cocorp seismic profiling of the appalachian orogen beneath the coastal plain of georgia a southeastward extension onto the coastal plain of an earlier cocorp traverse which confirmed large scale thin skinned thrusting of crystalline rocks of the southern appalachians has provided some of the most spectacular reflections yet seen in crustal seismic data most of the reflectors can be interpreted as either fault surfaces or as metamorphosed strata of late precambrian early paleozoic age they are consistent with the hypothesis that a major detachment extends eastward beneath this part of the orogen although other interpretations with a more complex pattern of detachments or sutures are also possible large scale overthrusting provides a mechanism for incorporating sedimentary rocks into the lower crust and may help to explain many of the layered features on crustal seismic data reflections from deep beneath the coastal plain indicate that the structural configuration of the rocks is complex and that the remains of a collision zone are being observed several east dipping horizons which bear strong similarities to thrust faults in valley and ridge sedimentary rocks are seen in the basement at shallow and mid crustal levels beneath the coastal plain the augusta fault for example displays a reflection which extends at a low angle some 80 km or more southeast of its surface position in conjunction with surface geologic information these new data demonstrate that late paleozoic compressive deformation was pervasive and resulted in lateral movements in the upper crust extending from the valley and ridge to the crystalline rocks beneath the coastal plain a distance of 400 km or more a large antiform cresting at about 2 3 sec or about 6 km below the surface and other structures beneath the coastal plain of georgia deserve further consideration for petroleum exploration although metamorphism may have eliminated petroleum from these rocks refracted arrivals and fault geometries indicate two triassic rift basins beneath coastal plain sedimentary rocks one of which has apparently not been recognized previously

crustal structure beneath the southern appalachians nonuniqueness of gravity modeling gravity models computed for a profile across the long wavelength paired negative positive bouguer anomalies of the southern appalachian mountains show that the large negative anomaly can be explained by a crustal root zone whereas the steep gradient and positive anomaly east of the root may be explained equally well by three different geometries a suture zone a mantle upwarp or a shallow body seismic data support the existence of a mountain root but are inadequate to resolve differences among the three possible geometries for the positive anomaly the presence of outcropping mafic and ultramafic rocks in the southern appalachians and the inferred tectonic history of the appalachian orogen are most consistent with the suture zone model crust similar to continental crust probably exists beneath the coastal plain and inner continental shelf where the gravity anomalies return to near zero values

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

the role of premagmatic rifting in shaping a volcanic continental margin an example from the eastern north american margin both magmatic and tectonic processes contribute to the formation of volcanic continental margins such margins are thought to undergo extension across a narrow zone of lithospheric thinning 100 k

clastic patterned ground in lomonosov crater mars examining fracture controlled formation mechanisms the area surrounding lomonosov crater on mars has a high density of seemingly organised boulder patterns these form seemingly sorted polygons and stripes within kilometre scale blockfields patches of boulder strewn ground which are common across the martian high latitudes several hypotheses have been suggested to explain the formation of clastic patterned ground on mars it has been proposed that these structures could have formed through freeze thaw sorting or conversely by the interaction of boulders with underlying fracture polygons in this investigation a series of sites were examined to evaluate whether boulder patterns appear to be controlled by the distribution of underlying fractures and test the fracture control hypotheses for their formation it was decided to focus on this suite of mechanisms as they are characterised by a clear morphological relationship namely the presence of an underlying fracture network which can easily be evaluated over a large area it was found that in the majority of examples at these sites did not exhibit fracture control although fractures were present at many sites there were very few sites where the fracture network appeared to be controlling the boulder distribution in general these were not the sites with the best examples of organization suggesting that the fracture control mechanisms are not the dominant geomorphic process organising the boulders in this area

a geophysical study of the earth s crust in central virginia implications for appalachian crustal structure a regional seismic reflection line i 64 across the virginia piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region a highly reflective upper crust an allochthonous blue ridge province underlying thrust sheets including the blue ridge master decollement and a basal decollement at a depth of about 9 km 3 s are confirmed on the seismic data immediately east of the blue ridge province appalachian structures plunge to as much as 12 km 4 s depth the evington group hardware terrane and chopawamsic metavolcanic rocks carolina terrane crop out in the piedmont province and numerous eastward dipping reflections originate from these rocks in the subsurface these eastward dipping reflectors overlie a gently west dipping 10 15 highly reflective zone that varies in depth from 1 5 s 4 5 km beneath the goochland terrane to 4 s 12 km beneath the rocks of the evington group some of the overlying eastward dipping reflections apparently root in this zone the zone may include decollement surfaces along which the overlying rocks were transported relatively few reflections originate from within autochthonous grenville basement at the western end of the profile the goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the chopawamsic metavolcanic rocks a broad zone of east dipping 20 45 reflections bounds the goochland terrane on the east these reflections may originate from deformation zones and continue to moho depths they appear to be correlative with similar events seen on other appalachian lines the pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent piedmont gravity high suggest continuity of crustal features along the length of the appalachians a major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in virginia that is the appalachian gravity gradient and the piedmont gravity high the crust thins from about 52 km beneath the appalachian mountains to about 35 km beneath richmond virginia and then rethickens by up to 10 km beneath the zone of east dipping reflections mylonites east of richmond the i 64 seismic data also contain a sequence of reflections at about 9 12 s indicative of lower crustal layering the base of this zone of reflections coincides almost exactly with the mohorovicic discontinuity interpreted from earlier refraction work the layering extends about 70 km west from richmond virginia and is interpreted as a lower crustal transition zone that is believed to persist across most of virginia

wide angle three component seismic reflection profiling of the crust along the east coast gravity high southern appalachians using quarry blasts inversion of refraction wide angle reflection travel times for two dimensional velocity structure along the axis of the east coast gravity high in the carolina terrane of georgia indicates much higher crustal p wave velocities than previously modeled thirty nine timed quarry blasts recorded with a 20 element array of three component recorders at 330 m spacings provide reversed and overlapping coverage along the profile which crosses consortium for continental reflection profiling georgia line 5 ray paths are concentrated in high grade rocks of the charlotte and kiokee belts shot gathers show strong first arrivals moho reflections and numerous reflections from within the crust shear wave arrivals are particularly strong with prominent direct arrivals sg and reflections from the moho sms observed for all quarries three alternative models show average crustal p wave velocities of 6 5 6 6 km s and moho depths between 37 and 39 km p wave velocities increase from 6 0 6 2 km s at 1 km depth to 6 7 6 8 km s at a first order discontinuity at a depth of 24 26 km alternative models for the lower crust feature both a slight increase in velocity and velocity reversals at the 25 km discontinuity with velocities steadily increasing to between 6 9 and 7 5 km s at the base of the crust low average v p v s ratios for the crust 1 70 1 75 estimated from travel time ratios for sms and pmp reflections suggest that the high compressional wave velocities derived by inversion are associated with rocks of high metamorphic grade and granitic to intermediate not mafic average composition over most of the crust the low average ratios can also be explained in part by the presence of a layer of low p wave velocity in the midcrust possibly associated with a package of quartz rich platform and rift basin sediments overthrust late in the alleghanian orogeny by the carolina terrane allochthon as suggested by earlier workers crustal models incorporating the new estimate for crustal thickness beneath the carolina terrane show that the long wavelength component of the gravity gradient in georgia can be explained in terms of crustal thickness variations aeromagnetic data indicate that density variations responsible for local steepening of the gradient are confined largely to the lower half of the crust

crustal profile of mountain belt cocorp deep seismic reflection profiling in new england appalachians and implications for architecture of convergent mountain chains relatively shallow reflections from beneath the taconic allochthon and green mountain anticlinorium are suggested to mark a zone of detachment that carries precambrian grenville basement of the green mountains in its upper plate the data suggest that tectonic thickening of the shelf sedimentary and synorogenic flysch sequence beneath the taconic allochthon has occurred and that major thrust fault systems champlain thrust of western vermont which imbricate shelf lithologies project downplunge into the seismic profile eastward dipping reflections beneath the east flank of the green mountains appear to project across the connecticut river valley into new hampshire because some of these reflections can be projected into an exposed zone of probable thrust imbricated precambrian grenville basement and younger metasediments there is a strong implication that similar rock types exist in the deeper crust beneath new hampshire the gross geometry of the eastward dipping zone is a large thrust ramp that facilitated faulting of continental basement in the green mountains to higher structural levels thus suggesting the anticlinorium is a large hanging wall structure or ramp anticlinorium highly deformed satellitic massifs of grenville basement such as the chester dome and sub silurian and silurian devonian rocks of the bronson hill anticlinorium and merrimack synclinorium are probably transported along intracrustal detachments as well data suggest the possiblity of a tectonically buried continental margin transition zone much like that interpreted for the southern appalachians this zone would have been preserved from prior rifting of the proto atlantic ocean and its inherited geometry could have controlled subsequent convergent deformation during appalachian orogenesis although regional deep seismic reflection profiling has not been carried out across the alps existing data imply lateral changes in crustal reflection character similar to those observed in the appalachians deep seismic reflection data from convergent mountain belts raises the possibility of such preserved transition zones beneath transported rocks and provides an alternative interpretation to lithosphere penetrating sutures in explaining lateral geologic and geophysical contrasts in the crust modified journal abstract

towards an understanding of the southern appalachian piedmont crustal transition a multidisciplinary approach abstract the piedmont of the southern appalachians is characterized by significant geophysical and geological anomalies which indicate there is a major crustal transition multiple hypotheses including a suture zone and a subducted continental margin have been presented to explain the variations although crustal seismic reflection data have provided significant constraints there are ambiguities inherent in the interpretation of such data the ambiguities can be reduced by careful consideration of related geophysical and geological observations although the importance of correlating crustal reflection data with known geologic features by tracing reflections to drill holes or to the surface cannot be overestimated only rarely are such correlations possible in almost all interpretations of crustal reflection structure it is necessary to constrain the model with methods such as seismic refraction gravity magnetics conductivity and surface geology including palinspastic reconstructions when information from these techniques is incorporated into interpretations of the piedmont crustal structure the model which appears to be most consistent with the observations is one in which the upper crust of the piedmont is decoupled from the lower crust and in which the lower crust thins eastward the lower crust may be a subducted precambrian continental margin and its associated transition toward thinner basinal crust

ages of regional metamorphism and ductile deformation in the central and southern appalachians abstract a summary of available geochronology of deformational fabrics formed at greenschist and higher grades in the central and southern appalachians is presented as a map showing the ages of regional metamorphism and ductile deformation the ca 480 435 ma taconic event affected most of the piedmont and blue ridge provinces the ca 380 340 ma acadian event was milder and was centered in the west central piedmont and eastern blue ridge the 330 230 ma alleghanian event exhibits an abrupt ductile deformation front in the eastern piedmont and extends an unknown distance eastward below the atlantic coastal plain

noise attenuation by vibroseis whitening vsw processing reflection seismic data recorded on the atlantic coastal plain to examine suspected faulting of the shallow 200 m basement were contaminated by severe ground roll noise with an apparent velocity of 587 m sec 25 hz dominant frequency and 23 m apparent wavelength a receiver interval of 5 m and a total spread length of 285 m were used to obtain 24 fold coverage although the noise problem was severe overlapping source and or receiver arrays on the surface were not used for the recording geometry in order to avoid a decrease in resolution caused by the smoothing effect of overlapping subsurface coverage instead the large amplitude surface waves were attenuated by a process called vibroseis whitening vsw vsw is based on the application of time varying amplitude scaling before vibroseis crosscorrelation a conventional method of automatic digital gain control was found to be effective for this purpose this scaling results in a signal to noise ratio improvement equal to the gain expected from crossc

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

new cocorp profiling in the southeastern united states part ii brunswick and east coast magnetic anomalies opening of the north central atlantic ocean new cocorp profiles on the coastal plain of georgia and northern florida support the hypothesis that the brunswick anomaly marks a late paleozoic suture they do not support the alternate view that this anomaly is caused by a mesozoic rift basin the trend of the brunswick anomaly relative to the appalachian gravity gradient indicates that in westernmost georgia and adjacent alabama african basement suwannee terrane is in proximity to autochthonous north american basement grenville farther east one or more paleozoic accreted terranes intervene between the north american and african sides of the orogen offshore the brunswick anomaly closely parallels the east coast magnetic anomaly this relationship implies that the east coast magnetic anomaly marks not only the present continental oceanic crustal transition but also the northward continuation of the late paleozoic suture between north america and africa transitional crust beneath the carolina trough baltimore canyon trough and correlative parts of the african atlantic margin is thus likely to have formed within a preexisting suture zone the dramatic change in character of the u s atlantic margin southward from the carolina trough to the blake plateau probably reflects the fact that two different types of prerift crust are juxtaposed in this region

deep velocity structure of rifted continental crust u s mid atlantic margin from wide angle reflection refraction data we present new ocean bottom wide angle seismic data collected during the 1990 edge mid atlantic multichannel seismic experiment which provide a measurement of the deep velocity structure of rifted appalachian continental crust beneath the u s east coast continental margin reflections from the entire crust and moho are visible from offsets of zero to 100 km one dimensional inverse traveltime modeling reveals a 34 km thick crust consisting of four layers beneath the post rift sediments with velocities of 5 9 6 3 6 4 and 6 85 km s this velocity structure is indistinguishable from that found beneath several appalachian terranes in the northern appalachians of new england the average velocity of 6 85 km s in the lower crust limits the landward extent of the high velocity 7 2 7 4 km s lower crust found farther seaward and supports the interpretation of that layer as the result of rift related magmatic underplating rather than as thinned pre existing lower continental crust

appalachian carboniferous dextral strike slip faults an example from brookneal virginia the northern appalachian dextral fault system of late paleozoic age is also present in the central and southern appalachians an example of a dextral strike slip fault is the 4 km wide brookneal shear zone in the southwest virginia piedmont the shear zone is in part superimposed on the melrose granite where an s c mylonite was produced by dynamic recrystallization of all constituent minerals the arvonia metasedimentary and charlotte belt metavolcanic rocks contain spaced dextral shear bands at a consistent 24 3 to the shear zone boundary a minimum displacement estimate of 17 km was obtained from rotated foliation measurements in the melrose granite the age of movement on the brookneal shear zone has been constrained by isotopic dating to between 324 and 300 ma other faults in the southern appalachians including the nutbush creek and modoc zones show similar ages and relative offsets possible plate tectonic models that could account for the late paleozoic dextral fault system throughout the appalachians include 1 tectonic escape resulting from the collision of a plate with north america to the north of the canadian appalachians 2 postcollision interplate readjustments involving counterclockwise rotation of africa relative to north america and 3 oblique convergence of eastern north america with an oceanic plate moving west

character of the alleghanian orogeny in the southern appalachians part iii regional tectonic relations geological and geochronological studies in the eastern piedmont of the southern appalachians have documented a polyphase late paleozoic deformational chronology d 2 d 4 that is contemporaneous with the alleghanian orogeny recorded in the western appalachian foreland the scale and vergence of d 3 anticlinoria in the eastern piedmont are similar to those of anticlinoria in the western appalachians which are related to ramping of underlying thrust surfaces we suggest that the appalachian decollement extends southeastward beneath the entire piedmont province and that all of the piedmont rocks were displaced at least 175 km northwestward relative to north america during the alleghanian orogeny late paleozoic deformational effects in the eastern piedmont thus are considered cogenetic with alleghanian foreland deformation we interpret the alleghanian orogeny to be the result of oblique dextral convergence and collision of gondwana and laurentia

results from regional vibroseis profiling appalachian ultra deep core hole site study summary in 1985 180 km of regional vibroseis profiles were acquired in the carolinas and georgia southeastern united states as part of the appalachian ultra deep core hole adcoh site study the data quality is excellent with large amplitude reflections from faults and crystalline rocks lower palaeozoic shelf strata and from within autochthonous grenville basement the profiles image the subsurface more clearly than other available data and allow the possibility of alternative interpretations of important elements of the tectonic framework of the southern appalachians the major points in the interpretation are 1 the blue ridge master decollement is at a depth of 2 3 km beneath the blue ridge this thrust increases in dip just nw of the brevard fault zone 2 the brevard fault zone appears to splay from the master decollement at 6 km 2 2 s near westminster s c and defines the base of the crystalline inner piedmont allochthon 3 below the blue ridge thrust sheet are images of duplex and imbricate structures duplex tuning wedges connected by other thrust faults that duplicate shelf strata to a thickness of 4 5 km 4 subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower palaeozoic strata as suggested by others 5 eocambrian cambrian rift basins in the grenville basement are also imaged the adcoh data were originally recorded with 14 56 hz bandwidth and 8 s length but an extended vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust below 8 s reflections from within the grenville basement become weak but are observable as late as 13 s however these moho reflections are generally short segments

geophysical anomalies along strike of the southern appalachian piedmont bouguer gravity anomalies and aeromagnetic anomalies display a northward increase of values in the charlotte and carolina slate belts from georgia to virginia analysis of these data with constraints imposed by seismic reflection seismic refraction conductivity measurements and surface geology imply that this increase is due to a corresponding increase in the percentage of mafic rocks in the upper crust a regional trend in the bouguer field is determined for an observed eastward decrease in crustal thickness based on seismic refraction measurements a comparison of the calculated regional with surface boundaries shows that in places the crustal thinning occurs more than 50 kilometers west of the inner piedmont charlotte belt transition the comparison also shows that the crustal thinning may take place west of exposed grenville basement the evidence thus indicates that the southern appalachian gravity gradient is not likely related to a crustal or lithospheric suture the evidence further indicates that because the upper crustal and deep crust upper mantle variations are independent in terms of spatial position the upper crust of the charlotte slate belts is decoupled from the lower crust perhaps at subhorizontal boundaries seen on reflection data

seismicity seismic reflection gravity and geology of the central virginia seismic zone part 3 gravity bouguer gravity anomalies at 1 870 locations on the central piedmont of virginia from 37 37 n to 37 52 n and 77 44 w to 78 23 w display patterns of variation produced by upper crustal density contrasts and thickening of the crust in a west northwest direction no other deep sources are evident upper crustal density contrasts are associated with rock units known from geologic mapping the subsurface distribution of these rock units interpreted from seismic reflection data was confirmed by measured variations in gravity a two dimensional model analysis indicates the following average in situ density values for the principal formations arvonia formation 2 77 gm cc columbia granite 2 75 gm cc tonalite and 2 73 gm cc pegmatite chopawamsic volcanics 2 77 gm cc felsic units and 2 79 gm cc malic units catoctin lynchburg evington 2 82 gm cc maidens gneiss 2 78 gm cc and grenville basement 2 71 gm cc gravity anomalies are consistent with the existence of a major thrust fault at depths between 9 km and 16 km that probably separates grenville basement rocks from younger catoctin lynchburg rocks these anomalies confirm that the slight eastward dip of this thrust fault beneath the western part of the area increases significantly east of 78 05 w

rift basins of the long island platform four buried early mesozoic rift basins on the long island platform region of the united states atlantic continental margin were mapped using seismic reflection magnetic and gravity data low angle normal border faults and high angle cross faults control the structure of three of the basins and some of these faults can be traced deep into the crust faulted basement blocks characterize the fourth basin adjacent basins form pairs in which the border faults dip toward each other and may have acted to localize crustal thinning two phases of rifting took place an early stage characterized by widespread faulting leading to basin formation and a late stage characterized by uplift and erosion near the hinge zone and subsidence seaward of the hinge zone the new york bight basin at the western end of the platform may be an offshore extension of the hartford basin paleozoic tectonic and structural trends are inferred to be the primary controls on the location of these basins although our data are not specific enough to address local structural details

stratigraphy and tectonics of the virginia north carolina blue ridge evolution of a late proterozoic early paleozoic hinge zone late proterozoic 690 570 ma through ordovician rocks of the virginia north carolina blue ridge and valley and ridge record the evolution of an eastward facing passive margin and its subsequent deformation during continent arc collision taconic orogeny comparison of blue ridge stratigraphy with modern rifts and passive margins suggests that late proterozoic rifting involved substantial crustal attenuation in the eastern blue ridge the axial zone of the blue ridge from central virginia southward is occupied by a major fault system hayesville fries rock fish valley fault contrasts between rocks deposited east and west of this fault suggest that it is a reactivated hinge zone separating highly attenuated continental crust to the east from continental crust of more normal thickness to the west during rifting normal faulting and rapid subsidence of attenuating crust in the eastern blue ridge produced rift basins in which deep water clastic and volcanic rocks accumulated lynchburg group ashe formation landward of the hinge zone in the western blue ridge mainly terrestrial to shallow water clastic and volcanic rocks were deposited in a series of subaerial rift basins swift run catoctin grandfather mountain mount rogers formations the hinge zone not only controlled depositional patterns throughout the rift and passive margin history but it was also reactivated during the taconic collision when attenuated crust and its cover were thrust westward over the shelf edge turbidite dominated rift sequences of the eastern blue ridge are unlike typical rift deposits which are characterized by predominantly subaerial and shallow water sedimentation deep water rifts may be common on attenuated crust as in the bay of biscay however they are poorly known from the stratigraphic record because of their susceptibility to destruction during orogeny

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

a regional crustal velocity model for the southeastern united states arrival time data from a southeastern united states earthquake are analyzed to formulate an average regional crustal velocity model the event studied is the magnitude 3 7 mb shallow earthquake 2 km depth that occurred on 26 august 1979 near the lake jocassee reservoir in northwestern south carolina over 90 seismograms were collected and read by at least two independent interpreters some 250 phases have been timed including 60 pn 80 pg 50 sn and 70 sg lg arrival times the arrival time data can be interpreted in terms of a single layer crust with average velocities of 6 25 0 04 km sec for compressional waves and 3 65 0 11 km sec for shear waves the average upper mantle velocities are 8 11 0 19 km sec p waves and 4 59 0 13 km sec s waves the mohorovic ic discontinuity is found to lie at varying average depths throughout the region 31 km coastal plain 33 km piedmont 40 km valley and ridge central united states a regional velocity model derived from the above results has the aforementioned average velocities a crustal thickness of 40 km and a set of time corrections for mantle refraction phases coastal plain pn 1 68 sec sn 2 74 sec piedmont pn 1 40 sec sn 2 28 sec valley and ridge and central united states no corrections this new velocity model is tested for precision by relocating several regional earthquakes and comparing the resulting travel time residuals with those from the earlier locations the tests demonstrate that the new model can be used to develop good results

simulations of the indian summer monsoon using a nested regional climate model domain size experiments seasonal simulations of the indian summer monsoon using a 50 km regional climate model rcm are described results from three versions of the rcm distinguished by different domain sizes are compared against those of the driving global general circulation model agcm precipitation over land is 20 larger in the rcms due to stronger vertical motions arising from finer horizontal resolution the resulting increase in condensational heating helps to intensify the monsoon trough relative to the agcm the rcm precipitation distributions show a strong orographically forced mesoscale component similar in each version this component is not present in the agcm the rcms produce two qualitatively realistic intraseasonal oscillations isos associated respectively with monsoon depressions which propagate northwestward from the bay of bengal and repeated northward migrations of the regional tropical convergence zone the rcm simulations are relatively insensitive to domain size in several respects 1 the mean bias relative to the agcm is similar for all three domains 2 the variability simulated by the rcm is strongly correlated with that of the driving agcm on both daily and seasonal time scales even for the largest domain 3 the mesoscale features and isos are not damped by the relative proximity of the lateral boundaries in the version with the smallest domain results 1 and 2 contrast strongly with a previous study for europe carried out with the same model probably due to inherent differences between mid latitude and tropical dynamics

seismicity seismic reflection studies gravity and geology of the central virginia seismic zone part i seismicity the central virginia seismic zone is an area of persistent low level seismicity in the piedmont province of that state its north south dimension is 120 km and it extends some 150 km in an east west direction from richmond to lynchburg the results of instrumental monitoring there by a seismic network during the past 5 yr are reported herein to provide a useful constraint on geologic structural models considerable care must be exercised in the location of earthquakes and in the specification of the accuracy of those locations our procedures for accomplishing those tasks were as follows first a velocity model specific to the network locale was developed from seismic refraction and reflection data second evaluation of the detection capability of the network indicated that the detection of seismic events should be virtually complete down to a magnitude of 1 5 third the locational capability of the network was assessed by locating blasts originating from three different quarries operating within the network area as if they were earthquakes of unknown location the average calculated locations when compared to the known blast sites indicated average differences of 2 km horizontally and 4 km vertically except for the northeastern portion there the location capability degrades to fair for epicenters 4 km and to poor for estimation of focal depths 8 km the resultant decrease in locational capability is probably due to the presence of low velocity coastal plain sediments fourth the location of all of the earthquakes relative to each other was tested by use of special joint hypocenter calculations the recalculated locations differed from the original locations in a random manner on the average that difference was 3 km and it indicates the absence of serious bias introduced by combining earthquake locations derived from different subnetworks of seismographs ninety six pre network pre 1978 earthquakes for the most part felt events with noninstrumental locations and 34 network determined hypocenters 1978 1982 for mostly smaller not felt shocks form the basic data set of this study the spatial distribution exhibited by the 130 hypocenters is diffuse both vertically and horizontally such a pattern favors multiple rather than singular seismogenic structures seventy five percent of the focal depths are in the upper one third of the crust at depths of 11 km or less these multiple shallow sources will be shown in part 4 of this sequence of papers to be along and above a master detachment fault

character of the alleghanian orogeny in the southern appalachians part i alleghanian deformation in the eastern piedmont of south carolina the eastern piedmont province in south carolina contains a sequence of cambrian volcanic and sedimentary rocks that was penetratively deformed d1 and regionally metamorphosed m1 to the greenschist facies during the early and or middle paleozoic the eastern piedmont was subsequently affected by late paleozoic alleghanian polyphase deformation d2 d4 and regional metamorphism the earliest alleghanian event d2 is associated with amphibolite facies regional metamorphism and felsic plutonism in a mid crustal infrastructure at ca 295 315 ma the gradational interface between infrastructure and overlying suprastructure contained a steep m2 metamorphic gradient between amphibolite facies below and greenschist facies above numerous sheets of felsic orthogneiss and a deformation front marking the upper limit of intense d2 penetrative deformation d3 is represented by northwestward vergent folding of s1 and s2 foliations and m2 isothermal surfaces the kiokee belt in south carolina is interpreted as a d2 infrastructure exposed within the core of a d3 antiform this interpretation suggests that an alleghanian infrastructure may be present in the subsurface beneath much of the piedmont and coastal plain the polygenetic modoc zone forms the northwest boundary of the kiokee belt it is interpreted to represent an interface between d2 infrastructure and suprastructure which was rotated into a steep northwest dipping attitude during development of the kiokee belt antiform between ca 267 and 290 ma portions of the kiokee belt modoz zone and carolina slate belt were overprinted by ductile deformation d4 along steeply dipping northeast trending dextral shear zones

the alleghanian southern appalachian piedmont a transpressional model alleghanian dextral transpression produced multiple strike slip shear zones forming a contractional mega duplex in the central and southern appalachian piedmont structural upwelling along the zones is identifiable as crustal arching on seismic reflection profiles that roughly corresponds to the piedmont gravity high the structural upwelling coincides both temporally and spatially with areas of peak alleghanian plutonism and metamorphism the compositionally bimodal plutons generally occur in or around the fault zones and are isotopically primitive geobarometry of cognate xenocryst inclusions of coexisting plagioclase and pyroxene from a granitoid indicates 8 11 kbar or 31 to 42 5 km depth to proposed source region this depth roughly corresponds to the pre erosional location of multiple reflections beneath the arches on seismic profiles that are interpreted as the moho the granitoids are proposed to have formed by partial melting of the lowermost crust through heat from mafic intrusions and decompression resulting from crustal arching beneath the strike slip faults the granitoids ascended along the strike slip faults and were emplaced in or near them the crustal arching and melting processes appear to be important mechanisms in the development of strike slip orogens

tectonic evolution of the easternmost piedmont north carolina the northeastern north carolina piedmont is made up of a basement sequence of unknown age possibly a southern extension of the grenville age goochland terrane of eastern virginia structurally overlain by a volcanogenic sequence of probable late precambrian to early paleozoic age an eastward continuation of lithologies occurring in the carolina slate belt a model is proposed in which the basement and volcanogenic sequences are separated by a pre alleghanian taconic decollement northwestward and westward vergence of associated folds of the upper plate suggests that the volcanogenic sequence was thrust from the southeast or east over the basement terrane this deformation occurred during a greenschist grade m 1 metamorphic event alleghanian granites 313 to 285 ma intrude both the basement and volcanogenic sequences clearly cutting the inferred decollement in some areas this alleghanian plutonism was accompanied by regional heating to produce the upper greenschist to amphibolite grade m 2 assemblages of the raleigh belt the raleigh belt remained hot until late alleghanian compression produced regional scale f 3 folds and d 3 mylonite zones with the resultant uplift and cooling of this belt by 250 240 ma

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

a collisional crustal fabric pattern recognised from seismic reflection profiles of the appalachian caledonide orogen abstract deep seismic reflection profiles across the appalachians and across the once contiguous caledonides around britain and ireland show a broadly similar pattern of crustal reflection fabrics the pattern is of two adjacent fabric domains each characterized by dips towards the other bounded by an interface which dips in conformity with one of the fabrics the domain interface strikes parallel to the orogen suggesting a genetic relationship between the fabrics and the orogen from the north sea to the long island platform the domain boundary dips to the northwest in the southern appalachians the pattern appears with less consistency between the charlotte and carolina slate belts the pattern appears with the same polarity as further north but below the coastal plain it occurs with opposite polarity the interface dips to the southeast an attractive but not exclusive hypothesis for explaining the fabric domain contrast is that it results from detachment of all or most of the crust from underlying mantle lithosphere during collisional orogenesis

u s mid atlantic margin structure and early thermal evolution marine seismic experiments have revealed that the u s east coast rifted margin crustal structure is dominated by voluminous volcanics emplaced during continental breakup we present results from seismic data recorded onshore during the 1990 edge mid atlantic onshore offshore seismic experiment that extend the margin crustal profile westward 180 km across the coastal plain and place the offshore results into a broader tectonic context the onshore offshore crustal model defines the crustal thinning profile across the margin enabling calculation of total extension based on direct measurements of crustal thickness we find that the crust beneath the coastal plain is largely unextended has a uniform thickness of 35 km shows no evidence for magmatic additions of sufficient volume to affect seismic velocity and that the total half extension of the crust is 30 to 40 km less than previous estimates by a factor of 3 to 4 we also present results of depth migration of the offshore vertical incidence and wide angle seismic data that yield the most complete and accurate image to date of reflectivity across the u s east coast margin we use this image to analyze margin subsidence by exploiting the stratigraphy of the subaerially extruded basalts beneath the sediments we find that 1 margin subsidence is consistent with a petrogenetic model for initial seafloor spreading magmatism involving high average melting pressures 3 5 4 0 gpa and potential temperatures 1500 c for a present day thermal lid thickness of 225 km 2 it is difficult to explain margin subsidence with a present day lid thickness of much less than 150 km and 3 rapid subsidence following breakup suggests the initially hot mantle underlying the rifting margin cooled very quickly implying a limited and finite initial distribution of hot mantle rather than a continuous upwelling or a ubiquitously warm upper mantle

juxtaposed mesozoic diabase dike sets from the carolinas a preliminary assessment aeromagnetic anomaly maps of the north carolina piedmont provide confirming evidence of juxtaposed mesozoic diabase dike sets dikes in north carolina may be divided into four domains on the basis of dike orientations most dikes are olivine normative but there are apparent systematic differences in chemical compositions that are reflected in different orientations and ages two orientations dominate northwest and north south the northwest trending set dominates in the south carolina piedmont and the north south set extends through much of the central virginia piedmont and under the coastal plain in south carolina the north south dikes are part of a set that converges near charleston and georgetown south carolina and diverges northward into virginia preliminary information suggests that the north south dikes are younger than the more common northwest trending dikes current models proposed for stress regimes associated with the breakup of pangea can accommodate the northwest set but not the north south convergent set this convergent set may be evidence for crustal deformation above a mantle plume centered near charleston and georgetown in middle jurassic time it is possible that presumed mafic plutons under the coastal plain near charleston regional seismicity patterns and orientations of the diabase dikes are all related to an intersection of zones of crustal weakness inherited from mid mesozoic time the western margin of northwest trending dikes may be related to tectonic features inherited from pre mesozoic time

new cocorp profiling in the southeastern united states part i late paleozoic suture and mesozoic rift basin new cocorp profiling in the southeastern united states has revealed a broad zone of dipping reflections that extends downward through the crust beneath the coastal plain in western georgia the zone is over 50 km wide and most of the reflections dip moderately steeply toward the south regional considerations suggest that this feature marks the late paleozoic suture between north america and africa where crossed by the cocorp survey the suture occurs beneath the north flank of the triassic early jurassic south georgia basin the main depocenter of the south georgia basin occurs about 90 km to the south and is formed by a large half graben containing more than 5 km of rift basin fill farther south the paleozoic suwannee basin sequence beneath northern florida is poorly imaged on the cocorp profiles however weak reflections suggest that these strata including basal felsic volcanics may have an aggregate thickness of about 6 km in north central florida at the northwest end of the cocorp traverse a prominent horizon imaged in the upper crust beneath the inner piedmont probably marks the southern appalachian detachment the detachment appears to be cut off by the towaliga fault implying that the towaliga fault is in part a down to the north normal fault intermittent moho reflections occur at 11 12 s two way time along the length of the cocorp survey indicating that the crust in the region has a roughly uniform thickness of about 33 36 km assuming an average crustal velocity of 6 km s

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

stratigraphic model for timing the opening of the proto atlantic ocean in northern virginia theoretical rifting models suggest that rifting events have predictable stages time spans and characteristics the stratigraphy and sedimentary tectonics of northern virginia were compared with the theoretical models for the opening of ocean basins according to our model the rift valley stage of initial uplift and graben formation 0 10 m y after onset of rifting is represented in virginia by the lynchburg and equivalent formations the initiation of ocean crust formation immediately follows the rift valley stage 10 15 m y and is represented in virginia by the catoctin formation sedimentation on an unstable thermally subsiding continental margin youthful stage 10 60 m y is represented by the weverton and harpers formations the beginning of deposition on a stable continental margin occurs no later than 60 m y after onset of rifting according to the theoretical models and in virginia it is represented by the antietam formation a biostratigraphic age of early cambrian ca 570 ma for the antietam provides the datum for proposing that the rifting which opened the proto atlantic in virginia was initiated 60 m y before deposition of the antietam at most about 630 ma

reprocessed cocorp southern appalachian reflection data root zone to coastal plain cocorp consortium for continental reflection profiling seismic reflection data from the atlantic coastal plain and eastern piedmont in georgia have been reprocessed and interpreted dipping events correlated with the master decollement root zone may be the only features that can be correlated with other reflection lines farther southeast on the coastal plain the shallow section is improved to produce reflections corresponding to the base of the coastal plain a triassic basin is confirmed to be more than 2 km deep and is the only major basin accurately revealed by the seismic lines improvement of the shallow seismic data is important to understanding the deep data some apparently deep reflections are probably produced from within the shallow coastal plain even though these shallow data have been improved the augusta fault is not interpreted as a major reflective feature