded by three outer domal ridges composed of lower cretaceous conglomeratic sandstones and gypsum clays jurassic conglomerates forming the main part of the central annular ridge show effects of severe internal deformation the original population of pebbles cobbles and boulders appears moderately displaced and mostly broken but nowhere aligned along shear planes or foliated primary sedimentary features such as cross lamination or imbrication have been obliterated we explain this penetrative brecciation as a result of dissipative shearing caused by a strong and rapid singular event that in magnitude was beyond the range of the common crustal tectonics recorded elsewhere in this region disrupted and chaotically distributed conglomeratic sandstone beds in the central annular ridge dip in highly variable directions on a local scale but show an apparent se nw trend of bedding plane alignment further outside the tilted and uplifted upper jurassic to lower cretaceous strata of the domal area are overlain by the flat lying upper cretaceous which stratigraphically constrains the timing of deformation at the uneged uul structure to most likely the early cretaceous endogenic formation models such as magmatism and salt gypsum or mud diapirism fail to explain the nature of the uneged uul structure the uneged uul structure bears a set of geomorphic and structural features resembling those at some eroded complex impact structures on earth morphologically similar central peaks are observed at the spider and matt wilson impact structures in australia the central annular ridge reminds of that at gosses bluff in australia the outer domal ridges might correspond to ring like features as known from tin bider in algeria we therefore cautiously propose that an impact may have produced the uneged uul feature causing structural uplift 1000 m of basement rocks at its center so far no convincing evidence for shock metamorphism could be proven by field work and petrographic analyses however it is likely that at the time of the deformation event the unconsolidated conglomerates were highly porous and possibly immersed in groundwater buffering the propagation of sudden stress reducing deformation further studies will be in order to unravel the nature of the uneged uul structure which should be considered a promising possible impact structure

long duration 4 ma and steady state volcanic activity in the early cretaceous parana etendeka large igneous province new palaeomagnetic data from namibia there is long standing correlation between large igneous provinces lips and major mass extinction events in the geological record postulated to be due to the emission of large quantities of volcanic gases over a geologically short period of time causing major climatic perturbations within the earth system the 135 ma parana etendeka volcanic province of brazil and namibia represents something of an enigma amongst lips despite an erupted volume 1 mkm3 comparable to other lips associated with mass extinctions such as the siberian or deccan traps it is not linked to a known mass extinction event this suggests that the parana etendeka volcanic province was emplaced over longer timescales than other lips and or emitted a lower concentration of volatiles directly or indirectly during its emplacement we present a new detailed magnetostratigraphy for the etendeka portion of the province that suggests emplacement took place over longer timescales 4 ma than those associated with other lips palaeomagnetic analysis of 893 specimens from 99 sites in sections that encompass nearly the complete etendeka stratigraphy yielded high quality data from 70 sites 612 specimens these record 16 individual polarity intervals which can be correlated with chrons 15 to 11 of the geomagnetic polarity time scale gpts while also providing two new high quality palaeopoles for south africa at 130 135 ma our magnetostratigraphy reveals a minimum period of volcanic activity in excess of 4 myrs and importantly we find no evidence for major changes in the rates of volcanic activity through that time period in contrast to other lips where volcanism seems to be concentrated in major pulses this suggests that the anomalously feeble environmental impact of parana etendeka volcanism may be due to lower effusion rates reducing the atmospheric loading due to volcanogenic volatiles

geophysical images and a crustal model of intrusive structures beneath the messum ring complex namibia the early cretaceous messum complex in namibia is an excellent example of the type of composite mafic and alkaline igneous ring complexes associated with continental rifts worldwide this study integrates reflection seismic gravity and aeromagnetic data from a profile across messum and reveals a heavily intruded root zone extending vertically to moho depths 40 km the crustal section to either side of the messum complex is characterized by a seismically transparent upper crust to 20 km depth and a laminar reflective lower crust below the crust below the messum complex is seismically transparent at all depths but it shows a combination of high seismic reflection strength and weak coherency relative to the unintruded crust and p velocities about 0 2 0 3 km s higher at all depths the seismic attributes are best explained by a network of mafic intrusions and this is supported by steep sided positive magnetic and gravity anomalies centered on the complex modelling indicates that about 30 gabbroic material in the crustal column can explain the velocity and potential field data an increase in reflective strength below 20 km suggests the degree of intrusion is greater in the lower crust but the difference is not resolved in the velocity or gravity models the new geometric and compositional constraints on the crust below the messum complex contradict the hypothesis that messum was the sole eruptive center for an estimated 8600 km3 of crustally derived rhyodacitic magmas that now form part of the regional parana etendeka volcanic sequences to our knowledge this is the first study to provide a seismic image of the whole crust below a typical igneous ring complex and our conclusion that the cross sectional area of the complex is the same in the lower as in the upper crust could not have been predicted from surface studies the data resolution is insufficient to define intrusion geometry in detail but the cylindrical 3 d shape throughout the crust and the lack of major breaks in the vertical distribution of intrusions indicate depth independent controls on the nature of the intruded zone the suggestion is that the magma transport and emplacement took place in a brittle fashion at all levels of the crust in response to conditions of high fluid magma pressure and a high magma flux

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

variation of schmidt hammer values with imbrication direction in clastic sedimentary rocks abstract in this study schmidt hammer rebound hr values of clast supported conglomerates matrix supported conglomerates and normally graded gravelly sandstones were measured to investigate the effect of such anisotropy on strength the highest schmidt hr values were obtained from the gravelly sandstones with an average value of 39 decreasing with gravel content in clast supported conglomerates the average hr values were 34 and 38 parallel and perpendicular to the palaeocurrent direction respectively similar measurements were also made over the matrix supported conglomerates average hr values of matrix supported conglomerates were determined as 37 and 41 parallel and perpendicular to the palaeocurrent direction respectively in the clast supported conglomerates average hr value on perpendicular palaeocurrent side was 12 higher than those parallel to palaeocurrent this ratio was 9 for matrix supported conglomerates preferred orientation or clast imbrication creates more gravelly and less matrix covered area parallel to the palaeocurrent than the other directions this visible difference in the gravel matrix ratio is considered to be the basis of the strength anisotropy in clastic sedimentary rocks this study demonstrates the potential of the schmidt hammer for quantifying the degree of anisotropy in such rocks

the effect of composition and strain on quartz fabric intensity in pebbles from a deformed conglomerate twenty eight quartz c axis fabrics from individual pebbles from a single outcrop of deformed conglomerate have been measured these pebbles vary in composition and have undergone different amounts of strain contrary to expectations based on published results of computer simulated deformation by intracrystalline slip of pure quartzites it is observed that fabrics are not stronger in the more highly deformed pebbles instead the fabric strength is related to the quartz content of the pebbles pure quartzites show the strongest fabrics as the percentage of non quartz minerals increases up to 25 the strength of the fabrics rapidly decreases though the influence of composition becomes less marked when impurities exceed 25 these observations can be explained if the deformation of the impure quartzites included a contribution from a non intracrystalline mechanism such as grain boundary sliding

tectonically controlled piecemeal caldera collapse a case study of glencoe volcano scotland caldera collapse at glencoe scotland was incremental and involved complex movements of numerous fault blocks before formation of the ring fault for which the volcano is renowned intracaldera depocenters had the form of grabens or half grabens locally with downsag and bounding monoclinal flexures and these changed form and location for each of the five major ignimbrite eruptions that represent the early volcanic history this piecemeal caldera collapse is in contrast to previous interpretations in which all subsidence was thought to have involved a coherent crustal block moving on the ring fault collapse and magmatic plumbing were profoundly influenced by preexisting tectonic faults trending northwest and northeast a dominant northwest trending graben controlled the general location and form of major caldera depocenters and repeatedly channeled a major river through glencoe the main graben was transected orthogonally by two cross grabens each of the first three caldera eruption cycles involved initial phreatomagmatic explosivity which built tuff cones followed by magmatic fountaining that produced lava like silicic ignimbrites the three lava like ignimbrites total more than 300 m thick two later eruptions produced eutaxitic silicic ignimbrites together more than 300 m thick and contemporaneous progressive downsag was associated with the formation of extensional fractures crevasses hundreds of meters deep sills up to 100 m thick of mingled andesite and rhyolite between the intracaldera ignimbrites were accommodated by increments of subsidence unconformities and sedimentary layers within the stratigraphic succession record fluvial erosion and abrupt switches to alluvial and or lacustrine deposition between each ignimbrite eruption the changes in drainage and sedimentation and the development of coarse debris avalanche breccias reflect tectonic faulting during the periods between major eruptions the duration of the period including the five caldera forming eruptions was probably 0 5 m y and the magnitude of the tectonic faulting that occurred prior to and during the caldera developments 0 5 km m y normal displacement is similar to that of the most actively subsiding sedimentary basins glencoe shows that the piecemeal nature of calderas may be plainly evident only in deeply dissected systems which with knowledge that caldera volcanoes commonly overlie faults suggests that piecemeal calderas may be more common than previously recognized the research also shows that the emplacement of granites s l was episodic rather than synchronous throughout the magmatic province and it suggests that numerous plutons in the province e g in donegal ireland may have formed at sites of central volcanoes that are no longer preserved

strain discontinuities within the seve koli nappe complex scandinavian caledonides this paper utilizes the results of a regional survey of the state of finite strain carried out in an area built up of stacked thrust sheets to draw conclusions about the relative timing of penetrative deformation and thrust motion and to place constraints on the deformation models devised to explain the geometry of the individual thrust sheets a strain survey within the seve koli nappe complex reveals abrupt changes which represent discontinuities in the strain pattern one of these discontinuities coincides with the boundary between the seve and koli nappes and separates intense transversal stretching strain within the seve nappe from the longitudinal strains of lower intensity within the koli nappe the weight of geological evidence favours the interpretation of this strain discontinuity as a unconformity with significant deformation of the seve nappe occurring before rocks of the overlying koli nappe were deposited middle ordovician a second discontinuity marks the junction between the krutfjell nappe and the underlying lower grade koli rocks the strain distribution is incompatible with a previous interpretation of the discontinuous krutfjell nappe as a string of mega boudins an interpretation which implies considerable post thrust flattening of the nappe pile the strain results and other evidence favour earlier deformation of the krutfjell nappe before these rocks were thrust over the underlying tectonic units the lens shaped outcrop configuration of the krutfjell nappe is explicable in some cases if the geometry of this nappe is a westerly thinning wedge within the low grade koli nappe rocks north of the grong culmination the longitudinal stretching is difficult to explain in terms of easterly directed nappe movements whereas the clear transverse strains in other tectonic units appear to predate the main silurian thrusting event

quartz preferred orientations within a deformed pebble conglomerate from new hampshire u s a abstract the orientations of quartz c axes within a deformed pebble conglomerate from piermont new hampshire have been measured and compared to finite strain estimates for both the pebbles and the surrounding matrix quartz grains within the relatively pure orthoquartzite pebbles show a strong c axis preferred orientation while quartz grains within the matrix show a random distribution of c axes comparisons of the preferred orientation of the pebbles with computer simulated fabrics of lister and hobbs 1980 yield estimates of the principal finite strains i 1 00 0 00 0 50 which are consistent with previous strain estimates rumble and oertel 1979 derived from the pebble shapes 1 22 0 13 0 48 the random c axis pattern within the matrix however is inconsistent with the strain estimates derived from fabric analysis of matrix phyllosilicates using the march model while it is believed that intracrystalline slip of quartz was as prevalent in the matrix as in the pebbles the presence of phyllosilicates in the matrix appears to have prevented the development of a quartz preferred orientation it is proposed that slip and or grain boundary sliding of phyllosilicates in the matrix may have been responsible for quartz grain rotations which were random with respect to quartz c axes

phase boundary mobility in naturally deformed high grade quartzofeldspathic rocks evidence for diffusional creep abstract grain shape fabrics and optical microstructures of some quartzofeldspathic rocks deformed under upper amphibolite facies conditions in the southwestern grenville province ontario canada suggest that quartz and feldspar have accommodated intracrystalline plastic strains by both diffusional and dislocation creep in these rocks quartz and feldspar form polycrystalline domains separated by gently curved and locally cuspate phase boundaries whose morphology is similar in certain respects to the phase boundary morphology of rocks annealed experimentally under hydrostatic stress conditions in the naturally deformed rocks however phase boundary cusps consistently point along the foliation and parallel to the mineral fibre lineation i e in directions of inferred finite extension which implies that phase boundary motion and cusp formation occurred during deformation optical microstructures in feldspar and crystallographic preferred orientations in quartz are consistent with the accommodation of some intracrystalline plastic strains by dislocation creep however the morphology of quartz feldspar phase boundaries cannot be explained by either dislocation creep or static annealing alone we propose that phase boundary motion resulted from a diffusion assisted process involving dissolution at foliation parallel quartz feldspar phase boundaries mass transfer over length scales of the order of feldspar domain size 200 m or greater and precipitation at quartz feldspar phase boundary cusps this study extends the range of natural deformation conditions under which diffusional creep has been identified in quartzofeldspathic rocks it also has important implications for the natural rheological behavior of the mid and lower continental crust

quartz latite rheoignimbrite flows of the etendeka formation north western namibia the etendeka formation of north western namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the karoo igneous province in southern africa the age of the etendeka formation is approximately 130 135 ma the quartz latites make up a significant proportion of the stratigraphic succession 25 of the total stratigraphic thickness and form as much as 60 of the outcrop area in the southern etendeka apart from some systematic differences between pitchstones and devitrified quartz latite largely explained by alteration processes individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern etendeka area enabling the reconstruction of the etendeka formation stratigraphy in this region individual quartz latite units occur as voluminous 400 2600 km3 widespread up to 8800 km2 sheet like deposits typically between 40 and 300 m thick each unit consists of basal main and upper zones the main zone generally constitutes over 70 of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite in contrast the basal and upper zones of the flow are characterised by flow banding pitchstone lenses and breccia with rare occurrences of pyroclastic textures the quartz latites are sparsely porphyritic 10 phenocrysts with glassy or devitrified groundmass textures the phenocrysts consist of plagioclase pyroxene titanomagnetite and rare ilmenite pyroxene geothermometry indicates high 1000 1100 c temperatures of crystallisation which coupled with the absence or primary hydrous phases indicates that the quartz latites were relatively hot h2o undersaturated magmas the quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style the local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high temperature rheomorphic ignimbrites i e rheoignimbrites a combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the etendeka area

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

short lived magmatic activity in an anorogenic subvolcanic complex 40ar 39ar and ion microprobe u pb zircon dating of the erongo damaraland namibia abstract the erongo subvolcanic center in namibia is the largest composite bimodal complex in the mesozoic etendeka igneous province of namibia this study of 40ar 39ar and high spatial resolution u pb zircon dating demonstrates that emplacement of the various igneous units at erongo took place within a time span equivalent to or shorter than geochronologically resolvable age differences ca 2 ma and at the peak of regional flood basalt activity in the etendeka parana province the erongo complex comprises a series of felsic volcanic and intrusive units that overlie or intrude basaltic lavas attributed to the etendeka group the stratigraphically oldest rhyodacite erongorus unit yielded u pb zircon ages of 131 8 1 1 ma 1 the overlying ombu rhyodacite was previously dated at 135 0 1 6 ma and our new ages for a resurgent intrusion of the same magma type ombu granodiorite in the vent area are 132 6 1 1 ma 40ar 39ar and 132 3 1 9 ma u pb zircon respectively the youngest felsic magmas at erongo are the high silica ekuta rhyolite and compositionally equivalent erongo granite these units yielded u pb zircon ages of 131 9 2 9 and 130 3 1 4 ma respectively concordant 40ar 39ar biotite ages from two samples of the granite average 132 2 0 8 ma the final stage of magmatism at erongo involved emplacement of basic alkaline plugs and dikes phlogopite and kaersutite from foidite plugs of this series yielded 40ar 39ar ages of 130 8 1 0 and 132 0 1 0 ma in a regional context these age results indicate that silicic magmatism in the largest complexes of the damaraland erongo brandberg paresis messum began simultaneously with the peak of flood basalt effusion at about 132 ma throughout the etendeka province and ceased by about 130 ma the silicic magmas are hybrid with varying contributions of crustal and mantle derived melts and the age constraints suggest that crustal melting was caused by a short lived thermal pulse related to the main flood basalt event basic magmatism in the damaraland complexes continued sporadically thereafter to about 123 ma but lacked the power to create further crustal melting

the effect of hydrous mineral content on competitive strain localization mechanisms in felsic granulites abstract this study investigates the grain scale mechanisms that facilitated strain localization in the lower crust within the east athabasca mylonite triangle of northern saskatchewan canada there two distinct lithotectonic units the upper deck and northwestern domain are juxtaposed across a km scale shear zone that has been previously interpreted to record thrust and or normal sense displacement our study reports new quartz c axis opening angle ti in biotite and ti in quartz geothermometry and amphibole plagioclase geothermobarometry within syn kinematic phases indicating that both units were deformed and metamorphosed at upper amphibolite to granulite facies 725 c and 0 8 gpa while sharing similar conditions the dominant mechanisms of deformation across the two units are strongly linked to modal proportion of syn kinematic hydrated mineral phases detailed microstructural and crystallographic preferred orientation cpo analyses reveals that specimens poor in hydrated minerals 10 pressure solution and re precipitation processes enhanced phase mixing and promoted grain boundary sliding resulting in weakly developed or absent quartz cpo these results indicate a strong mineralogical and fluid control on deformation and therefore which strain softening process es may dominate in lower crustal shear zones

the cretaceous messum igneous complex s w etendeka namibia reinterpretation in terms of a downsag cauldron subsidence model abstract the messum igneous complex mic lies within the ene trending zone of lower cretaceous 132 ma damaraland intrusive complexes in namibia intruded into pan african damara basement it is defined by a roughly circular structure 18 km in diameter the bounding ring fault exposed along the eastern sector encircling messum are the volcanic sequences of the goboboseb mountains comprising a lower basalt series tafelkop and tafelberg types followed with intervening basalts by four voluminous quartz latite ql eruptions goboboseb and springbok ql units inferred stages of development are a an initial very broad basaltic lava shield comprising the tafelberg and tafelkop basalts and messum crater basalts mcb possibly ponded in near vent lava lakes embedded within the lower basaltic sequence is a localised rhyolite dominated eruptive centre ca 5 km in diameter interpreted as a funnel caldera located towards the centre of the mic b downsagging extending northwards from messum following the goboboseb ql eruptions 2300 km3 ponding of overlying basaltic units c climactic springbok ql eruption 6300 km3 producing further downsag together with the inward radial dip of all volcanic units towards the mic ring fault initiation d cauldron subsidence emplacement of a granitoid suite forming the mic moat area between the ring fault and the core region e intrusion of gabbroic cone sheets into incompletely solidified granitic melts within the southeastern moat resulting hybridisation and magma mingling produced extensive development of heterogeneous granitoid and hybrid dioritic lithologies f cone sheet intrusions of the eastern gabbros into more highly solidified granitoids of the southeastern moat g intrusion of thick 1 2 km western gabbro cone sheets exhibiting local fine scale layering into solidified granitoids mainly within the western moat minor late stage granitic intrusions h 2 3 ma quiescent period followed by quartz and ne syenite intrusions and finally basanite dykes emplaced within the mic core accompanying differential uplift of the core uplift resurgence within the mic has accompanied intrusion of the moat granitoids and mafic cone sheets thereby juxtaposing volcanic and intrusive sequences phases of both subsidence and uplift have characterised the mic the nw scotland tertiary central igneous complexes and messum show evidence of a number of parallel developments but also important differences the mic differs markedly from caldera systems within the western usa and circum pacific messum is therefore suggested to represent a distinct class of intrusive extrusive central complex although probably common in large igneous provinces

shearing of magma along a high grade shear zone evolution of microstructures during the transition from magmatic to solid state flow abstract syntectonic plutons may record short lived geological events related to crustal melting and deformation of the continental crust therefore interpretation of microstructure and orientation of fabrics is critical in order to constrain space time temperature deformation relationships during pluton crystallization here we describe the transition from magmatic to solid state deformation in the late variscan diorite granite suite dgs emplaced along the santa lucia shear zone the systematic collection of meso microstructural and quartz axis data allow us to examine the spatial distribution and the mode of superposition of different fabrics we identify three magmatic microfabric types thought to reflect the microstructural evolution at decreasing melt content during pluton crystallization our data suggest that diffusion creep dislocation creep and grain scale fracturing cooperated in accommodating the shearing of the partially molten quartzofeldspathic aggregate we suggest that the switch from upward to horizontal magmatic flow occurred at melt fractions gt 0 40 and that most of the hypersolidus fabrics formed during horizontal flow reflecting the stress field imposed by the shear zone and preserving no memory of the ascent stage

oxygen isotope geochemistry of the mesozoic anorogenic complexes of damaraland northwest namibia evidence for crustal contamination and its effect on silica saturation fifty six new oxygen isotope analyses of minerals separated from nine mesozoic anorogenic complexes of damaraland in northwest namibia have been used to estimate the 18o values of the original magmas magma these complexes range in composition from nepheline syenite to alkaline and peraluminous granites often with a variety of rock types present at a single centre the silica undersaturated rock types show a relatively small spread of values of magma from 6 0 to 7 0 mean 6 6 which is consistent with their derivation from the mantle with little or no subsequent crustal contamination the silica oversaturated rocks show a wide range of magma values from 4 9 to 12 0 with a mean value of 8 3 the high values of magma in the silica oversaturated rocks can only be explained with considerable involvement of the continental crust in their petrogenesis and those rocks with magma 10 are interpreted as essentially crustal melts it is generally accepted that the damaraland complexes were generated as a result of rifting across the tristan plume with the plume providing both mantle derived magma and with it the heat required for crustal melting in addition to their mantle like oxygen isotope ratios the undersaturated rocks of the damaraland complexes possess the trace element characteristics e g low zr nb ratios of ocean island basalt suggesting that their parental magmas were produced from the plume itself in contrast the oversaturated complexes generally have higher zr nb ratios that are consistent with a larger crustal input the highest values of magma in the damaraland complexes are found in granitic rocks that intrude the central zone of the pan african damara orogen where presumably there is a substantial component of sedimentary origin in the lower to middle crust

structures and lithofacies of inferred silicic conduits in the parana etendeka lip southernmost brazil abstract extensive silicic units in the parana etendeka lip have been long interpreted as pyroclastic density currents rheomorphic ignimbrites derived from the messum complex in namibia in recent literature however they have been characterized as effusive lava flows and domes in this paper we describe structures and lithofacies related to postulated silicic lava feeder conduits at mato perso sao marcos and jaquirana cambara do sul areas in southern brazil inferred conduits are at least 15 25 m in width and the lithofacies include variably vesicular monomictic welded and non welded breccias in the margins to poorly vesicular banded spherulitic and microfractured vitrophyres in the central parts flat lying coherent vitrophyres and massive obsidian are considered to be the subaerial equivalents of the conduits large scale regional tectonic structures in southern brazil include the ne sw aligned porto alegre suture leao and acotea faults besides the antas lineament a curved tectonic feature accompanying the bed of antas river south of the antas lineament smaller scale nw se lineaments limit the exposure areas of the inferred conduits ne sw and subordinate nw se structures within this smaller scale lineaments are represented by the main postulated conduit outcrops and are parallel to the dominant sub vertical banding in the widespread banded vitrophyre lithofacies upper lava flows display flat lying foliation pipe like and spherical vesicles and have better developed microlites petrographic characteristics of the silicic vitrophyres indicate that crystal poor magmas underwent distinct cooling paths for each inferred conduit area the vitrophyre chemical composition is defined by the evolution of trachydacitic dacitic vitrophyres with 62 65 wt sio 2 to rhyodacite and rhyolite with 66 68 wt sio 2 the more evolved rocks are assigned to the latest intrusive grey vitrophyre outcropping in the center of the conduits degassing pathways formed during fragmentation and fracturing episodes within the conduits may have helped to inhibit the explosivity of the eruptions based on the documented lithofacies architecture we attribute the source of the silicic lava flows in the studied localities to tectonic controlled local conduits rather than pyroclastic density currents from distant vent areas

3 d 40ar39ar geochronology in the parana continental flood basalt province new 40ar39ar analyses of borehole samples from the parana etendeka continental flood basalt province provide critical evidence of eruption rates in unexposed regions when combined with surface samples the new data clarify the duration of this major volcanic province new ages from both surface and borehole samples confirm a 10 12 million year duration of magmatism which post dates the tithonian mass extinction and provide a unique 3 dimensional picture of the spatial and temporal eruption of the lava pile chemically defined magma types are diachronous and the onset of magmatism occurred 500 1000 km inland migrating southeast towards the incipient south atlantic ocean calculated eruption rates increased from 0 03 km3 yr 1 between 138 and 135 ma through 0 13 km3 yr 1 between 135 and 133 ma to 0 21 km3 yr 1 between 133 and 131 ma supporting the notion that the rate of magmatism increased with time and proximity to the developing south atlantic after 131 ma the continental eruption rates dropped dramatically to 0 01 km3 yr 1 presumably because rifting had occurred and eruption centred in the oceanic rio grande rise with eruption rates of around 0 3 0 5 km3 yr 1 given the available constraints on the potential temperature of the plume thickness of the continental lithosphere and degrees of extension through time predicted melt production rates within the plume seriously underestimate the calculated eruption rates on the continent however the eruption rates can be reconciled if melting occurred by conductive heating of volatile enriched mantle the diachronous nature of the magma types supports the idea that melting occurred over a wide area and that the different magma types reflect different source regions rather than the temporal evolution of magmas from a single source given that 10 million years is appropriate to the timescales for conductive heating above the mantle plume it is suggested that these distinct source regions were located within the lithospheric mantle consistent with geochemical studies

quartz and sheet silicate preferred orientations of low symmetry pikikiruna schist new zealand abstract the pikikiruna schist of nelson new zealand displays a fabric in which the patterns of quartz c axes the poles to planes of inequidimensional quartz grains and the statistical maxima of poles to sheet silicate cleavages are oblique to each other the quartz c axes patterns consist of type 1 and type 2 crossed girdles the triclinic fabric can be explained in terms of one complex rotational deformation of an essentially plane strain nature rotation of approximately 90 about the intermediate strain axis was combined at a late stage with subsidiary rotations about the extension axis the quartz c axes patterns can be related to the kinematic framework rather than the finite strain axes on the other hand the dimensional quartz preferred orientation may be closely related to the finite strain axes though the quantity of strain can not be measured because of recrystallisation

coherent intrusion complexes in large basaltic volcanoes a new structural model abstract highly concentrated coherent intrusion complexes consisting of thousands of small mafic intrusions occur in probably all major basaltic volcanoes and play an important role in volcano development magma excursions from the high level chamber travel laterally along a surface of neutral buoyancy at the margin of a complex and cause the complex to grow the limited distance however that narrow intrusions can propagate before becoming blocked causes complexes to be wedge like intrusive dike complexes underlie rift zones and asymmetric growth of the dike wedge causes rift zones of shield volcanoes to become non collinear and may initiate a third rift zone in the obtuse angle downbowing of stress trajectories across complexes causes dikes to be non vertical and results in axial subsidence intrusive sheet complexes form instead of dike complexes in volcanic systems that have a restricted ability to expand laterally and accommodate intrusions by expanding vertically instead downbowing of stress trajectories causes sheets to be non horizontal and this combined with subsidence increasing toward the thicker part of the sheet wedge produces the inward and inwardly increasing dip that characterizes cone sheet complexes this mechanism for cone sheets differs considerably from previously proposed mechanisms successive injections of sheets at the top of a sheet complex probably offers the most efficient means of powering a high intensity geothermal field such as the 5000 mw grimsvotn system in iceland it is inferred that similar mechanisms to those in major basaltic edifices operate in spreading ridges study of basaltic edifices has the potential to contribute significantly to the understanding of spreading ridges

tectonic influences in piecemeal caldera collapse at glencoe volcano scotland glencoe volcano is renowned as the archetype for caldera collapse that results from piston like subsidence of a coherent crustal block on a ring fault however the caldera floor rocks and the geometry of seven intracaldera units that record large volume explosive eruptions show that the collapse involved incremental and haphazard subsidence of numerous crustal blocks before development of the ring fault some subsidence involved flexure downsag with associated development of extensional crevasses hundreds of metres deep a system of orthogonal faults and related grabens records the influence of two intersecting basement discontinuities one of which is inferred to have linked with the active great glen fault substantial extensional and or transtensional faulting with distinct related sedimentary responses occurred without eruptions the tectonic framework substantially controlled the magmatic plumbing and the locations of vents caldera depocentres and through going rivers the related sediments influenced contrasting eruption styles e g transitions between phreatomagmatic and magmatic and the shallow level emplacement of sills

comparison of quartz microfabric with strain in recrystallized quartzite abstract the relationship between quartz c axis microfabric and strain is examined in six specimens of recrystallized quartzite conglomerate in which strain was measured using pebble shapes four rocks subjected to plane strain display a direct relationship between the strength of preferred orientation and the strain intensity the c axis distributions in these rocks as well as a rock subjected to moderate extensional strain are crossed girdles with maxima near the intermediate principal strain axis and connecting girdles at acute angles to the direction of maximum shortening a rock subjected to moderate flattening strain has several maxima clustered near the direction of maximum shortening and a weak connecting girdle through the intermediate principal strain axis these results are generally similar to those of other studies comparing strain and tectonite fabrics and also with experimental and computer simulation studies of fabrics the degree of preferred orientation is related to total strain and therefore microfabrics in quartzites may be cautiously interpreted as qualitative indicators of strain intensity uncertainties are greater however for correlations of fabric patterns with shapes of the strain ellipsoid an observed increase in recrystallized grain sizes with increasing strain suggests that flow stress was lower in the more strained rocks