tectonics of the antarctic scotia plate boundary near elephant and clarence islands west antarctica over 5000 km of new bathymetric data collected from near the northern antarctic peninsula 60 s 63 5 s latitude 53 5 w 63 w longitude show the morphology of an irregular segment of the antarctic scotia plate boundary and nearby shetland microplate the irregular plate boundary is formed by an oblique intersection 70 of the sinistral transpressional shackleton fracture zone sfz and the sinistral transtensional south scotia ridge transform ssr near elephant ei and clarence ci islands mapped boundaries of the shetland microplate include the south shetland trench and the volcanic rift axis of bransfield strait marginal basin bathymetric data single channel seismic reflection profiles and geosat ers 1 free air gravity data show a southeast trending fault zone on the northeast side of a prominent ridge in the sfz the fault zone is defined by scarps that affect ocean floor sediments fault bounded subbasins rotated sedimentary layers angular unconformities linear gravity trends and transtensional followed by contractional deformation southeast of a termination of the sfz ridge at the south shetland trench the fault zone subdivides into segments displaying steep scarps up to 23 and canyons on the northeast margin of the ei platform these features become east west trending nearer to the western ssr south of the islands southwest trending extensional or transtensional fault zones disrupt the bransfield strait volcanic rift axis these data suggest that 1 recent 4 ma changes in the configuration of the antarctic plate near the antarctic peninsula caused a segment of the sfz transform to adjust to a more stable rectilinear geometry with the ssr transform and 2 diffuse transtension resulting from current antarctic scotia relative motion is dissecting the shetland microplate near ei and ci and transferring slivers of the scotia plate onto the antarctic plate

rift propagation detachment faulting and associated magmatism in bransfield strait antarctic peninsula multichannel seismic data from bransfield strait a young 4 ma marginal basin located at the nw tip of the antarctic peninsula reveal a complex array of rifting styles short of clearly defined seafloor spreading the strait is undergoing basinwide extension while enriched mid ocean ridge basalts have been dredged from topographic highs in the ne part of the basin seismic evidence does not support the formation of oceanic crust there acoustic signatures of volcanic crust at known axial deep highs extend laterally subseafloor their distribution and style support propagation of volcanism and associated extension from ne to sw active extensional structures along both margins control a strong physiographic asymmetry the south shetland islands margin is composed of a few closely spaced large offset normal faults the gentler antarctic peninsula margin exhibits broader distributed extension to the ne normal fault polarity reversal structures nfprs dominate listric faults sole into nw dipping low angle reflectors interpreted as detachments to the sw larger nfprs and convex up geometries up to 20 km across suggest broad uplift rather than detachment based extension such uplift may precedes seafloor volcanism and rapid associated deflation along detachments nw se trending accommodation zones also segment the basin along strike bransfield strait extension may result both from plate boundary tectonics at the south shetland trench and at the evolving antarctic scotia plate boundary extension was probably initiated by onset of subduction rollback at the trench as a slab window opened from the sw augmented extension in the ne part of the basin may be a response to left lateral transtension associated with reorganization of the triple junction at the shackleton fracture zone south scotia ridge intersection

diatom based reconstruction of the subantarctic front migrations during the late miocene and pliocene abstract the antarctic circumpolar current acc is an influential component of the global climate system and therefore it is essential to clarify the history of the oceanic fronts associated with it however there have been only a few attempts to reconstruct their history during the neogene as most previous studies focused on quaternary ages this study uses the late miocene to pliocene ca 9 3 ma fossil records of diatoms from the atlantic sector of the southern ocean deep sea drilling project site 513 and ocean drilling program site 689 to reconstruct the paleo location of the subantarctic front saf which represents the northern limit of the acc the abundance of subtropical diatoms at site 513 shows significant fluctuations during ca 9 5 ma which strongly suggest that there have been repetitive north south migrations of the saf across site 513 during at least ca 9 5 ma in order to reconstruct the saf migration history in more detail changes in abundance of a typical subtropical diatom i e thalassionema nitzschioides var parva heiden emend moreno ruiz recorded at sites 513 and 689 are compared to abundance patterns of the same taxon at several drilling sites ocean drilling program sites 697 699 and 704 published in previous data reports results suggest several phases of saf migration involving the following shifts in saf position direction of shift is indicated in brackets ca 8 5 ma northward 8 0 ma northward 7 0 ma southward 6 1 ma northward 5 4 ma the direction differs region by region 4 9 ma northward 4 6 ma southward 3 8 ma northward the results also show that saf has experienced north south migrations with a latitudinal amplitude of ca 5 or more comparison between changes in fossil diatom assemblages at site 689 and the inferred saf shifts suggest that the northward southward shift of the saf could be induced by the cooling warming in the high latitude regions

western scotia sea margins improved constraints on the opening of the drake passage 1 we present a revised tectonic interpretation from 28 ma to 3 2 ma of the western sector of the scotia sea incorporating new multichannel seismic reflection profiles and magnetic anomaly identifications for the continental margin off the tierra del fuego island and available complementary data for the conjugate margin of the northwestern flank of the south scotia ridge seismic profiles show a remarkable diversity of the pair of conjugate passive margins of the western scotia sea in both their morphology and structural framework the tierra del fuego continental margin can be related to a classic rifted passive margin while the southwestern margin of the scotia sea is characterized by steep slopes mostly generated by subvertical faults that abruptly separate the continental crust of the south scotia ridge from the oceanic crust of the western scotia sea this structural difference was caused by intense strike slip tectonism mostly concentrated along the modern south scotia ridge since the early development of the western scotia sea we find evidence for a previously unrecognized magnetic anomaly 10 28 ma at the foot of the tierra del fuego continental margin the same anomaly is present at the conjugate northern flank of the south scotia ridge the timing of events leading to the earliest development of the western scotia sea which determined the opening of the drake passage is important because this gateway opening had a profound effect on global circulation and climate the thickness and the distribution of the sedimentary cover overall in the abyssal plain off the two western scotia sea margins is different this is due to the different regimes of the bottom current flows which affected the western scotia sea both in the past and in the present time

upper mantle seismic anisotropy beneath antarctica and the scotia sea region summary recent investigations on shear wave splitting from recordings of permanent and temporary antarctic seismological stations have lead to a greater understanding of the upper mantle dynamics of the scotia sea region and the continental margin in the eastern weddell sea in terms of their tectonic evolution the analysis of shear wave splitting from teleseismic core sks skks pks and direct s waves reveals the seismic anisotropy and the strain field of the upper mantle similar to the caribbean anisotropy structures in the antarctic peninsula and scotia sea regions are assumed to be influenced by mantle flows in easterly directions around the subducting nazca plate in general anisotropy polarization directions in the scotia sea do not contradict this hypothesis with polarizations oriented nearly e w and therefore aligning with the suggested mantle flow patterns anisotropy strengths decrease from delay times of dt 1 8 s pmsa palmer station in the west towards the east with delay times of dt 0 3 s beneath hope south georgia and cand candlemas south sandwich islands nevertheless a lithospheric and therefore fossil origin cannot be ruled out only the exceptionally high delay times at pmsa probably originate in part from recent asthenospheric flow around the subduction slab of the former phoenix plate beneath the northwestern margin of the antarctic peninsula the continental margin of western dronning maud and coats land plays a crucial role in understanding the early processes during the break up of gondwana upper mantle seismic anisotropy with delay times well over dt 1 s in this region gives new constraints on ancient deformation processes during break up and former episodes two layer modelling reveals archaean anisotropy in the upper layer corresponding well to polarization directions of the south african kaapvaal craton lower layer anisotropy is assumed to have been created during early gondwana rifting stages

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

autopsy on a dead spreading center the phoenix ridge drake passage antarctica new bathymetric and magnetic anomaly data from the phoenix ridge antarctica show that extinction of all three remaining segments occurred at the time of magnetic chron c2a 3 3 0 2 ma synchronous with a ridge trench collision south of the hero fracture zone this implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision spreading rates slowed abruptly at the time of chron c4 7 8 0 3 ma probably as a result of extinction of the west scotia ridge which would have led to an increase in slip rate and transpressional stress across the shackleton fracture zone spectacular high relief ridges flanking the extinct spreading center mapped for the first time using multibeam swath bathymetry are interpreted as a consequence of a reduction in spreading rate involving a temporary magma oversupply immediately prior to extinction

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

magmatism as an essential driving force for formation of active metamorphic core complexes in eastern papua new guinea the d entrecasteaux islands in eastern papua new guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation the plutons do not appear to have been intruded by diapiric processes as previously suggested late relatively undeformed plutons form flat lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography it is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes the source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the woodlark seafloor spreading center into continental crust it is suggested that a localized heat source of this type which can provide heat and magmatic material to the crust is essential for the development of metamorphic core complexes

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

tectonic segmentation of the northern antarctic peninsula discrete segments of the southeast pacific ocean crust were subducted beneath the antarctic peninsula in late cretaceous and cenozoic time the segment boundaries of the subducted ocean crust are reflected in the overlying continental crust by transverse megafractures these are usually obscured by contemporaneous magmatism but may be revealed by later fault movements the transverse fracture zones divide graham land and its offlying islands into segments that have different histories of magmatism uplift and erosion and postorogenic sedimentation metalliferous mineralization is controlled in part by the transverse fracture zones

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

segmentation and along strike asymmetry of the passive margin in socotra eastern gulf of aden are they controlled by detachment faults on the island of socotra the southern passive margin of the gulf of aden displays along its strike two different types of asymmetric structures western socotra is made up of a series of southward tilted blocks bounded by consistently northward dipping normal faults eastern socotra consists of a broad asymmetric anticline with a steep northern limb and a gently dipping southern limb a zone of ne sw striking strike slip and normal faults separates the two areas the overall structure is interpreted as representing two rift segments separated by a transfer zone the along strike juxtaposition of crustal scale asymmetric structures on the southern margin of the gulf of aden is complemented by the asymmetry of the conjugate margins on either side of the gulf whereas the western socotra margin is narrow and characterized by oceanward dipping normal faults the conjugate oman margin is broader and dominated by horsts and graben considering that asymmetric structures in the upper crust are often associated with synthetic shear zones at deeper ductile levels we propose that the western and eastern socotra margin segments were controlled at depth by two detachment faults with opposite dips and senses of shear the normal faults of western socotra would sole out into a top to the north ductile shear zone whereas the asymmetric anticline of eastern socotra would be associated with a top to the south detachment fault

backarc basin evolution and cordilleran orogenesis insights from new ocean bottom seismograph refraction profiling in bransfield strait antarctica bransfield strait a backarc basin off the northwestern antarctic peninsula is a modern analog for cretaceous basins inverted in the compressional tectonic regime that initiated the andean cordillera eight new refraction ocean bottom seismograph profiles in the strait demonstrate that crustal thickness in the deep central basin increases from northeast to southwest from 10 km to 14 16 km this confirms multichannel seismic interpretation of upper crustal structures suggesting that the bransfield basin is opening by northeast to southwest rift propagation within arc crust of the antarctic peninsula a process also recorded in the obducted cretaceous rocas verdes basin of the southernmost andes thinning is most prominent along the axis of the strait where the crust is 9 11 km thick in contrast thicknesses beneath the antarctic peninsula margin and the inactive south shetland islands pedestal are 18 km and 24 km respectively seismic velocities and thicknesses suggest that new oceanic crust is not yet being generated extension is focused along the northwest margin imparting the physiographic asymmetry to the strait comparing the bransfield basin with the inverted rocas verdes basin and intraoceanic counterparts in the western pacific suggests that rift propagation and trench side focusing of extension may be fundamental features of young backarc basins resultant asymmetry may facilitate observed obduction of backarc basin floor and arc rocks onto continental margins during compressional orogenesis

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

seismotectonics and relative plate motions in the scotia sea region source parameter inversion of p and sh waveforms and amplitudes for 20 shallow earthquakes using a singular value decomposition technique provides important constraints on the tectonics and relative plate motions of the scotia sea region the derived focal mechanisms show both thrust and strike slip faulting along the north scotia ridge and normal and strike slip faulting along the south scotia ridge two thrust faulting mechanisms south of tierra del fuego indicate diffuse convergence of the antarctic plate along the western margin of the scotia plate earthquakes are consistent with active subduction and back arc spreading along the south shetland trench and bransfield strait the depths of two earthquakes 35 km and 55 km and one shallow thrust faulting event suggest continued subduction along the south shetland trench but the underthrusting is largely aseismic due to the young plate age and slow subduction rate the magnitude and depth of normal faulting earthquakes along bransfield strait and the south scotia ridge are suggestive of diffuse extension rather than typical organized mid ocean spreading which is generally associated with smaller and shallower earthquakes eight strike slip and thrust faulting events provide well constrained slip vectors showing scotia plate motion relative to the surrounding larger plates using a four plate model scotia sc south sandwich ss antarctica an and south america sa and a fixed sa an euler vector we inverted these data together with published centroid moment tensor slip vectors along the south sandwich trench and published marine magnetic data from the ss back arc spreading center to derive the first quantitative plate motion model for the scotia region the sc sa euler pole is located in the south atlantic and shows large latitudinal uncertainties the sc an pole is better constrained and located in the weddell sea these results predict left lateral strike slip motion with a component of compression along the north scotia ridge left lateral strike slip with a component of extension along the south scotia ridge and east west compression in drake passage although no rate data have been published connecting the scotia plate with the global plate system euler vector magnitudes are obtained through closure the results though poorly constrained suggest relative motion rates of 0 5 cm yr along the north scotia ridge and 1 0 cm yr along the south scotia ridge and in drake passage

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

deep crustal structure of bransfield strait initiation of a back arc basin by rift reactivation and propagation 1 we present the results of a wide angle seismic survey consisting of a grid of five strike and three dip profiles conducted in bransfield strait antarctica we used a combination of two dimensional seismic tomography and ray tracing to constrain velocity structure in the region thinnest crust is located within the interpreted neovolcanic zone but the crustal thickness of 10 15 km indicates that rifting has not progressed to the formation of normal oceanic crust crust thickens to 20 26 km along the south shetland islands pedestal and to 14 20 km along the antarctic peninsula margin along strike crust generally thickens from the ne to the sw in the across strike direction crustal thinning is observed over a broader area in the ne than in the sw the observed crustal thickness pattern is consistent with propagation of extension from the ne toward the sw upper mantle velocities of 7 45 0 2 km s indicate either the presence of a small amount of partial melt or normal upper mantle anisotropy with the slow direction parallel to the rift axis large lateral changes in upper crustal velocities along the antarctic peninsula margin correspond with seismically imaged structural boundaries there is no correlation between these structural boundaries and large velocity changes or morphologic steps observed within the present day neovolcanic zone we suggest that the rift topography now characterizing the antarctic peninsula margin formed prior to the formation of the modern basin

punctiform initiation of seafloor spreading in the red sea during transition from a continental to an oceanic rift transition from a continental to an oceanic rift occurs in the red sea by initial emplacement of oceanic crust in regularly spaced hot points which serve as nuclei for axial propagation into segments of oceanic crust accretion and for initiation of seafloor spreading hot points develop above upwelling mantle diapirs caused by density viscosity inversion beneath continental and oceanic rifts

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

response of the lithosphere to in plane force variations the earth s lithosphere deforms when subjected to vertical and horizontal tectonic forces because it is not perfectly rigid the deformation has both elastic flexural and inelastic brittle and ductile creep components we use the yield strength envelope formulation of a depth dependent rheology to predict how the lithosphere will deform in response to applied in plane force brittle deformation of the upper lithosphere is influenced by the distribution of existing faults and fractures in the crust and their orientation with respect to the applied tectonic force the sense of fault reactivation indicates the sign of the applied in plane force the induced flexural deformation is governed by the pre existing lithospheric deflection the effective elastic thickness of the lithosphere when the force was applied and the magnitude of the applied force using this framework we compare the predicted response of the lithosphere to in plane force variations with that observed in the central indian ocean and the jeanne d arc basin offshore newfoundland the brittle deformation in both regions is readily observed in seismic reflection data the upper oceanic crust in the central indian ocean is broken into blocks bounded by high angle reverse faults the dip of these basement faults and their orientation parallel to the marine magnetic lineations suggest that they represent a reactivated crustal fabric imparted during the formation of oceanic crust the reverse sense of fault reactivation is consistent with the north south compression determined from earthquake focal mechanisms in the jeanne d arc basin episodes of northwest southeast extension reactivated the basin bounding faults in a normal sense during late barremian early aptian and late aptian time in the late albian the same crustal blocks were reactivated and uplifted in response to north northeast in plane compression flexural deformation due to applied in plane force is observed only in the central indian ocean case where the oceanic crust and most of the overlying bengal fan sediments were deformed into broad east west trending undulations with wavelengths of 100 300 km and amplitudes up to 2 km in the jeanne d arc basin the flexural deformation which should accompany the fault reactivation is difficult to recognize from the preserved stratigraphy consequently brittle deformation within extensional basins which appears to be the most recognizable response of the lithosphere to in plane force variations obscures the effects of the flexural deformation however it is this flexural deformation that has been postulated as a tectonic alternative to glacial eustasy in explaining the 3rd order sea level cycles because it is difficult to recognize the effects of the flexural deformation in the preserved stratigraphy we surmise that it is not a viable alternative to glacial eustasy in generating either regional or global unconformities instead we propose that the flexural and brittle deformation due to applied in plane force may locally enhance or diminish the effects of other processes responsible for unconformity generation

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

seasonal changes of dissolved nutrients within and around port foster deception island antarctica abstract temporal and spatial distribution of dissolved macronutrients ammonia nitrate phosphate and silica and productivity were investigated within and around port foster the flooded 160 m deep caldera of deception island antarctica this study was part of the erupt project which included five seasonal cruises over a complete annual cycle during 1999 2000 seawater samples were collected and physical properties were monitored from seven stations within port foster and 12 stations in the adjacent bransfield strait in addition shallow water and beach interstitial water samples were collected along the shorelines of the peripheral coves port foster macronutrient depth profiles were typical for a normal shallow seawater column in a polar region the water column in early austral spring was well mixed and changed to a stratified water column with a weak thermocline during the summer by early winter the thickness of the well mixed surface layer increased until the entire water column returned to well mixed conditions this early winter transition from stratified conditions to well mixed conditions occurred in june and appeared to be abrupt during the seasons of light limitation and low primary productivity local currents were effective at redistributing dissolved biochemical components throughout the bay during the summer season the dissolved nutrient and oxygen distributions reflected consumption of nutrients by primary producers the mid depth maximum observed in the ammonia profiles implies the excretion of metabolites from resident animal populations residence time of dissolved ammonia must have been shorter than the circulation time within port foster because ammonia is not as uniformly distributed during the summer months as it was during the winter and spring dissolved nitrate concentrations in the bransfield strait during this study were similar to those measured in previous studies the mean concentrations of phosphate nitrate and silica in the beach interstitial samples were significantly higher 2 8 9 5 times than in the shore offshore and ctd samples possible sources for the high phosphate nitrate and ammonia concentrations in the beach interstitial and shore waters include decaying organic matter and bird and mammal excrement elevated silica concentrations appear to be associated with hydrothermal heating of beach and near shore waters however the elevated macronutrient concentrations measured in the beach interstitial water were not traceable beyond 5 m of the shoreline phytoplankton biomass in port foster exhibited temporal variability similar to other coastal and continental shelf zones ccz of the antarctic peninsula blooms in february and november 2000 were dominated by the centric diatoms thalassiosira spp and rhizosolenia spp chlorophyll a chl a values of 15 and 19 mg m 3 in port foster during these blooms were comparable to maximum recorded levels in western antarctic peninsula ccz 30 40 mg chl a m 3 while chl a values from november 1999 and june 2000 non bloom conditions corresponded to historical monthly chl a averages of western antarctic peninsula ccz 3 during blooms phytoplankton standing stock could account for about 15 m nitrate which corresponds to the observed surface nitrate depletion

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

active margin processes along the antarctic peninsula abstract the antarctic peninsula has a remarkable record of active margin processes which include subduction with progressive ridge trench collisions margin segmentation by major fracture zones rifting in a hybrid back arc and sheared plate margin context fore arc basin development and glacial marine controlled trench fill processes several facets of these active margin processes both of internal crustal dynamic and external origin climate controlled have been documented by a geophysical survey during the antarktis vi 2 cruise of r v polarstem october december 1987 reflection seismic profiles have been shot over the rift basin of bransfield strait over an elongated sediment filled trough interpreted as a fore arc basin over accretional and progradational slopes over recent and ancient trench environments and over the facing oceanic domain in this oceanic domain different fracture zones have highly contrasting morphological and geophysical expressions the subduction of a fracture zone like hero f z characterized by a significant relief possibly related to the presence of buoyant serpentinite ridges may have been a factor of subduction termination for the last segment of the aluk drake plate it may also have played a role in the separation of a blueschist bearing fragment smith island from the base of the accretionary plate margin and in its lift to the surface the magnetic anomaly pattern of the oceanic slabs facing the northwestern peninsula margin shows evidence of an intriguing spreading acceleration which apparently preceded ridge trench collision the same anomaly pattern provides a clue to the stratigraphie interpretation of the oceanic sediment cover and of the frontal part of the prograding now passive margin south of the south shetland island arc an apparently broken and tilted oceanic plate fragment squeezed between the south shetland trench and shackleton fracture zone may argue for the role of transpression associated with the oblique convergence of the antarctic and scotia plates

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

geomagnetic secular variation of bransfield strait western antarctica from analysis of marine crossover data summary tracking the secular variation of the geomagnetic field in the past is severely limited in some cases by factors relating to the remoteness of the sites this is maximal in the antarctic where the remote location and severe climate lead to logistic limitations that make it difficult to keep a continuous record of magnetic field variations from the magnetic information available from historical marine expeditions it is possible to infer this time varying component from the comparison of readings at crossovers this study focuses on this technique discusses the impact of the different error sources and proposes a simple mathematical procedure to infer average secular variation rates the result is validated by comparing it with local data from the arctowski and livingston magnetic observatories sited in the area additionally using a high quality data set from a local area in the neighbourhood of deception island we have detected a systematic distribution in its secular variation this dichotomy has been interpreted in terms of a volcano magnetic signal this fact and the nature of its principal mechanisms are analysed and discussed

back arc extension in the southern andes a review and critical reappraisal the interpretation that the mafic rocas verdes green rocks complex of the southern andes represents part of the uplifted floor of a late jurassic to early cretaceous back arc basin has proved particularly useful in understanding the geological evolution of the southern andes the north scotia ridge and the antarctic peninsula clear field evidence of the back arc setting of the 9rocas verdes gabbro sheeted dyke pillow lava ophiolitic assemblages has encouraged fruitful petrological and geochemical comparison with mid ocean ridge and marginal basin basalts other onshore ophiolite complexes and archaean greenstone belts uncertainty still surrounds estimates of the original width and depth of the basin as well as the proportion of new mafic crust compared with relict sialic crust in the basin floor these questions are unresolved owing mainly to the considerable lower cretaceous turbiditic basin infill and the effects of mid cretaceous compressional deformation while the field relations clearly indicate that the rocas verdes basin is not an older piece of ocean floor trapped behind a volcanic arc it is not yet clear whether the basin is directly subduction related or falls in the category of back arc leaky transforms like the proto gulf of california or apparent rip off features like the andaman sea

the tydeman morphology and seismic structure of old fracture zone crust the tydeman fracture zone was identified 160 km se of the azores in 1977 from a detailed bathymetry and magnetic survey between 22 309 and 23 409w twigt et al 1979 the fracture zone forms a linear trough here with a width of 20 km water depth of 4 7 km and orientation of 085 this study extends the path of this fracture zone between 18 and 30 w though not yet to the ridge crest using a larger dataset of bathymetric seismic reflection and magnetic profiles a discontinuity in the fracture zone valley indicates an interruption in its formation possibly during the formation of the azores platform two separate wide angle seismic experiments carried out over old crust in the active section of the tydeman fracture zone show low crustal velocities compared with normal oceanic crust of comparable age an absence of an identifiable layer 3 and a shallower than normal moho overlying a region of anomalously low upper mantle velocities estimates of poisson9s ratio suggest that hydrothermal circulation may continue for as long as 70 ma after crustal formation

volcanic stratigraphy and geochronology of the cretaceous lancones basin northwestern peru position and timing of giant vms deposits a 10 km thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the cretaceous lancones basin in northwestern peru and underlies part of the huancabamba deflection the marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin the earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar quartz porphyritic felsic volcanic rocks these volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide vms deposits which are inferred to have formed in a localized but relatively deep marine setting u pb zircon dating of felsic volcanic rocks associated with vms deposits at tambogrande indicates ages from 104 8 1 3 to 100 2 0 5 ma for the ore bearing volcanic sequence the timing of onset of rift related volcanism is not well constrained but is therefore of middle albian age or older subsequent latest albian to turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields u pb zircon ages of 99 3 0 3 to 91 1 1 0 ma these later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the lancones basin finally the basin was intruded by late cretaceous to tertiary granitoids of the coastal batholith the genesis of the cretaceous lancones basin and other equivalent volcanic rift related marginal basins in western south america including the western peruvian trough is related tectonically to the break up of gondwana early volcanism and associated vms deposits formed in the lancones basin during the albian coincided with the initial rifting stage prior to active oceanic spreading between south america and africa during this time the relatively stationary western margin of continental south america was undergoing extension and rifting due to a westward and oceanward retreating arc resembling a mariana arc type setting the mochica orogeny marks the termination of rifting subsidence and related volcanism along the western margin of south america this orogenic event also broadly coincides with the onset of spreading of the south atlantic and westward drift of the south american continent subsequent volcanism in the lancones basin was more continental arclike under an andean type scenario

crustal diapirism in bransfield strait west antarctica evidence for distributed extension in marginal basin formation new multichannel seismic data from bransfield strait west antarctica image complex fan shaped faulting patterns along the antarctic peninsula margin although a neovolcanic zone running along the strait9s axis has been presumed to be a nascent spreading center and the primary focus for extension the fan shaped faulting pattern can be interpreted instead as evidence for intracrustal diapirism suggesting separate diffuse zones of extension further more the diapiric trend 5 divergent from that of the neovolcanic zone supports temporal and spatial evolution of stress perhaps in response to complex plate interactions to the northeast