paleocene sea level and productivity changes at the southern tethyan margin el kef tunisia integrated analysis of quantitative distribution patterns of organic walled dinoflagellate cysts dinocysts and benthic foraminifera from the paleocene el kef section nw tunisia allows the reconstruction of sea level and productivity fluctuations our records indicate that the environment evolved from an initially oligotrophic open marine outer neritic to upper bathyal setting towards a more eutrophic inner neritic setting influenced by coastal upwelling by the end of the paleocene an overall second order change in paleodepth is reflected by both microfossil groups from the base of planktic foraminifera zone p4 onwards the main phase of shallowing is evidenced by an increase of inner neritic dinocysts of the areoligera group disappearance of deeper water benthic foraminifera and increasing dominance of shallow marine taxa several buliminids haplophragmoides spp trochammina spp the total magnitude of this shallowing is obscured by interaction with a signal of eutrophication but estimated to be around 150 m from 200 to 50 m superimposed on the overall trend third order sea level fluctuations have been identified and correlated to sequences in other regions paleoproductivity indicators notably protoperidinioid dinocysts and buliminids show a trend from an oligotrophic to a eutrophic setting d 2005 elsevier b v all rights reserved

structural analysis of sheath folds in the sylacauga marble group talladega slate belt southern appalachians remnant blocks of marble from the moretti harrah dimension stone quarry provide excellent exposure of meter scale sheath folds tubular structures with elliptical cross sections 4 ryz 5 are the most common expression of the folds the tubes are elongate subparallel to stretching lineation and are defined by centimeter scale layers of schist eccentrically nested elliptical patterns and opposing asymmetry of folds s and z are consistent with the sheath fold interpretation sheath folds are locally numerous in the moretti harrah quarry but are not widely distributed in the sylacauga marble group reconnaissance in neighboring quarries provided no additional observations the presence of sheath folds in part of the talladega slate belt indicates a local history of plastic non coaxial deformation such a history of deformation is substantiated by petrographic study of an extracted hinge from the moretti harrah quarry the sheath folds are modeled as due to passive amplification of initial structures during simple shear using both analytic geometry and graphic simulation as indicated by these models relatively large shear strains y 9 and longitudinal initial structures are required the shear strain presumably relates to nw directed displacement of overlying crystalline rocks during late paleozoic orogeny

mid to late cretaceous ductile deformation and thermal evolution of the crust in the northern dome rock mountains arizona abstract mid to late cretaceous basement involved deformation in the maria tectonic belt of southeastern california and west central arizona varies in style from low angle overthrusts to km scale fold nappes to establish the relationship between tectonite fabrics and thermal evolution of the rocks structural and petrographic analysis was conducted in the northern dome rock mountains where the two different styles converge a penetrative northeast dipping foliation forms the most conspicuous fabric in an 5 km wide zone of distributed ductile deformation within the broader zone of ductile deformation northeast dipping shear zones contain evidence for both southwest and northeast directed deformation textural analysis of mylonitic rocks reveals that southwest directed deformation occurred under prograde conditions whereas northeast directed deformation occurred under retrograde conditions southwest directed deformation resulted in tubular extreme sheath fold development on a kilometer scale in the footwall of a reverse sense shear zone northeast directed deformation cuts the map scale tubular fold and is inferred to have developed as a manifestation of gravitational collapse of rocks mechanically weakened by fluid infiltration and upper greenschist to lower amphibolite facies metamorphism we propose that the mode and degree of fluid infiltration rather than temperature or structural level alone controlled the style of deformation in the maria belt

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

inactive and relict rock glaciers of the deboullie lakes ecological reserve northern maine usa lobate talus slopes in the deboullie lakes ecological reserve dler of northern maine exhibit parabolic profiles characteristic of inactive and relict talus derived rock glaciers vegetated rock glacier surfaces suggest that the landforms are no longer active and lobes comprising two dler rock glaciers document periods of past growth observations of perennial subsurface ice are supported by datalogger temperature measurements indicating that sporadic permafrost exists throughout the dler we compare the dler rock glaciers along with similar features elsewhere in new england and adjacent quebec to the modern alpine permafrost distribution results indicate that a mean annual temperature cooling of 68c is required to promote active rock glacier growth ages of plant remains recovered from the basal sediments of a local pond constrain deglaciation to before 11 320 14 c a bp and core stratigraphy and organic content reveal that a periglacial environment persisted during the early postglacial era thus we hypothesise that the dler rock glaciers were active during lateglacial time despite the lack of glacier activity in the region we take this to suggest that north eastern us rock glaciers formed in response to mean annual temperatures skewed towards the frigid winters of the younger dryas chronozone copyright 2009 john wiley sons ltd

mid crustal cretaceous roots of cordilleran metamorphic core complexes thermobarometry for cretaceous to mid tertiary plutonism and deformation in the lower plate of whipple and santa catalina metamorphic core complexes shows that both crystalline terranes originated in the middle crust moreover they are characterized by a striking acceleration of tectonic decompression coincident with middle tertiary low angle detachment faulting leading to erosional and tectonic unroofing by mid miocene time depth estimates for emplacement of five intrusive suites within the whipple complex include 1 33 4 km for the peraluminous 89 ma whipple wash plutonic suite 2 29 1 km for the 73 ma axtel quartz diorite 3 16 5 km for mylonitization and synkinematic plutonism at 26 ma 4 6 2 1 9 km for the postkinematic 19 ma war eagle gabbro quartz diorite complex and 5 5 2 2 3 km for a 17 ma postkinematic granodiorite decompression initially occurred at a low rate of 0 3 mm yr from 89 to 26 ma and increased to approximately 2 mm yr during the late oligocene to middle miocene estimated depths for four pluton emplacement or deformational events in the santa catalina mountains include 1 21 1 km for the magmatic epidote bearing 68 ma leatherwood quartz diorite 2 15 3 km for the garnet two mica 47 ma wilderness granite 3 9 3 1 9 km for post wilderness mylonitzation and 4 6 3 2 6 km for the 27 ma catalina monzogranite post laramide decompression estimated at 0 3 mm yr accelerated to 1 3 mm yr prior to the cessation of detachment faulting whereas most batholithic terranes of the north american cordillera are representative of an upper crustal setting core complexes provide as a consequence of their tectonic evolution a petrological and structural view into middle crustal processes

a complete high resolution paleocene benthic stable isotope record for the central pacific odp site 1209 1 we have compiled the first stratigraphically continuous high resolution benthic foraminiferal stable isotope record for the paleocene from a single site utilizing cores recovered at pacific odp site 1209 the long term trend in the benthic isotope record suggests a close coupling of volcanic co2 input and deep sea warming over the short term the record is characterized by slow excursions with a pronounced periodic beat related to the short 100 kyr and long 405 kyr eccentricity cycle the phase relationship between the benthic isotope record and eccentricity is similar to patterns documented for the oligocene and miocene confirming the role of orbital forcing as the pace maker for paleoclimatic variability on milankovitch time scales in addition the record documents an unusual transient warming of 2 c coeval with a 0 6 carbon isotope excursion and a decrease in carbonate content at 61 75 ma this event which bears some resemblance to eocene hyperthermals marks the onset of a long term decline in 13c the timing indicates it might be related to the initiation of volcanism along greenland margin

danian selandian boundary criteria and north sea basin tethys correlations based on calcareous nannofossil and foraminiferal trends in sw france abstract high resolution calcareous nannofossil and foraminiferal investigations of the bidart and loubieng outcrop sections allow to define a time calibrated sequence of 47 bio events within the danian selandian d s boundary interval 61 2 59 7 ma of aquitaine sw france the d s boundary as originally defined in denmark start of clastic sedimentation at the base of the lellinge greensand formation is marked by the end of the acme of the nannofossil family braarudosphaeraceae this bio event dated at 59 9 ma has also been identified at the lithological change from limestone dominated lasseube formation to marly sedimentation latapy member of the pont labau formation in sw aquitaine and at the base of the red marls of the itzurun formation at zumaia spain recently designated as global stratotype section and point gssp for the d s boundary this implies contemporaneity of this lithological shift throughout europe and a gssp proposal which is consistent with the original boundary definition the braarudusphaeraceae event is believed to be due to the interruption of freshwater influx probably related to a sudden decrease in precipitation it is located at the top of nannofossil zone np4 and within planktonic foraminiferal zone p3b and bracketed between the lowest occurrence lo of morozovella velascoensis below and the lo of fasciculithus tympaniformis above it is coincident with the lo of bomolithus elegans the lcso cs consistent of fasciculithus janii and the lo of subbotina velascoensis the d s boundary as originally defined is 400 k y posterior to a major discontinuity recorded throughout the tethyan realm tunisia egypt and up to now erroneously considered to correspond to the d s boundary this break in sedimentation dated at 60 3 ma and coinciding with the p3a p3b boundary is due to a major sea level fall it is correlated with sedimentation changes in the aquitaine zumaia area start of development of marly interbeds and in the north sea basin transition bryozoan limestone calcisiltite in denmark transition shallow marine mons formation continental hainin formation in belgium the loubieng section supplements the zumaia section because of its rich and well preserved fossil content and continuous sedimentation it constitutes an excellent auxiliary section for the d s boundary

the bisbee basin and its bearing on late mesozoic paleogeographic and paleotectonic relations between the cordilleran and caribbean regions the bisbee group and its correlatives in southern arizona the new mexico panhandle and adjacent parts of mexico are composed dominantly of lower cretaceous nonmarine marginal marine and shallow marine deposits but apparently also include upper upper jurassic and lower upper cretaceous strata farther west in southwestern arizona and southeastern california lithologically similar nonmarine strata of the mccoy mountains formation and its correlatives occupy the same general stratigraphic position as the bisbee group but are poorly dated and may be older the rifted bisbee basin was a northwestern extension of the chihuahua trough a late mesozoic arm of the gulf of mexico depression basal zones of the bisbee group were deposited as alluvial fans marginal to active fault blocks during the rift phase of basin development and are intercalated locally with lavas and ignimbrites subsequent thermotectonic subsidence of thinned crust beneath the bisbee basin allowed intertonguing fluvial lacustrine deltaic strandline and marine shelf facies to invade the basin and bury the foundered fault block topography the upper lower cretaceous aptian albian mural limestone was deposited during the phase of maximum transgression differing sandstone petrofacies of quartzose arkosic and volcaniclastic character reflect derivation of clastic detritus from varied sources bordering the bisbee basin more the bisbee basin and chihuahua trough developed in relation to jurassic opening of the gulf of mexico and were associated with changing plate configurations and motions throughout the mesoamerican region less

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

thermal history of crystalline nappes of the maria fold and thrust belt west central arizona the thermal history of the crust in west central arizona as documented by 40ar 39ar step heating analysis of mineral suites from crystalline nappes of the maria fold and thrust belt is characterized by three distinct stages for late cretaceous and tertiary time regional heating of the crust occurred during basement involved folding and thrust faulting in middle late cretaceous time 80 90 ma and represents the earliest phase recorded by the 40ar 39ar data the second stage was dominated by uplift and slow cooling 5 10 c m y of the crust throughout latest cretaceous and early tertiary lime tectonic denudation in late oligocene to early miocene time resulted in rapid uplift and cooling of midcrastal rocks in the footwall of the whipple buckskin rawhide detachment system marking the final thermal signature in the region evidence for variable argon loss in the hornblende spectra implies exposure of differing structural levels within the thrust belt and or significant thermal heterogeneity in the crust during late cretaceous orogenesis partially outgassed hornblende in the northern granite wash mountains reflects temperatures of 450 c whereas complete resetting of hornblende at mesquite mountain suggests that this area attained temperatures in excess of 500 c in contrast data from the northern plomosa mountains indicate that this area did not attain temperatures sufficient to completely degas biotite or k feldspar 350 c during late cretaceous time the range of early tertiary muscovite and biotite ages in the region reflects slow cooling presumably associated with uplift and erosion following crustal thickening k feldspars yield saddle shaped age spectra with minimum apparent ages ranging from 22 28 ma and reflect quenching due to rapid exhumation of midcruslal to upper crustal rocks in the detachment terrain collectively the data are consistent with either of two models for the thermal evolution of the crust in middle to late tertiary time either 1 lower plate rocks were still sufficiently hot following late cretaceous heating that argon clocks were set simply due to rapid cooling or 2 the area experienced a renewed thermal input associated with extension causing argon loss from previously cool rocks based on consideration of diffusion domain behavior in k feldspars we infer that a renewed thermal pulse was associated with the early stages of extension and was related to the cause for crustal weakening

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

latest cretaceous and early tertiary orogenesis in south central arizona thrust faulting regional metamorphism and granitic plutonism rocks in a number of mountain ranges in southernmost central arizona are juxtaposed by thrust faults regionally metamorphosed and intruded by garnet two mica granites field relations and k ar and u pb isotopic geochronology indicate that thrust faulting metamorphism and granitic plutonism were closely related aspects of a latest cretaceous and early tertiary orogenic episode the regionally metamorphosed rocks chiefly greenschist facies quartzofeldspathic schists were derived from jurassic and cretaceous sedimentary volcanic and plutonic rocks in five ranges these metamorphic rocks are overlain along synmetamorphic thrust faults by precambrian gneiss or late jurassic or late cretaceous plutonic rocks in three additional ranges upward increase of textural grade within the metamorphic rocks and analogy with the areas of exposed thrust faults indicate the likelihood of a concealed thrust fault flanking the range seven of these eight thrust faults have single lower plates but one thrust is underlain by a duplex consisting of several imbricate structural sheets separated by mylonitic tectonic slides areal structural relations and the regional arrangement of distinctive jurassic igneous rock units strongly suggest that one large range the baboquivari mountains and several smaller ranges are fensters in a thrust fault system of regional extent and that several adjacent ranges are allochthonous the regionally metamorphosed rocks are intruded by synmetamorphic to postmetamorphic early tertiary leucocratic granites characterized by various combinations of accessory biotite muscovite and garnet at least two of these granites contain inherited precambrian zircon indicative of generation by crustal anatexis k ar and or u pb isotopic ages of premetamorphic granites metamorphic rocks thrust zone mylonites synmetamorphic granites and metamorphogenic fissure veins indicate that the orogenic episode commenced in late cretaceous time and culminated in early tertiary time 58 to 60 m y ago the observed and inferred close spatial and temporal relations between thrust faulting regional metamorphism and granitic plutonism lead to the following hypothesis for orogenesis in south central arizona crustal compression caused overthrusting of crystalline rocks resulting in crustal thickening and crustal thickening and conductive and magmatic heat flux from the mantle together set up a thermal regime within which regional metamorphism and the generation and emplacement of the anatectic granites took place

development of tongue shaped and multilobate rock glaciers in alpine environments interpretations from ground penetrating radar surveys abstract rock glaciers occur as lobate or tongue shaped landforms composed of mixtures of poorly sorted angular to blocky rock debris and ice these landforms serve as primary sinks for ice and water storage in mountainous areas and represent transitional forms in the debris transport system accounting for 60 of all mass transport in some alpine regions observations of active flowing alpine rock glaciers indicate a common association between the debris that originates from cirque headwalls and the depositional lobes that comprise them the delivery of this debris to the rock glacier is regulated primarily by the rate of headwall erosion and the point of origin of debris along the headwall these factors control the relative movement of individual depositional lobes as well as the overall rate of propagation of a rock glacier in recent geophysical studies a number of alpine rock glaciers on prins karls forland and nordenskioldland svalbard norway and the san juan mountains of southwest colorado usa have been imaged using ground penetrating radar gpr to determine if a relationship exists between the internal structure and surface morphology results indicate that the overall morphologic expression of alpine rock glaciers is related to lobate deposition during catastrophic episodes of rockfall that originated from associated cirque headwalls longitudinal gpr profiles from alpine rock glaciers examined in this study suggests that the difference in gross morphology between the lobate and tongue shaped rock glaciers can be attributed primarily but not exclusively to cirque geometry frequency and locations of debris discharge within the cirque and the trend and magnitude of valley gradient in relation to cirque orientation collectively these factors determine the manner in which high magnitude debris discharges which seem to be the primary mechanism of formation accumulate to form these rock glaciers

geologic setting and tertiary structural evolution of southwestern arizona and southeastern california volcanic and sedimentary rocks and structures record the tertiary structural evolution of the lower colorado river region in southwestern arizona and southeastern california a late eocene or early oligocene prior to 33 ma episode of faulting is indicated by medium to coarse grained arkosic rocks in the chocolate and southern trigo mountains much of the area was relatively quiescent tectonically during extrusion of volcanic rocks from 33 to 22 ma but the southernmost part was periodically uplifted and eroded into its underlying crystalline rocks a major episode of extensional deformation and tilting occurred after deposition of welded tuff about 22 ma and affected the entire area from the palo verde mountains on the west to the kofa mountains on the east extension related faulting quickly waned and had largely ceased by about 20 ma in the kofa mountains elsewhere the timing is poorly constrained by 13 ma thick alluvial fans filled many grabens and half grabens among tilted fault blocks throughout the area volcanism in the lower colorado river region may have been coincident with a broad structural depression now oriented east west the northern limit of the volcanic terrane defines a tilt domain boundary the northern boundary reaching from the new water mountains in arizona to the little chuckwalla mountains in california ultimately evolved to separate a terrane of relatively untilted crystalline horsts on the north from a series of east or northeast dipping fault blocks on the south the southern margin is less well defined but is subparallel to the northern boundary and to the chocolate mountains anticlinorium

stratigraphic relations and u pb geochronology of the upper cretaceous upper mccoy mountains formation southwestern arizona a previously unrecognized angular unconformity divides the jurassic and cretaceous mccoy mountains formation into a lower and an upper unit in the dome rock mountains and livingston hills of western arizona the lower unit of the mccoy mountains formation consists of generally fine grained quartzose and volcaniclastic strata that were deposited after cessation of middle jurassic explosive volcanism the basal contact of the lower unit is disconformable in most places but locally it has been interpreted to be gradational with the underlying silicic volcanic rocks the upper unit is a fining upward sequence of quartzo feldspathic and arkosic conglomerate and sandstone that records uplift of a northern source terrane a tuff in the lower part of the upper unit has a u pb crystallization age of 79 2 ma late cretaceous rocks of the lower unit are deformed by pre 80 ma thrust faults of the generally southward vergent maria fold and thrust belt which bounds the outcrop belt of the mccoy mountains formation on the north the upper unit is exposed only south of the fold and thrust belt we interpret the intraformation unconformity in the mccoy mountains formation to have developed where rocks of the lower unit were deformed adjacent to the southern margin of the maria fold and thrust belt the upper unit of the formation is interpreted as a foreland basin deposit that was shed southward from the actively rising and deforming fold and thrust belt the apparent absence of an equivalent unconformity in the mccoy mountains formation in adjacent california is presumably a consequence of the observed westward divergence of the outcrop belt from the fold and thrust belt continued southward shortening deformed the entire formation under greenschist and locally amphibolite facies conditions soon after the upper unit was deposited tectonic burial beneath the north vergent mule mountains thrust system in the latest late cretaceous 70 ma marked the end of mesozoic contractile deformation in the area

u th pb rb sr and ar ar mineral and whole rock isotopic systematics in a metamorphosed granitic terrane southeastern california mesozoic structural domes are developed in an older proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the halloran hills southeastern california isotopic analyses of whole rocks and mineral separates from these rocks by u th pb rb sr and ar ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history individual mineral isotopic systems have variably equilibrated with each other in response to mesozoic regional metamorphism and locally to later heating during mesozoic batholith emplacement discordant u th pb zircon data indicate that the granitic core rocks are 1 710 ma and that one dioritic phase may be slightly older rb sr whole rock model dates scatter about 1 700 ma rb sr amphibole whole rock and u th pb amphibole dates are also proterozoic potassium feldspars retain a 207 pb 206 pb signature of their proterozoic age ar ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1 450 ma to 1 100 ma and the dates become younger toward the structurally deeper core of the dome all remaining isotopic determinations yield mesozoic or younger dates for mineral whole rock systems rb sr whole rock apatite feldspar biotite analyses show nonequilibration of strontium isotopes with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 ma no single metamorphic age is indicated by the rb sr data rb sr whole rock biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water u pb sphene and apatite analyses from rocks that yield 1 710 ma zircon dates are nearly concordant at 140 ma an amphibole from the structurally deepest rocks of the main dome that yield 140 to 150 ma u pb sphene dates has an ar ar plateau date of 144 ma the u pb sphene and ar ar amphibole analyses are believed to be the best age estimate for the end of the highest temperature phase of regional metamorphism th pb sphene and apatite dates and ar ar biotite dates cluster at 90 5 ma as a consequence of regional cooling during late cretaceous time following extensive mesozoic plutonism in the region at 97 to 90 ma we interpret the discordant mineral date patterns to have resulted from metamorphism of 1 700 ma plutonic rocks during the jurassic 140 50 ma and subsequent uplift and cooling to 200 c at about 90 ma on the basis of this study the isotope dating systems ranked in decreasing order of resistance to resetting are u th pb zircon concordia intercept rb sr whole rock rb sr amphibole u th pb amphibole pb pb whole rock ar ar amphibole rb sr sphene u pb sphene and apatite rb sr plagioclase potassium feldspar apatite th pb sphene and apatite ar ar biotite u pb feldspars rb sr biotite

the pinto shear zone a laramide synconvergent extensional shear zone in the mojave desert region of the southwestern united states the pinto shear zone is one of several late cretaceous shear zones within the eastern fringe of the mesozoic magmatic arc of the southwest cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening we demonstrate an extensional origin for the shear zone by describing the shear zone geometry and kinematics hanging wall deformation style progressive changes in deformation temperature and differences in hanging wall and footwall thermal histories deformation is constrained between w74 and 68 ma by 40 ar 39 ar thermochronology of the exhumed footwall including multi diffusion domain modeling of k feldspar we discount the interpretations applied in other areas of the mojave desert region that widespread late cretaceous cooling results from refrigeration due to subduction of a shallowly dipping laramide slab or to erosional denudation and suggest alternatively that post intrusion cooling and exhumation by extensional structures are recorded widespread crustal melting and magmatism followed by extension and cooling in the late cretaceous are most consistent with production of a low viscosity lower crust during anatexis and or delamination of mantle lithosphere at the onset of laramide shallow subduction q 2005 published by elsevier ltd

in search of the latest danian event in a paleobathymetric transect off kasserine island north central tunisia danian lower paleocene sequences in north central tunisia are dominantly composed of marls and shales but a conspicuous indurated glauconite bearing marker bed is associated with the p3a p3b transition this glauconite bed is considered to correlate with the latest danian event lde described from the nile basin in egypt with the top chron c27n event atlantic and pacific oceans and with the cie ds1 zumaia spain the lde is thought to reflect a short period of global warming similar to the paleocene eocene thermal maximum but of lesser magnitude the presence of a glauconitic bed at the p3a p3b transition in tunisia suggests that the sequence is condensed which is confirmed by planktic foraminifer and nannofossil biostratigraphy and by the absence of the typical lde beds found in most egyptian sections benthic foraminiferal assemblages were quantitatively analyzed in three sections in north central tunisia sidi nasseur garn halfaya elles in order to characterize the paleoenvironmental evolution during the late danian and compare this with sections in egypt the benthic foraminiferal assemblages indicate that the tunisian sections were located along a depth and a paleoenvironmental gradient on the shelf north of kasserine island the reconstructed depth range is comparable with sections in egypt and encompasses middle neritic sidi nasseur section to shallow upper bathyal depths elles section with up to 16 gavelinella beccariiformis below the p3a p3b like in egypt assemblage shifts across the p3a p3b subzonal transition indicate shallowing and a transition to a more eutrophic paleoenvironment characterized by relatively high abundances of buliminids and stainforthia sp the 13c and 18o records generated on well preserved specimens of the ostracode species bairdia failed to demonstrate the presence of the carbon isotope excursion cie associated with the lde at the base of subzone p3b a condensed section or a hiatus associated with the glauconite bed would explain why the cie is not recorded in the tunisian sections

characterization of the latest danian event by means of benthic foraminiferal assemblages along a depth transect at the southern tethyan margin nile basin egypt abstract the latest danian event lde has been recognized on the southern tethyan margin egypt tunisia and in the atlantic zumaia spain and pacific oceans odp site 1209 based on a supraregional carbon isotope excursion and a negative shift in oxygen isotopes in the pacific it has been suggested that the lde is an early paleogene transient warming event so far the environmental effects of the lde have been observed in few sections and details on its impact and duration are scarce we present a quantitative study of benthic foraminiferal assemblages retrieved from five sections along a depth transect on the paleocene southern tethyan shelf nile basin egypt to assess paleoenvironmental change during the lde the lithologic sequences and foraminiferal assemblages indicate that the onset of the lde is related to widespread shelf dysoxia the organic rich laminated marls of lower lde bed i contain levels devoid of benthic foraminifera during the later stage of the lde dark gray shales of bed ii the shelf is repopulated by a neoeponides duwi benthic assemblage occurring in all sections initiating a gradual restoration of normal marine shelf environments q mode and r mode correspondence analysis assist in the interpretation of the n duwi assemblage which is related to disturbed conditions at the sea floor following oxygen depletion and increased organic loading the sharp lithologic boundary at the base of the lde suggests that the event coincides with a rapid transgression following a sea level fall with an estimated amplitude of 50 m or less comparison with the dan c2 and elpe mpbe two proposed transient warming episodes preceding and postdating the lde shows that the three paleocene events have several characteristics in common however the duration of the lde 200 kyr exceeds the estimated duration of the other events and a sea level cycle is only reported from the lde

constraints on the tectonics of the mule mountains thrust system southeast california and southwest arizona the mule mountains thrust system crops out discontinuously over a 100 km strike length in the blythe quartzsite region of southeast california and southwest arizona along the thrust system middle and upper crustal metamorphic and plutonic rocks of proterozoic and mesozoic age are thrust north northeastward 015 degree to 035 degree over a lower plate metamorphic terrane that formed part of the proterozoic north american craton its paleozoic sedimentary rock cover overlying mesozoic volcanic and sedimentary rocks and the intruding jurassic and cretaceous granitic rocks stratigraphic petrologic and pb isotopic ties for jurassic granitoids and for jurassic and cretaceous sedimentary rocks across the various parts of the thrust system indicate that related crustal blocks are superposed and preclude it from having large displacements the thick skinned thrust system is structurally symmetrical along its length with a central domain of synmetamorphic thrust faults that are flanked by western and eastern domains where lower plate domains where lower plate synclines underlie the thrusts deformation occurred under low greenschist facies metamorphic conditions in the upper crust movement along the thrust system was probably limited to no more than a few tens of kilometers and occurred between 79 plus minus 2 ma and 70 plus minus 4 ma the superposition of related rocks andmore the geometry of the thrust system preclude it from being a major tectonic boundary of post middle jurassic age as has been previously proposed rather the thrust system forms the southern boundary of the narrow zone of cretaceous intracratonic deformation and it is one of the last tectonic events in the zone prior to regional cooling less

rock glaciers and related periglacial landforms in the sierra nevada ca usa inventory distribution and climatic relationships abstract rock glaciers and related periglacial rock ice features rifs are abundant yet overlooked landforms in the sierra nevada california where they occur in diverse forms we mapped 421 rifs from field surveys and grouped these into six classes based on morphology and location these categories comprise a greater range of frozen ground features than are commonly described in rock glacier surveys mapped features extended from 2225 to 3932 m modern mean 3333 m occurred mostly on nnw to nne aspects and ranged in apparent age from modern to relict late pleistocene many of the smaller features mapped here are not readily discernible with remote e g air photo observation field surveys remain the best approach for their detection we interpreted the presence of outlet springs basal lakes suspended silt in outlet streams and fringing phreatophytic vegetation in addition to morphologic indications of current rock movement as evidence for interstitial ice either persistent or seasonal the six classes were distinct in their geographic settings and morphologic conditions indicating process level differences to assess modern climate we intersected mapped locations with the 30 arcsec prism climate model discriminant analysis indicated significant differences among the climate means of the classes with the first three canonical vectors describing 94 of the differences among classes mean annual air temperatures maat for modern features ranged from 0 3 to 2 2 c mean precipitation ranged from 1346 to 1513 mm we calculated differences between modern and pleistocene temperatures in two ways one based on elevation differences of modern and relict rifs 662 m and standard lapse rate the other using prism estimates for the first we estimate the difference in maat as 3 9 c range 2 2 to 7 9 c from prism the difference was 3 3 c range 1 0 to 6 1 c in that persistent snowfields and glaciers are retreating in the sierra nevada under warming climates rifs will likely become increasingly important in prolonging water storage during the warm season and providing small but distributed water reserves for biodiversity and runoff their presence and water contributions would benefit by further hydrologic study

sheath folds as discriminators of bulk strain type abstract nested elliptical closures defining eye folds represent classic y z cross sections through highly curvilinear sheath folds generated during intense ductile deformation in metamorphic rocks systematic analysis of 1425 such eye folds based on our own field observations and examples from the published literature reveals distinct and consistent differences in ellipticites measured from the outer r yz to the inner most r y z elliptical rings of individual sheaths the variation in overall aspect ratios from outer to inner rings is defined as r where r r yz r y z and may display a relative increase or decrease in ellipticity to define cats eye r bulls eye r 1 fold patterns respectively layer thicknesses may also be measured along the y axis parallel to the axial surface t y and at 90 to this along the z axis t z to define the ratio of t yz sheath folds generated during broadly simple shear deformation k 1 display mean r yz 4 61 t yz 3 31 and cats eye folds r 0 69 sheath folds generated during general shear also display cats eye folds with identical mean r 0 69 values but greater thickness variations and elliptical ratios t yz 4 35 r yz 5 76 thus within both simple and general shear dominated deformations the overall variation in layer thickness t yz and ellipticity of eye folds r yz increases with increasing deformation and a greater component of pure shear whilst the r value remains constant and reflects original fold patterns sheath folds formed during constrictional k 1 deformation display markedly lower aspect ratios r yz 2 42 and thickness variations t yz 2 94 together with distinctive bulls eye folds r 1 23 these empirical relationships suggest fundamental and universal constraints on curvilinear fold generation across this broad spectrum of deformation types and allow sheath folds to act as both effective 95 consistent and robust discriminators of bulk strain type

the three dimensional shape and localisation of deformation within multilayer sheath folds abstract sheath folds are widely believed to develop by the passive geometric amplification of folds in which layering has no mechanical influence during non coaxial deformation where layering becomes rheologically significant then active sheath folds may form in which the inner nose of the sheath decouples along detachments and undergoes a translation relative to the outer folded layers that surround and envelope the nose we present a systematic 3 d analysis of multiple folded layer geometries in a serially sectioned natural sheath fold weak layers which are reactivated as detachments define different shaped sheath folds relative to other layers with the sense of cut off along detachments reversing across the axial surface as the inner fold nose has protruded into the surrounding sheath envelope detachment layers are more tightly folded meaning that such active sheath folds are non similar shapes the obliquely oriented bifurcating en echelon nature of the fold hinges developed in adjacent layers suggest that pre cursor folds formed with pronounced 3 d obliquity relative to the subsequent shear plane mineral lineations folded around sheath closures display asymmetric star burst patterns consistent with recrystallisation during active folding and hinge rotation we show that the eye fold shapes exposed in any 2 d y z slice can be used to predict the geometry of marker horizons back along the x axis in the third dimension this self similarity may be of value when tracing stratiform mineralised horizons in large scale sheath folds

subsurface investigation of a rock glacier using ground penetrating radar implications for locating stored water on mars 1 the discovery of rock glacier like features on mars suggests the presence of flowing or once flowing ice rock mixtures these landforms which include lobate debris aprons concentric crater fill and lineated valley fill hold significant promise as reservoirs of stored water ice that could be used as fuel sources for human exploration of mars and provide a frozen record of the climatic history of the planet to understand the deformation and distribution of ice within these landforms fundamental descriptions of their internal structure and development are required to this end a ground penetrating radar investigation was initiated using rock glaciers in the san juan mountains of colorado as surrogates for similar martian landforms results obtained from one of these rock glaciers show that the interior of the landform is composed of a layered permafrost matrix of ice sediment and ice lenses that comprise thicker depositional units formed through high magnitude debris falls folds in the uppermost layers correspond to the surface expression of ridges and furrows suggesting that compressive stresses originating in the accumulation zone are transmitted downslope through the rock glacier rock glacier features on mars may also consist of layered permafrost which would suggest a history of development involving seasonal frost accumulation and or water influx from below in terms of water storage within martian analogs consideration must include the possibility that some water ice may be stored in relatively pure form within lenses and vein networks such as observed in the surrogate rock glacier of this study

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

successive development of plane noncylindrical folds in progressive deformation in the history of superposed deformations of the iron formations at the western border of the kolar gold field in s india an important event was the successive growth of broadly coaxial plane noncylindrical folds in course of a progressive deformation concomitant with development of ductile mesoscopic shear zones the noncylindrical folds were initiated as active folds by the creation of a buckling instability at successive stages on newly developed foliation surfaces the nucleation of noncylindrical folds and the subsequent axial plane folding of the tightened mature folds are explained by the mechanical inhomogeneity of the rocks and the heterogeneous character of strain the correlation between increasing tightness and increasing noncylindricity of the folds indicates that the initial curvatures of hinge lines were accentuated by an extension parallel to the subhorizontal stretching lineation from the patterns of deformed lineations over folds of varying tightnesses it is concluded that the passive accentuation of hinge line curvatures was mostly achieved when the folds had already become isoclinal or very tight

tubular folds and sheath folds definitions and conceptual models for their development with examples from the grapesvare area northern sweden abstract two parameters are used in the classification of highly non cylindrical folds the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross section of the structure at the position where the hinge angle was measured if in a non cylindrical fold a cross section can be selected so that the hinge angle w measured where the cross section cuts the cone is less than 90 and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross section the fold is a sheath fold a tight sheath fold with w x y 1 can be termed a tubular fold a tubular fold is thus an extreme type of sheath fold while sheath folds with w 20 but x y 0 25 but tubular folds from the grapesvare area comply to the definition given above and they occur in a structural setting that is consistent with the models described

the depth dependency of planktonic benthic foraminiferal ratios constraints and applications abstract study of the ratio between planktonic and benthic foraminifera in a great number of areas shows that variation of this ratio with depth can be described using organic matter flux equations essentially the share of benthic foraminifera in the total association is inversely proportional to depth as is the amount of organic matter resulting from primary productivity reaching the sea bottom variation in the p b ratio from area to area appears to be strongly dependent on the amount of inbenthic living foraminifera if the ratios are corrected for this the regressions between the proportion of planktonic foraminifera p and depth appear to be near identical in the gulf of mexico the gulf of california the west coast of the usa and the adriatic sea this regression can be described by depth e 3 58718 0 03534 p a study of the behaviour of this function in nine cores from the adriatic upper quaternary reveals that the palaeodepth reconstructions are not influenced by fluctuations in productivity and temperature but that they are significantly influenced by redeposition of fine grained deposits

progressive deformation and superposed fabrics related to cretaceous crustal underthrusting in western arizona u s a abstract in the maria fold and thrust belt a newly recognized e trending cretaceous orogenic belt in the southwestern united states ductile thrusts large folds and superposed cleavages record discordant emplacement of crystalline thrust sheets across previously tilted sections of crust style of deformation and direction of thrusting are in sharp contrast to those of the foreland fold thrust belt in adjacent segments of the cordillera the net effect of polyphase deformation in the maria belt was underthrusting of paleozoic and mesozoic metasedimentary rocks under the proterozoic crystalline basement of north america the structure of the maria belt is illustrated by the granite wash mountains in west central arizona where at least four non coaxial deformation events d 1 d 4 occurred during the cretaceous sse facing d 1 folds are associated with s directed thrusts and a low grade slaty cleavage d 1 structures are truncated by the gently dipping hercules thrust zone d 2 a regional sw vergent shear zone that placed proterozoic and jurassic crystalline rocks over upturned paleozoic and mesozoic supracrustal rocks exposures across the footwall margin of the hercules thrust zone show the progressive development of folds cleavage and metamorphism related to thrusting d 3 and d 4 structures include open folds and spaced cleavages that refold or transect d 1 and d 2 folds the d 2 hercules thrust zone and a d 3 shear zone are discordantly crosscut by late cretaceous plutons

tectonics of mid tertiary extension along a transect through west central arizona large magnitude miocene extension in west central arizona occurred primarily along three imbricate northeast dipping normal faults the structurally highest of these faults the gently dipping buckskin rawhide detachment fault accommodated approximately 66 km of crustal extension whereas the two structurally lower faults accommodated a total of about 20 km extension due to this large magnitude extension an area at the earth s surface that was 10 to 20 km wide is now over a 100 km wide and crystalline rocks with mid tertiary mylonitic fabrics uncovered by detachment faulting are exposed over roughly 2000 km in the harcuvar metamorphic core complex most of the upper plate of the buckskin rawhide detachment fault was largely undeformed by internal extension only the thin tapered end of the upper plate was highly extended during extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded forming extension parallel folds in the lower plate low flexural strength characterized the lower plate during denudation and a highly mobile low viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere

the geometry and topology of natural sheath folds a new tool for structural analysis abstract curvilinear sheath folds are classically depicted as displaying symmetrical geometries about two orthogonal mirror planes centred along the x y axial surface and the x z medial culmination depression surface which bisects the fold nose however 10 000 geometric analyses of minor folds and fabrics formed during ductile thrusting in the caledonides of northern scotland reveals that major dome and basin sheath folds can display distinct and predictable asymmetries across both axial and medial surfaces the strain is typically heterogeneous so that structural fabrics and younging evidence are preserved within sheath folds at varying stages of development this allows an analysis of the evolution of such structures from tongue folds to more extreme tubular forms geometric relationships between measured orientations of fold hinges axial planes extension lineations and foliations are compared on fabric topology plots ftps which provide an effective tool for monitoring planar and linear fabric rotations with increasing progressive non coaxial deformation they consistently display systematic variation from regions of lower to higher strain on passing from upper to lower fold limbs across major axial surfaces and on crossing medial surfaces from short to long hinge line segments axial and medial surfaces effectively therefore divide major sheath folds into quadrants with different amounts senses and combinations of planar and linear fabric rotation within each domain such heterogeneous deformation implies that models of intense non coaxial deformation uniformly affecting pre existing folds may overestimate bulk displacement and shear strain variable fold hinge line rotation about medial surfaces also provides an effective mechanism for the closure of major sheaths which may otherwise project for unfeasible distances in the x direction bedding cleavage intersections are developed at greater angles to the transport direction than fold hinges which they transect in a consistent and predictable sense thereby confirming the direction of fold rotation even in areas which lack information on fold facing in cross section asymmetric tear drop eyes indicate the sense of fold hinge line vergence whilst fold limb and hinge line asymmetry may be combined on 3 d vergence ranking and rotation grids which allow location and relative strain states of minor structures to be accurately predicted within the overall sheath framework systematic fabric analysis on ftps may be applied to the investigation of ductile deformation across a broad range of scenarios

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

deformation of granitoids at low metamorphic grade ii granular flow in albite rich mylonites abstract the high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite rich polyphase aggregates these aggregates formed by retrograde breakdown reactions from intermediate plagioclase and k feldspar the predominant deformation mechanism changes in granitoids as the grain size is reduced coarse grained low strain examples are deformed by a combination of intracrystalline plasticity quartz and fracturing feldspar in the mylonites intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non cataclastic granular flow of polyphase aggregates consisting largely of albite and quartz deformation appears to be stable probably because grain growth in albite quartz mixtures is inhibited these fine grained aggregates are mechanically weaker than pure quartz aggregates thus the change in deformation mechanism mainly due to feldspar breakdown reactions appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar the rheological behaviour of albite dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar dominated lithologies 1 feldspar in the presence of aqueous fluids can be an important mineral controlling the rheology at low metamorphic grades that is below amphibolite facies p t conditions 2 the occurrence of granular flow of fine grained polyphase aggregates in low grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz

surface area control of organic carbon accumulation in continental shelf sediments the relationship between organic carbon oc and grain size found in most continental shelf sediments is here reinterpreted in terms of the surface area of the sediments cores from many north american shelf environments show downcore decreases in oc to similar refractory background concentrations if expressed relative to the surface area of the sediments this consistent concentration is 0 86 mg oc m 2 which is equivalent in concentration to a monolayer of organic matter coating all mineral surfaces a more global collection of sediment water interface samples show that this relationship is even more extensive with exceptions occurring in areas of very high riverine sediment input organic pollution or low oxygen water columns density separations indicate that organic matter is largely adsorbed to mineral grains the microtopography of surfaces was examined with n2 sorption and most surface area was found to be inside pores of 10 nm width these data lead to a hypothesis that organic matter is protected by its location inside pores too small to allow functioning of the hydrolytic enzymes necessary for organic matter decay such protection would likely work in concert with other protection mechanisms such as humification this consistent surface area correlation with oc concentration may explain control of spatial and temporal variations in oc burial rates by sedimentation rates the pore protection hypothesis provides a causal mechanism for this observed control