seismic velocities in the uppermost oceanic crust age dependence and the fate of layer 2a it has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to typical crustal velocities 5 2 km s 1 over a period of 30 40 million years with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation some recent seismic studies however suggest that the seismic velocities actually rise rapidly nearly doubling in 5 km s 1 and velocities 3 km s 1 are not observed in crust older than 5 ma there is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years median and average velocities increase from 2 3 to 4 4 km s 1 by 7 5 ma and remain unchanged thereafter if layer 2a is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks layer 2b 5 2 km s 1 it follows that layer 2a does not disappear but is a persistent feature of the oceanic crust these results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation

half graben versus large offset low angle normal fault importance of keeping cool during normal faulting 1 we study the effect of temperature in the crust on the style of faulting during the extension of a model lithosphere two distinct fault patterns are seen depending on whether the brittle layer thickness hl is greater or less than a critical value hc for a thin layer hl hc a rift develops with multiple faults bounding an evolving set of grabens numerical results show that the value of hc depends on how the thermal structure and therefore the thickness distribution of the lithosphere evolve during extension in the region where the displacement occurs along the fault the brittle lithosphere thins and we show that this promotes development of multiple faults in models with heat transfer only by rock motion and conduction the advective thinning effect can greatly reduce hc with rift velocities greater than 1 cm yr 1 hc is reduced by a factor of 5 to unreasonably small values however inclusion of simulated hydrothermal cooling increases hc this allows large offsets to develop in model lithospheres with brittle layers of order 10 km thick extension of layers thicker than 10 km tends to produce a rift with multiple graben structures finally we compare the result of a simplified analytic model of extension to data we find that the style of faulting seen in a variety of continental rifts is broadly consistent with our model predictions

control on the emplacement of the andesite lava dome of the soufriere hills volcano montserrat by degassing induced crystallization lava solidification is controlled by two mechanisms external cooling and gas exsolution the latter inducing crystallization due to increasing liquidus temperature the andesite lava dome of the soufriere hills volcano montserrat is an extrusion dominated by crystallization caused by gas exsolution where cooling is unimportant in controlling emplacement in the magma chamber the magma has an estimated viscosity of 7 106 pa s during ascent gas exsolution caused the magma to extrude in a highly crystalline state with only 5 15 residual melt viscosities in the range 1013 1014 pa s and mechanical strength 1 mpa deformation can be heterogeneous with extrusion along shear zones rheological stiffening in the upper conduit also causes large overpressures shallow seismicity and cyclic patterns of dome extrusion gas rich porphyritic andesites tend to be the least mobile kind of lava because transition from magma into hot crystalline material was reached during ascent

lower crustal variability and the crust mantle transition at the atlantis massif oceanic core complex 1 seismic refraction data provide new constraints on the structure of the lower oceanic crust and its variability across the atlantis massif oceanic core complex 30 n on the mid atlantic ridge a 40 km long spreading parallel profile constrains p wave velocities to depths of up to 7 km beneath the seafloor two shorter spreading perpendicular lines provide coverage to 2 km depth the anomalous character of the massif s central dome crust is clear compared to the neighboring rift valley and similar age crust on the opposite ridge flank the domal core of the massif unroofed via detachment faulting has velocities 7 0 km s at depths below 2 5 km sub seafloor increasing to 7 5 7 8 km s over the depth range 4 8 6 8 km within the core complex the moho does not appear to be sharp as no pmp arrivals are observed within the axial valley velocities do not reach mantle transition zone values in the uppermost 6 km we infer that crust there is of normal thickness but that a thinner than average mafic section is present in the central massif near iodp hole u1309d located on the central dome there is a low velocity gradient interval at 1 3 km depth with velocities of 6 6 6 8 km s that coincides with a 3 5 km wide region where shallower velocities are highest given the predominantly gabbroic section recovered from the 1 4 km deep drillhole this seismic structure suggests that the mafic body extends a few km both laterally and vertically

the structure of oceanic core complexes controlled by the depth distribution of magma emplacement extension at mid ocean ridges can be accommodated by detachment faults forming oceanic core complexes that develop under low rates of magma intrusion modelling reveals that oceanic core complexes can also form under high rates of magma intrusion if the magma is injected into the lower ductile layer of the crust

modeling polarimetric radar scattering from the lunar surface study on the effect of physical properties of the regolith layer 1 a theoretical model for radar scattering from the lunar regolith using the vector radiative transfer theory for random media has been developed in order to aid in the interpretation of mini sar data from the chandrayaan 1 and lunar reconnaissance orbiter missions the lunar regolith is represented as a homogeneous fine grained layer with rough upper and lower parallel interfaces that possesses embedded inclusions with a different dielectric constant our model considers five scattering mechanisms in the regolith layer diffuse scattering from both the surface and subsurface volume scattering from buried inclusions and the interactions of scattering between buried inclusions and the rough interfaces both the lunar surface and subsurface multiple scattering between buried inclusions and coherent backscatter opposite effect are not considered in the current model the modeled radar scattering coefficients are validated using numerical finite difference time domain simulations and are compared with incident angle averaged earth based radar observations of the moon both polarized and depolarized radar backscattering coefficients and the circular polarization ratio cpr are calculated as a function of incidence angle regolith thickness surface and subsurface roughness surface slope abundance and shape of buried rocks and the feo tio2 content of the regolith simulation results show that the polarized opposite sense radar echo strength at s and x bands is mostly dominated by scattering from the rough surface and buried rocks while the depolarized same sense radar echo strength is dominated by scattering from buried rocks or ice inclusions finally to explore the expected polarimetric signature of ice in the polar permanently shadowed areas four parametric regolith models are considered and the possibility of detecting diffuse ice inclusions by the cpr is addressed our study suggests that detection of ice inclusions at the lunar poles using solely the cpr will be difficult given the small dielectric contrast between the regolith and ice

regolith thickness over the lunar nearside results from earth based 70 cm arecibo radar observations the inversion of regolith thickness over the nearside hemisphere of the moon from newly acquired earth based 70 cm arecibo radar data is investigated using a quantitative radar scattering model the radar scattering model takes into account scattering from both the lunar surface and buried rocks in the lunar regolith and three parameters are critically important in predicting the radar backscattering coefficient the dielectric constant of the lunar regolith the surface roughness and the size and abundance of subsurface rocks the measured dielectric properties of the apollo regolith samples at 450 mhz are re analyzed and an improved relation among the complex dielectric constant bulk density and regolith composition is obtained the complex dielectric constant of the lunar regolith is estimated globally from this relation using the regolith composition derived from lunar prospector gamma ray spectrometer data to constrain the lunar surface roughness and abundance of subsurface rocks from radar data nine regions are selected as calibration sites where the regolith thickness has been estimated using independent analysis techniques for these sites scattering from the lunar surface and buried rocks cannot be perfectly distinguished and a tradeoff relationship exists between the size and abundance of buried rocks and surface roughness using these tradeoff relations as guidelines for globally representative parameters the regolith thickness of four regions over the lunar nearside is inverted and the inversion uncertainties caused by calibration errors of the radar data and model input parameters are analyzed the regolith thickness of the maria is generally smaller than that of highlands and older surfaces have thicker regolith thicknesses our approach cannot be applied to regions where the surface roughness is very high such as with young rocky craters and regions in the highly rugged highlands

constraints on the shallow seismic structure at ocean drilling program site 1107 ninetyeast ridge from implosive bottom sources and airgun shots on ninetyeast ridge the seismological community drilled ocean drilling program odp site 1107 coincident with odp site 757 for the future host of a long term seismological observatory in the central indian ocean to study the shallow seismic structure at the drill site we developed a unusual on bottom seismic tool five implosive sources were placed along a 2 km line four ocean bottom sensors recorded seismic waves over epicentral distances of 400 m to 2200 m revealing seismic velocities of the immediately subjacent sediments velocities within this portion of crust are generally not detectable by traditional marine seismic techniques airgun shots however can be used to determine velocities of basement rocks one dimensional travel time inversion of both bottom sources and water path corrected surface shots yields a velocity structure which is in excellent agreement with odp drilling results three lithological units are imaged a 150 m thick sediment section with velocities of 1600 1720 m s underneath a layer of 230 m thickness with velocities ranging from 2080 to 2250 m s corresponding to volcanic ashes and tuffs the volcanic basement occured at 380 m below seafloor velocities rapidly increase from 3500 m s to values of more than 4500 m s within a few hundred metres

the 3 d geometry of detachment faulting at mid ocean ridges seismic images of cretaceous slow spreading crust from the eastern central atlantic provide new constraints on the process of seafloor spreading and the importance of detachment faulting the seismic depth sections image detachment faults that appear to have exhumed footwall massifs of similar geometry to massifs of plutonic and mantle rocks mapped at the present mid atlantic ridge the detachments are consistent with the structure and microearthquakes of the mid atlantic ridge at 26 n with the footwall rotation inferred from paleomagnetic data and with numerical modeling of oceanic detachments other seismically imaged detachments have similar dimensions and geometry but are covered by a layer of small fault blocks the detachment types differ in whether or not the fault locks up in the subsurface probably controlled by the fault strength the elastic thickness and whether the exhumed footwall is partly covered by basalts toward the segment middle decreasing mantle serpentinization decreasing elastic thickness and thicker median valley basalts all increase the likelihood that the fault locks up and a new fault propagates upwards from the still active root zone transferring a slice of the hanging wall to the footwall to be rafted with the footwall out of the median valley as a result an oceanic detachment fault exhuming the footwall at a segment end to form an oceanic core complex may disappear laterally beneath rafted blocks detachment faulting may be more widespread at slow spreading ridges than interpreted from seafloor mapping

regolith composition and structure in the lunar maria results of long wavelength radar studies radar measurements at 70 cm and 7 5 m wavelengths provide insight into the structure and composition of the upper 5 100 m of the lunar regolith and crust we combine high resolution 3 5 km 70 cm radar data for the nearside with earlier calibrated full disk observations at the same wavelength to provide a reasonable estimate of the lunar backscatter coefficient these data are tested against models for echoes from a buried substrate and mie scattering from surface and buried rocks these mechanisms are expected to dominate the 70 cm radar echo with their relative importance determined by the rock population regolith depth substrate roughness and the loss tangent of the soil results indicate that the 70 cm radar echo for the maria comes largely from mie scattering by rocks buried within the fine soil radar scattering from a buried substrate is not likely to greatly affect the observed return we also compared the 70 cm and 7 5 m radar images to infrared eclipse temperature maps crater population age estimates for the maria and to tio2 and feo abundances inferred from earth based telescopic and clementine multispectral observations these data imply that 1 the tio2 ilmenite content of the regolith controls variations in 70 cm depolarized echo strength among mare units with higher titanium abundance leading to lower echoes 2 changes in the average 70 cm return for a given tio2 abundance between maria of different ages do occur but uncertainties in the current radar data do not allow us to uniquely distinguish between variations in rock population with age and calibration effects 3 the 7 5 m radar echoes are controlled by the age of the mare basalt flows with older deposits having a greater degree of fracturing and higher backscatter future mapping at 12 6 cm and 70 cm wavelengths will help to resolve some of the issues raised here

seismic evidence for large scale compositional heterogeneity of oceanic core complexes long lived detachment faults at mid ocean ridges exhume deep seated rocks to form oceanic core complexes occs using large offset 6 km multichannel seismic data we have derived two dimensional seismic tomography models for three of the best developed occs on the mid atlantic ridge our results show that large lateral variations in p wave velocity occur within the upper 0 5 1 7 km of the lithosphere we observe good correlations between velocity structure and lithology as documented by in situ geological samples and seafloor morphology and we use these correlations to show that gabbros are heterogeneously distributed as large tens to 100 km2 bodies within serpentinized peridotites neither the gabbros nor the serpentinites show any systematic distribution with respect to along isochron position within the enclosing spreading segment indicating that melt extraction from the mantle is not necessarily focused at segment centers as has been commonly inferred in the spreading direction gabbros are consistently present toward the terminations of the detachment faults this suggests enhanced magmatism during the late stage of occ formation due either to natural variability in the magmatic cycle or to decompression melting during footwall exhumation heat introduced into the rift valley by flow and crystallization of this melt could weaken the axial lithosphere and result in formation of new faults and it therefore may explain eventual abandonment of detachments that form occs detailed seismic studies of the kind described here when constrained by seafloor morphology and geological samples can distinguish between major lithological units such as volcanics gabbros and serpentinized peridotites at lateral scales of a few kilometers thus such studies have tremendous potential to elucidate the internal structure of the shallow lithosphere and to help us understand the tectonic and magmatic processes by which they were emplaced

comment on regolith layer thickness mapping of the moon by radar and optical data by y g shkuratov and n v bondarenko in a recent icarus article shkuratov and bondarenko 2001 propose a model for the depth of the lunar regolith based on 70 cm radar observations and near infrared spectroscopy this model treats the regolith as a single homogeneous layer of lossy dust and ignores scattering by buried rocks there is also an implicit assumption that the regolith substrate is everywhere of uniform morphology radar scattering from this substrate is treated using averaged fresnel coefficients with little explanation of how such a model might produce the observed lunar polarization properties taken together these issues weaken the validity of the published regolith depth map

fuji dome a large detachment fault near 64 e on the very slow spreading southwest indian ridge a continuous domed detachment surface fuji dome has been imaged on the very slow spreading southwest indian ridge using deep towed side scan sonar and has been investigated by manned submersible and sea surface geophysics the dome is morphologically similar to other oceanic detachments core complexes or mega mullions in addition to bathymetric mullions observed in ship borne bathymetry finer scale spreading parallel striations were imaged with the side scan on the detachment surface metabasalt crops out near the termination probably as part of a thin fault sliver gabbro and troctolite probably crop out near the summit of the dome the rest of the detachment surface is covered with sediment and rubble which is basaltic except for a single sample of serpentinite most of the detachment surface dips toward the ridge axis at 10 20 but near the breakaway it is strongly rotated outward and dips away from the axis at up to 40 normal undeformed volcanic seafloor crops out adjacent to the detachment modeling of sea surface magnetic data suggest the detachment was active from 1 95 ma for about 1 ma during a period of reduced and asymmetric magmatic accretion modeling of sea surface and seafloor gravity requires laterally fairly uniform but high density material under the dome and precludes steeply dipping contacts between bodies with large density contrasts at shallow levels under the dome

the cooling history and the depth of detachment faulting at the atlantis massif oceanic core complex 1 oceanic core complexes occs are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults we combine previously reported u pb zircon crystallization ages with u th he zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the atlantis massif occ 30 n mar and help establish cooling rates as well as depths of detachment faulting and gabbro emplacement we present nine new u th he zircon ages for samples from iodp hole u1309d ranging from 40 to 1415 m below seafloor these data paired with u pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at atlantis massif as 2895 1276 1162 c myr 1 from 780 c to 250 c the lower 600 m of the borehole cooled more slowly at mean rates of 500 125 102 c myr 1 from 780 c to present day temperatures rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of 300 c myr 1 possibly due to hydrothermal circulation to 4 km depth through the detachment fault zone assuming a fault slip rate of 20 mm yr from u pb zircon ages of surface samples and a rolling hinge model for the sub surface fault geometry we predict that the 780 c isotherm lies at 7 km below the axial valley floor likely corresponding both to the depth at which the semi brittle detachment fault roots and the probable upper limit of significant gabbro emplacement

continental geotherm and the evolution of rifted margins models of melting accompanying mantle upwelling predict far more melt than is observed at nonvolcanic margins the discrepancy may be explained if the paradigm of a uniform asthenosphere is incorrect work on the velocity structure of the continents has shown that the convecting sublithospheric mantle may have a potential temperature as low as 1200 c 100 c cooler than that beneath the oceans the continental geotherm derived from studies on xenoliths brought up by kimberlites is also compatible with a cool sublithospheric mantle except where perturbed by mantle plumes upwelling of such cool mantle during rifting leads to little melt production even for rapid extension rates explaining the formation of amagmatic margins away from mantle plumes however the transition to seafloor spreading and the development of normal thickness oceanic crust requires the invasion of the amagmatic rift by hotter oceanic asthenosphere and or plume material this influx may cause a transient thermal uplift recorded as a breakup unconformity conversely at volcanic margins the onset of seafloor spreading is accompanied by the escape of the hot plume puddle along the mid ocean ridge system away from the volcanic margin leading to a pulse of rapid subsidence

interplay between magmatic accretion spreading asymmetry and detachment faulting at a segment end crustal structure south of the ascension fracture zone highlights magmatism detachment faulting and changing symmetry of crustal accretion at a ridge segment end pmps from an occ argue for constant magma production during detachment faulting refraction seismic modelling and pmp events reveal very thin 4 km oceanic crust at a segment end a wide angle seismic section across the mid atlantic ridge just south of the ascension transform system reveals laterally varying crustal thickness and to the east a strongly distorted moho that appears to result from slip along a large offset normal fault termed an oceanic detachment fault gravity modelling supports the inferred crustal structure we investigate the interplay between magmatism detachment faulting and the changing asymmetry of crustal accretion and consider several possible scenarios the one that appears most likely is remarkably simple an episode of detachment faulting which accommodates all plate divergence and results in the westward migration of the ridge axis is interspersed with dominantly magmatic and moderately asymmetric most on the western side spreading which moves the spreading axis back towards the east following the runaway weakening of a normal fault and its development into an oceanic detachment fault magma both intrudes the footwall to the fault producing a layer of gabbro subsequently partially exhumed

fault structure and detailed evolution of a slow spreading ridge segment the mid atlantic ridge at 29 n we present preliminary results of a detailed near bottom study of the morphology and tectonics of the 29 n broken spur segment on the slow spreading mid atlantic ridge using principally the tobi deep towed instrument the survey covered two thirds of the segment length including all of its southern non transform boundary and extended off axis of 40 km 3 3 ma on either side we obtained nearly complete near bottom sidescan sonar coverage and deep towed three component magnetic observations along 2 km spaced e w tracks sidescan data reveal new details of fault structure and evolution faults grow by along axis linkage in the inside corner they also link in the axis normal direction by curving to meet the next outer older fault this leads to wider spaced faults compared to segment centre or outside corner outward facing faults exist but are rare the non transform offset is characterised by faults that are highly oblique not parallel to the spreading direction and show cross cutting relations with ridge parallel faults to the north suggesting along axis migration of the offset almost all volcanic activity occurs within 5 km of the axis most fault growth is complete within 15 km of the axis 1 2 ma though large scarps continue to be degraded by mass wasting beyond there crustal magnetisation is strongly three dimensional the current neovolcanic zone is slightly oblique to earlier reversal boundaries and its magnetisation rises to a maximum of 30 a m 1 near its southern tip the central magnetisation high tapers southwards and is asymmetric with a sharp western but gradual eastern boundary we infer a highly asymmetric accretion of layer 2 near the segment end older magnetic anomalies are kinked and sometimes missing we interpret these observations as evidence of a rapid 18 km southward migration of the segment boundary during the past 1 8 ma and present a series of reconstructions illustrating this tectonic history

models of ocean ridge lithospheric deformation dependence on crustal thickness spreading rate and segmentation we use three dimensional 3 d temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid ocean ridge lithospheric structure we find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along axis gradients in crustal temperature and a pita pocket shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle in contrast fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis the overthickened crust produced by ridge hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling creating an extremely weak lithosphere in a steady state model we apply a simple two dimensional 2 d cyclic faulting model to across axis sections of temperature and rheological structure slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing in contrast fast spreading and hotspot affected segments are predicted to have much smaller variations in faulting styles except in the immediate vicinity of a major transform fault the presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness spreading rate and the ability of rift bounding normal faults to propagate along axis for fast spreading and hotspot affected segments the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma chamber supported isostatic features this is in contrast to most slow spreading segments where axial rift valleys are predicted to be a dominant morphological feature for a slow spreading segment of great length however the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center

direct geological evidence for oceanic detachment faulting the mid atlantic ridge 15 45 n from a detailed survey and sampling study of corrugated massifs north of the fifteen twenty fracture zone on the mid atlantic ridge we demonstrate that their surfaces are low angle detachment fault planes as proposed but not previously verified spreading direction parallel striations on the massifs occur at wavelengths from kilometers to centimeters oriented drill core samples from the striated surfaces are dominated by fault rocks with low angle shear planes and highly deformed greenschist facies assemblages that include talc chlorite tremolite and serpentine deformation was very localized and occurred in the brittle regime no evidence is seen for ductile deformation of the footwall synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low angle fault surface at very shallow levels directly beneath the ridge axis strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient

geophysical signatures of oceanic core complexes author posting the authors 2009 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 178 2009 593 613 doi 10 1111 j 1365 246x 2009 04184 x

crustal structure of north atlantic fracture zones seismic studies have established that large offset transforms along the slow spreading mid atlantic ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust seismically fracture zone crust in the north atlantic is extremely heterogeneous in both thickness and internal structure it is frequently quite thin 1 2 km thick and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3 a more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the mid atlantic ridge the geological nature of the seismically anomalous crust found within atlantic fracture zones and how this crust forms are still controversial one interpretation consistent with available seismic observations is that the crust within north atlantic fracture zones consists of a thin intensely fractured and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust the existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large scale detachment faults that form preferentially in the cold brittle lithosphere near the ends of segments