the rock elm meteorite impact structure wisconsin geology and shock metamorphic effects in quartz the rock elm structure in southwest wisconsin is an anomalous circular area of highly deformed rocks 6 5 km in diameter located in a region of virtually horizontal undeformed sedimentary rocks shock produced planar microstructures pms have been identified in quartz grains in several lithologies associated with the structure sandstones quartzite pebbles and breccia two distinct types of pms are pres ent p1 features which appear identical to planar fractures pfs or cleavage and p2 features which are interpreted as possible incipient planar deformation features pdfs the latter are uniquely produced by the shock waves associated with meteorite impact events both types of pms are oriented parallel to specific crystallographic planes in the quartz most commonly to c 0001 11 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 2 end document 2 and r z 10 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 1 end document 1 the association of unusual structurally deformed strata with distinct shock produced microdeformation features in their quartz bearing rocks establishes rock elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact preliminary paleontological studies indicate a minimum age of middle ordovician for the rock elm structure a similar age estimate 450 400 ma is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks

rubidium strontium dating of ore deposits hosted by rb rich rocks using calcite and other common sr bearing minerals the authors have tested a rb sr technique that permits ore deposits to be dated using common gangue minerals such as calcite and fluorite the only conditions the deposit must meet are that 1 it have minerals with a low rb sr ratio and 2 it be enclosed by wall rock with a high rb sr ratio because hydrothermal minerals acquire a strontium isotope composition that is usually similar to that of the wall rock minerals with low rb sr ratio should record and retain the isotopic composition that the wall rock had at the time of mineralization the difference between that ratio and that of the wall rock at present is a function of time and the rb sr composition of the wall rock the technique was tested fusing fluorite and calcite from three deposits ranging in age from tertiary to precambrian in all cases the age determined here closely resembles that obtained by conventional k ar and rb sr dating methods the precision however can be poor and depends chiefly on the strontium isotope heterogeneity of the wall rock and its rb sr enrichment 36 references 1 figure 1 table

constraints on the emplacement age of the heart mountain slide northwestern wyoming abstractthe heart mountain slide is the largest terrestrial landslide deposit as yet recognized on earth the slide covers an area of at least 3400 km2 and the upper plate rocks include 2 4 km of paleozoic carbonate and eocene volcanic rocks thrust out over 45 km of eocene landscape the precise age and duration of sliding is critical to emplacement models as well the slide s effect on regional eocene river systems to address the timing issues we sampled zircons from the basal fluidized layer 2 km from the slide s breakaway fault silver gate mt and 40 km downslope nearer the slide s toe white mountain wy within this basal layer we have identified mineral content and features consistent with a partially solidified magma we interpret these observations to be consistent with the slide catastrophically dismembering an active magma body that mixed with the basal fault layer the results yield remarkably similar u pb zircon crystallization ages at the proximal and distal locations 48 78 0 51 ma

geophysical investigations and crustal structure of the north american midcontinent rift system abstract geophysical investigations have had a prominent role in studying the 1100 ma midcontinent rift system because of limited surface exposures and deep drilling of the rift rocks gravity and magnetic anomaly data delineate the rift system its central volcanic filled graben and overlying and adjacent post rift sedimentary basins and the major faults of the system over its 2000 km length in central north america crustal seismic studies indicate a high velocity crust that is thickened to as much as 55 km along the rift deeply penetrating seismic reflection data obtained across the rift show that the crust was thinned to less than one half of its original thickness during rifting and the present crust has been thickened by addition of up to a total of 30 km of mantle derived volcanic rocks within grabens and clastic sediments deposited in overlying sag basins as well as by intrusions into the crust and crustal underplating these data also confirm the occurrence of a post rift regional compressional event that locally caused broad folding of the rift rocks and reactivation of the graben normal faults as high angle reverse faults with vertical throws of several kilometers geophysical data indicate that the rifting event had a profound effect on the entire vertical range of the crust and that structures and thus the tectonic evolution are similar along the entire length of the rift

the kapuskasing uplift a geological and geophysical synthesis over the past decade the kapuskasing uplift has been the subject of intense geological and geophysical investigation as lithoprobe s window on the deep crustal structure of the archean superior province enigmatic since its recognition as a positive gravity anomaly in 1950 the structure has been variably interpreted as a suture rift transcurrent shear zone or intracratonic thrust diverse studies including geochronology geothermobarometry and various geophysical probes provide a comprehensive three dimensional image of archean 2 75 2 50 ga crustal evolution and proterozoic 2 5 1 1 ga cooling and uplift the data favour an interpretation of the structure as an intracratonic uplift related to hudsonian collision eastward across the southern kapuskasing uplift erosion levels increase from 10 km in the michipicoten greenstone belt through the wawa gneiss domain 10 20 km into granulites 20 30 km of the kapuskasing structural zone juxtaposed against the low grade swayze greenstone belt al

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

structural interpretation of the midcontinent rift in eastern lake superior from seismic reflection and potential field studies integrated interpretations of potential field and glimpce and industry seismic reflection data in eastern lake superior reveal the structural and stratigraphic complexity of the midcontinent rift in this region projection of the keweenaw fault into southeastern lake superior suggested by early potential field studies is confirmed by seismic reflection data analysis of seismic data in conjunction with aeromagnetic anomalies and regional gravity data also reveals a continuous section of basalt in the footwall of the keweenaw fault the lateral dimensions of this section vary along the strike of the rift from the center of the basin towards the southern flank spatially extensive anticlinal and synclinal features reverse faults and related drag folds imaged by the reflection and enhanced potential field data attest to the influence of a late stage compressional event in this region east northeast trending gradients and displacements associated with observed potential field anomalies and fault traces mapp

proterozoic reactivation of the southern superior province and its role in the evolution of the midcontinent rift major reverse faults associated with the late compressional phase of the 1 1 ga midcontinent rift in the western lake superior region appear to cut across the rift at the eastern end of the lake and join with reverse faults on the eastern shoreline defined on the basis of geological and potential field data the continuation of the faults across eastern lake superior is inferred on evidence drawn from nearshore shipborne magnetic surveys together with new interpretations of published bathymetric and glimpce aeromagnetic data in the archean superior province about 100 km east of lake superior paleomagnetic and petrographic data from the 2 45 ga matachewan dyke swarm show that the kapuskasing zone a narrow belt of uplifted crust can be extended to within 50 km of the lake superior shoreline and has bounding reverse faults that are almost continuous with two faults of similar dip and sense of displacement that define the inversion of the midcontinent rift in the central and western parts of the lake si

pre glacial and interglacial pollen records over the last 3 ma from northwest canada why do holocene forests differ from those of previous interglaciations we synthesize pollen spectra from eleven dated stratigraphic sections from central and northern yukon palaeomagnetic and tephra dating indicates the earliest assemblages representing closed canopy pinus and picea forest are middle late pliocene age more open forest conditions indicated by increased poaceae and with evidence of permafrost are dated at ca 3 ma while pinus pollen is abundant at 3 ma it is reduced in records after 2 6 ma and subsequent pleistocene interglacial forest records are repeatedly dominated by picea along with alnus and small but significant amounts of abies surface sample comparisons indicate that abies was more widespread and abundant in past interglaciations than at present and that middle pleistocene piceaeabies forest grew in the northern yukon porcupine basin 500 km beyond modern abies limits in contrast pinus which occurs today in southern and central yukon was not a significant component of these pleistocene interglacial forests late holocene pollen assemblages with rare abies and high pinus are the most distinct in the past 2 6 ma possible factors driving holocene difference are paleoclimate paludification changes in megafaunal herbivory and an unusual fire regime anthropogenic burning is a factor unique to the holocene and if it is shown to be important in this case it would challenge our notion of what constitutes boreal wilderness

comment on paleozoic reactivation structures in the appalachian ouachita marathon foreland far field deformation across pangea by j craddock d malone r porter j compton j luczaj a konstantinou j day and s johnston earth science reviews 169 2017 1 34

closing of the midcontinent rift a far field effect of grenvillian compression the midcontinent rift formed in the laurentian supercontinent between 1109 and 1094 ma soon after rifting stresses changed from extensional to compressional and the central graben of the rift was partly inverted by thrusting on original extensional faults thrusting culminated at about 1060 ma but may have begun as early as 1080 ma on the southwest trending arm of the rift the crust was shortened about 30 km on the southeast trending arm strike slip motion was dominant the rift developed adjacent to the tectonically active grenville province and its rapid evolution from an extensional to a compressional feature at ca 1080 ma was coincident with renewal of northwest directed thrusting in the grenvllle probably caused by continent continent collision a zone of weak lithosphere created by rifting became the locus for deformation within the otherwise strong continental lithosphere stresses transmitted from the grenville province utilized this weak zone to close and invert the rift

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

crustal scale thrusting and origin of the montreal river monocline a 35 km thick cross section of the midcontinent rift in northern michigan and wisconsin a structurally simple 35 km thick north facing stratigraphic succession of late archean to middle proterozoic rocks is exposed near the montreal river which forms the border between northern wisconsin and michigan this structure the montreal river monocline is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the keweenawan supergroup middle proterozoic along the south limb of the midcontinent rift and disconformably underlying sedimentary rocks of the marquette range supergroup early proterozoic these rocks lie on an archean granite greenstone complex about 10 km of which is included in the monocline this remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension the faults responsible for the monocline are newly recognized components of a well known regional fault system that partly closed and inverted the midcontinent rift system resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 20 ma and followed very shortly after extension that formed the midcontinent rift system

post keweenawan compressional faults in the eastern lake superior region and their tectonic significance glimpce aeromagnetic data in eastern lake superior are characterized by a series of strong easterly and northeasterly oriented gradients that relate to mapped post keweenawan faults occurring along the eastern shore the reversed nature of three of the faults is established through field observations and potential field modelling middle keweenawan volcanic rocks at mamainse point are in fault contact on their south side with upper keweenawan sandstone of bayfield jacobsville type gravity modelling suggests that the fault is a low angle thrust dipping to the north field observations and high resolution aeromagnetic data show that it extends inland along the southern margin of the batchawana greenstone belt for at least 17 km to the west the mamainse point fault may extend across eastern lake superior to the keweenaw peninsula linking several offsets in the seismic data that are consistent with the same attitude and sense of displacement along the south side of batchawana bay at havilland sandstones of bayfield jacobsville type are isoclinally folded against a package of upthrust older rocks that include drag folded middle keweenawan volcanics at grindstone point north of cape gargantua a reverse fault separating isoclinally folded upper keweenawan sandstones from archean basement may on aeromagnetic evidence be an eastward extension of the michipicoten island fault these faults mark a significant change in the style of late compressional tectonism observed within the midcontinent rift all cut keweenawan rocks across strike the inference is that broad north south or northwest southeast compression consistent in timing and orientation with the grenville orogeny led to a reversal of movement along the major graben faults in western lake superior and was taken up in the eastern region by reverse faults oriented normal to the extensional axis of the rift

consortium for continental reflection profiling michigan surveys reprocessing and results reprocessing of consortium for continental reflection profiling data from central michigan with emphasis on deconvolution and velocity filtering before stack and more detailed velocity analysis has resulted in greatly improved imaging of a previously reported keweenawan rift buried beneath the paleozoic of the michigan basin the seismic stratigraphy of the sub paleozoic units can now be related to the basin filling middle keweenawan volcanic sequence and the upper keweenawan clastic assemblage in greater detail important structural and stratigraphic features can be seen for the first time for example reprocessing has imaged an unconformity within the upper keweenawan it is possible some of these newly imaged features are associated with a late keweenawan compressive episode furthermore reprocessing also reveals previously unrecognized sub keweenawan structures including a deeply penetrating reflection which may represent a major crustal fault and possible weak reflections from midcrustal 25 km and moho 43 km depths

rock elm structure pierce county wisconsin a possible cryptoexplosion structure the rock elm structure is a nearly circular feature in western wisconsin flat 44 43 n long 92 14 w in an area of otherwise little deformed cambrian and ordovician shallow marine sediments the structure has a ring boundary fault with at least 50 m of vertical displacement a sediment filled ring basin and central denial uplift the ring basin fill consists of a shale sandstone sequence of probable ordovician age not found outside of the structure blocks of deformed lower ordovicisin carbonates are generally in angular unconformity beneath the basin fill but locally are in fault contact against basin fill rocks the central dome consists of conglomeratic sandstones tentatively identified as the upper cambrian mount simon sandstone that have been uplifted more than 250 m although the deformation of the rocks is unusual for the region there have been no shock metamorphic features yet found associated with the structure the rock elm structure has the essential features of a cryptoexplosion structure as described by dietz and by mccall conclusive evidence is lacking as to whether the rock elm structure formed as a result of a bolide impact explosive volcanic event or other localized violent geologic event

an interpretation of gravity anomalies over the midcontinent rift lake superior constrained by glimpce seismic and aeromagnetic data cross sections of the midcontinent rift in lake superior derived from glimpce seismic reflection images provide unprecedented structural details of the rift and a new constraint for modelling associated gravity anomalies in turn gravity modelling constrained also by new high resolution aeromagnetic data has permitted critical examination of the seismic models the latter generate gravity anomalies having limited agreement with observed anomalies when appropriate rock densities are assigned good agreement may be achieved generally by making comparatively local changes to the models while retaining their larger scale attributes gravity modelling thus enhances and supports glimpce seismic models modifications to seismic models include revisions of initial densities within the geometrical framework of the models leading to a redefinition of lithologies for example in some segments of the rift mafic volcanics are substituted for keweenawan sedimentary and sedimentary volcanic sequences and for l

crustal structure of the central graben of the midcontinent rift beneath lake superior abstract travel times and amplitudes of refracted and reflected arrivals observed during glimpce great lakes international multidisciplinary program on crustal evolution from the midcontinent rift in lake superior have been modeled using two dimensional ray tracing techniques forward modeling was used to iteratively refine an initial velocity model that was constructed from results of tau sum analysis of the travel time data at each station combined with information from a coincident common midpoint cmp reflection profile when converted to time the resulting model agrees quite well with the cmp reflection profile within the central graben of the midcontinent rift approximately 2 km of sedimentary rock 2 8 4 6 km s overlie an 8 km thick sequence of volcanics and interflow sediments 5 0 6 5 km s within the central rift graben observed on the reflection data beneath this sequence is a 6 8 km thick sequence of 6 6 7 0 km s material interpreted to represent metamorphosed volcanics the velocity of the material at the base of the rift graben is not well constrained approx 7 0 7 2 km s but probably comprises an additional 10 12 km of metavolcanic rocks and intrusions that extend to the base of the graben as imaged on the coincident seismic reflection profile the central graben is ranked by reverse faults the isle royal and keweenawan faults that probably represent normal faults associated with graben formation that were reactivated during the late precambrian a 50 100 ms time advance anomaly associated with the isle royal fault is observed at every station this anomaly has been modeled as shallow high velocity blocks located directly beneath the fault the blocks correlate well with the walls of a steep sided bathymetric trough and are believed to represent highly indurated upper keweenawan sediments resulting from hydrothermal alteration a marked decrease in the apparent velocity and amplitude of the first arrivals is observed on reversed sections at offsets exceeding 100 km and has been modeled as pre rift continental crustal rocks with a velocity of about 6 5 km s adjacent to the 7 0 km s material in the graben calculation of the gravity response of the seismic model demonstrates that the gravity high centered over the rift can be entirely attributed to high density rocks occupying the central half graben imaged on the cmp profile wide angle reflections from about 15 30 km depth beneath the flanks of the graben indicate the presence of velocity discontinuities that may represent rift related detachment surfaces and or pre rift structures within the pre rift crust additional wide angle reflections define the top and bottom of high velocity lower crustal layer the high velocity lower crustal layer is thickest beneath the rift graben and is interpreted to represent magmatic material underplated to the crust during rifting as discussed by trehu et al 1991 total crustal thickness beneath the lake varies from about 55 km beneath the graben to about 35 km beneath the southern shore and 40 km beneath the northern shore

evidence for grenville age seismicity and thick skinned deformation in northern wisconsin u s a abstract one of the great continental rifts in all of earth s history the ca 1 1 ga midcontinent rift of the lake superior region was closed and inverted less than 100 m yr after its initiation when rift bounding normal faults were reactivated as reverse faults this was a major crustal event involving thick skinned deformation but its tectonic importance has not been fully appreciated owing in part to the lack of good outcrop along the traces of the rift closing faults a rare exposure of one of these faults occurs near the falls of the marengo river in southeastern bayfield county wisconsin the atkins lake marenisco fault is a crustal scale structure that juxtaposes archean granitic gneisses with paleoproterozoic and neoproterozoic metasedimentary and metavolcanic rocks pseudotachylytes cataclasites and microbreccias in the fault zone record large documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont

correction to evidence for and implications of a bering plate based on geodetic measurements from the aleutians and western alaska 1 in the paper evidence for and implications of a bering plate based on geodetic measurements from the aleutians and western alaska by ryan s cross and jeffrey t freymueller journal of geophysical research 113 b07405 doi 10 1029 2007jb005136 2008 the magnitudes of the estimated angular velocities for the bering plate were off by two orders of magnitude or by a factor of 100 because of a unit conversion error the same error affected all forward and inverse calculations so only the bering plate rotation rate was incorrect the pole location and all other calculations in the paper are correct the correct angular speed for the best model is 0 064 0 033 degrees ma an updated table 3 is given below with the correct rates for all models tested

model of the midcontinent rift system in northeastern kansas recent drilling of the midcontinent rift system in northeastern kansas reveals a transposed keweenawan stratigraphy of mafic volcanic rocks overlying thick clastic sedimentary rocks a reprocessed version of cocorp kansas line 1 indicates that the drillhole penetrated a series of west dipping reflectors associated with a reverse fault bounding the east side of the rift basin reanalysis of the original cocorp line also reveals a west bounding reverse fault and evidence of crustal thickening this information is integrated with gravity and magnetic data to define a model of the rift in northeastern kansas the model consisting of an asymmetric basin bounded by reverse faults with thickened crust beaneath the rift is similar to models proposed for northern segments of the rift system and argues for homogeneity in the structural style and tectonic evolution along the length of the rift

imaging the midcontinent rift beneath lake superior using large aperture seismic data the authors present a detailed velocity model across the 1 1 billion year old midcontinent rift system mrs in central lake superior the model was derived primarily from onshore offshore large aperture seismic and gravity data high velocities obtained within a highly reflective half graben that was imaged on coincident seismic reflection data demonstrate the dominantly magic composition of the graben fill and constrain its total thickness to be at least 30 km strong wide angle reflections are observed from the lower crust and moho indicating that the crust is thickest 55 60 km beneath the axis of the graben the total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north above the moho is a high velocity lower crust interpreted to result from syn rift basaltic intrusion into and or underplating beneath the archean lower crust the lower crust is thickest beneath the axis of the main rift half graben a second region of thick lower crust is found approximately 100 km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting the crustal model presented here resembles recent models of some passive continentalmore margins and is in marked contrast to many models of both active and extinct phanerozoic continental rift zones it demonstrates that the moho is a dynamic feature since the pre rift moho is probably within or above the high velocity lower crust whereas the post rift moho is defined as the base of this layer in the absence of major tectonic activity however the moho is very stable since the large abrupt variations in crustal thickness beneath the mrs have been preserved for at least a billion years less