volcanic arc emplacement onto the southernmost appalachian laurentian shelf characteristics and constraints in the southernmost appalachians the hillabee greenstone an ordovician volcanic arc fragment lies directly atop the outermost laurentian devonian earliest mississip pian shelf sequence at the structural top of the greenschist facies talladega belt the frontal metamorphic allochthon along this orogenic segment the hillabee greenstone was emplaced between latest devonian and middle mississippian time it and the upper most laurentian section were later repeated together within a series of map scale imbri cate slices of a postmetamorphic dextral transpressional alleghanian thrust duplex system that placed the high grade eastern blue ridge allochthon atop the talladega belt geochemical and geochronologic u pb zircon studies indicate that the hillabee greenstone s interstratifi ed tholeiitic meta basalt and calc alkaline metadacite rhyolite formed within an extensional setting on con tinental crust ca 460 470 ma palinspastic reconstructions of the southern appalachian ordovician margin place the hillabee green stone outboard of the present position of the pine mountain terrane and suggest links to ordovician plutonism in the overlying east ern blue ridge and possibly to widespread k bentonite deposits within ordovician plat form units the tectonic evolution of the hil labee greenstone exhibits many unusual and intriguing features including 1 premeta morphic emplacement along a basal cryptic thrust which is remarkably concordant to both hanging wall and footwall sequences across its entire extent 230 km 2 forma tion transport and emplacement of the arc fragment accompanied by minimal deforma tion of the hillabee greenstone and under lying outer margin shelf rocks 3 emplace ment temporally coincident with the adjacent collision of the younger tectonically indepen dent ouachita volcanic arc with southeastern laurentia these features highlight strong contrasts in the ordovician taconian evolu tion of the southern and northern parts of the appalachian orogen

supplemental material zircon and monazite geochronology in the palmer zone of transpression south central new england usa constraints on timing of deformation high grade metamorphism and lithospheric foundering during late paleozoic oblique collision in the northern appalachian orogen analytical methods for zircon u pb and monazite th pb sims geochronology zircon u pb la icp ms geochronology data tables and supporting previous work

acadian dextral transpression and synorogenic sedimentary successions in the appalachians the successive seboomook littleton northern appalachians and catskill pocono central appalachians clastic wedges suggest oblique convergence and southwestward migration of acadian orogeny beginning in early devonian and continuing into early mississippian wrench fault movement in maritime canada coincided with deposition of all but the earliest components of the catskill pocono clastic wedge and continued into the pennsylvanian contrasts between a thin lower to middle devonian shallow shelf facies in the alabama appalachian fold thrust belt and a time equivalent thick shallowing upward sedimentary to volcanic succession in the adjacent talladega slate belt are interpreted to reflect a wrench fault basin a wrench fault setting for devonian rocks in alabama integrated with manifestations of oblique convergence during the acadian orogeny in the central and northern appalachians can be accommodated in dextral transpression along the entire length of the acadian appalachian orogen

post taconian shortening inversion and strike slip in the stephenville area western newfoundland appalachians deformed terrigenous and carbonate sedimentary rocks representing the early paleozoic laurentian continental margin form a series of elongate fault bounded blocks that plunge north beneath the humber arm allochthon in the stephenville area west newfoundland appalachians the continental shelf succession was folded and thrust faulted after emplacement of the humber arm allochthon in the west of the area table mountain is cut by a pop up structure bounded by downward converging reverse faults structures at the east margin of table mountain indicate both dextral and reverse slip the phillips brook structure farther east contains multiple fault bounded carbonate slices one slice carried by the west blanche brook fault is thrust over the humber arm allochthon the western edge of the indian head massif consisting of grenville basement is also a thrust contact these reverse faults and thrusts which cut the carbonate succession and postdate emplacement of the humber arm allochthon are in turn

structural styles of paleozoic intracratonic fault reactivation a case study of the grays point fault zone in southeastern missouri usa abstract a pronounced subparallel set of northeast striking faults occurs in southeastern missouri but little is known about these faults because of poor exposure the commerce fault system is the southernmost exposed fault system in this set and has an ancestry related to reelfoot rift extension recent published work indicates that this fault system has a long history of reactivation the northeast striking grays point fault zone is a segment of the commerce fault system and is well exposed along the southeast rim of an inactive quarry our mapping shows that the grays point fault zone also has a complex history of polyphase reactivation involving three periods of paleozoic reactivation that occurred in late ordovician devonian and post mississippian each period is characterized by divergent right lateral oblique slip faulting petrographic examination of sidewall rip out clasts in calcite filled faults associated with the grays point fault zone supports a minimum of three periods of right lateral oblique slip the reported observations imply that a genetic link exists between intracratonic fault reactivation and strain produced by paleozoic orogenies affecting the eastern margin of laurentia north america interpretations of this link indicate that right lateral oblique slip has occurred on all of the northeast striking faults in southeastern missouri as a result of strain influenced by the convergence directions of the different paleozoic orogenies

role of the birmingham basement fault in thin skinned thrusting of the birmingham anticlinorium appalachian thrust belt in alabama the birmingham anticlinorium is a northeast striking northwest verging thrust related fold associated with the jones valley and opossum valley frontal thrust ramps in the southern appalachian thin skinned sedimentary thrust belt in alabama the amplitude of the southeastern trailing limb of the anticlinorium cahaba synclinorium is greater than that of the forelimb coalburg syncline the depth to the top of basement rocks beneath the decollement is greater beneath the cahaba synclinorium than beneath the coalburg syncline commensurate with vertical separation of the top of basement along the down to southeast birmingham basement fault at the northwest boundary of the birmingham basement graben the frontal ramp that controls the geometry of the birmingham anticlinorium rises northwestward over the down to southeast birmingham basement fault the geometry kinematics and mechanics of structure of the birmingham anticlinorium reflect stratigraphic variations in mechanical properties within the cambrian to pennsylvanian stratigraphic succession the stratigraphic variations record episodic synsedimentary movement on the birmingham basement fault the regional decollement at the birmingham anticlinorium is in the middle to lower upper cambrian conasauga formation which is dominantly a massive carbonate and less than 800 m thick northwest of the birmingham basement graben but is a succession of shale and thin bedded limestone more than 2 000 m thick within the graben ductile deformation of the thick shale dominated succession localized the palmerdale and bessemer mushwads ductile duplexes along strike beneath the birmingham anticlinorium the massive carbonate regional stiff layer the cambrian ordovician knox group forms the mushwad roof the upper part of the knox group was eroded as a consequence of inversion of the graben during flexural subsidence associated with taconic tectonic loading of the southeastern margin of laurentia small scale synsedimentary structures indicate minor structural reactivation from late ordovician through early mississippian thickness and facies variations in upper mississippian lower pennsylvanian synorogenic clastic wedge deposits document large scale reactivation of the down to southeast birmingham basement fault the birmingham basement fault localizes both the leading edge of the mushwads of shale dominated conasauga formation and frontal ramps of the regional decollement

late acadian alleghenian transpressional deformation evidence from asymmetric boudinage in the casco bay area coastal maine abstract asymmetric competent layer and foliation boudinage in the casco bay area of coastal maine formed during regional f 2 hinge parallel elongation and distributed dextral strike slip movement it consists of backward rotated oblique shear fracture boudinage and forward rotated extension fracture boudinage initially symmetric quartz filled extension fractures were intermittently developed as boudin partings orthogonal to the prominent ductile stretching lineation due to fault related fluctuations in fluid pressure and tectonic stresses modification by concurrent non coaxial deformation related to dextral strike slip movement resulted in the clockwise rotation sinistral slip and vein parallel elongation reflected in the forward rotated asymmetric boudinage discordant planar pegmatite veins and variably oriented boudinaged pegmatite sheets common in these exposures underwent a similar evolution the development of this asymmetric boudinage is attributed to progressive brittle ductile non coaxial deformation related to orogen parallel dextral transpression along the norumbega nonesuch river strike slip fault system during the later stages of the acadian orogeny and subsequent alleghenian deformation

paleozoic age of the walden creek group ocoee supergroup in the western blue ridge southern appalachians implications for evolution of the appalachian margin of laurentia in the foothills of the great smoky mountains in tennessee low grade metamorphic rocks of the walden creek group ocoee supergroup contain a fossil assemblage composed of trilobites bryozoans ostracodes pelmatozoans algae and agglutinated foraminifers indicating a silurian or younger age the new paleontological data contradict older ideas on the neoproterozoic age of the walden creek group the walden creek group was not deposited in a continental rift basin preceding the opening of the iapetus ocean but possibly in a pull apart basin formed in a transtensional event during the acadian orogeny there is no field evidence indicating that the walden creek group is separated from the lower units of the ocoee supergroup by a basin wide unconformity and hiatus a tentative geodynamic model is presented for the ocoee supergroup basin the model is dependent on paleontological data indicating a paleozoic age for the walden creek group and concentrates on lithologic petrographic and basin characteristics the model allows for the possibility that the entire ocoee supergroup was deposited in a paleozoic transtensional basin

early jointing in coal and black shale evidence for an appalachian wide stress field as a prelude to the alleghanian orogeny early ene striking joints present coordinates within both pennsylvanian coal and devonian black shale of the central and southern appalachians reflect an approximately rectilinear stress field with a dimension 1500 km this appalachian wide stress field awsf dates from the time of joint propagation when both the coal and shale were buried to the oil window during the 10 15 m y period straddling the pennsylvanian permian boundary the awsf was generated during the final assembly of pangea as a consequence of plate boundary tractions arising from late stage oblique convergence where maximum horizontal stress s h of the awsf was parallel to the direction of closure between gondwana and laurentia after closure the awsf persisted during dextral slip of peri gondwanan microcontinents when sh appears to have crosscut plate scale trans current faults at around 30 sup o following 10 m y of dextral slip during tightening of gondwana against laurentia the awsf was disrupted by local stress fields associated with thrusting on master basement decollements to produce the local orocline shaped alleghanian map pattern seen today

strain variations and three dimensional strain factorization at the transition from the southern to the central appalachians abstract the transition between the southern and the central appalachian foreland thrust system is a region subjected to at least two non coaxial deformations where typical two dimensional analyses such as cross section balancing or plane strain retro deformation cannot be applied three dimensional factorization can simulate the non coaxial strain history simulation results for compaction and layer parallel shortening lps were compared to finite strains of 16 quartz arenite samples from the tuscarora sandstone successful simulations indicate in the southern appalachians 30 35 total compaction by volume loss including a 4 5 9 e w horizontal shortening during lithification and 5 lps along 150 330 by plane strain or axial symmetric flattening in the central appalachians 30 35 total compaction by volume loss including a 3 7 n s horizontal shortening during lithification and 10 lps along 120 300 by plane strain or axial symmetric flattening in the transition zone 30 compaction by volume loss including a 3 e w horizontal shortening during lithification 5 southern lps along 150 330 by axial symmetric flattening and 5 10 central shortening along 130 or 140 by plane strain or axial symmetric flattening at 10 to the normal central trend from the simulations horizontal shortening in the southern appalachians during lithification in the mississippian was greater than the later alleghanian deformation this shortening probably represents a very early lps developed in unlithified sand that required less deviatoric stress to deform than did the cemented quartz arenites during later tectonic deformation also the oblique and weaker central tectonic lps in the transition zone probably represents behavior at the limits of central appalachian deformation finally the lps simulations indicate that axial symmetric flattening oblate strain is more successful than plane strain as the representative behavior so caution is recommended in assuming plane strain as the dominant strain behavior in orogenic forelands

hyperpycnal transport of carbonaceous sediment example from the upper devonian rhinestreet shale western new york u s a abstract the upper devonian rhinestreet shale of the western new york region of the appalachian basin contains centimeter scale composite beds comprised of a thin black shale layer overlain by several cm of gray shale that are interpreted to have been deposited from hyperpycnal turbidity currents evidence for this explanation includes 1 the presence of planar parallel and cross lamination within some layers and 2 zr al profiles consistent with an inversely graded lower unit overlain by normally graded deposits diminished k al values of the inferred hyperpycnites suggest that they were generated during periods of increased precipitation and consequent intensified continental weathering of the source region within the tectonically active acadian highlands the transition to a wetter climate reflected in these deposits perhaps a response to astronomically induced variations in tropical monsoonal dynamics would have increased the frequency and magnitude of floods within the catskill delta complex thereby enhancing the likelihood that hyperpycnal flows would have been produced organic matter contained within the hyperpyncites probably reflects the entrainment of carbonaceous detritus perhaps much of it in the form fecal pellets from estuarine and shallow marine settings during flood related discharges post depositional oxygen depletion within the organic rich sediment and related bacterial sulfate reduction resulted in the precipitation of abundant diagenetic pyrite intense bioturbation of gray shale intercalated with the dark layers suggests that oxygenated conditions prevailed during accumulation of the sturgeon point section alternatively these deposits reflect a history of repeated early diagenetic burn down events that resulted in the strongly reworked nature of the organic lean gray shale and the sharp upper contacts of the black shale layers the present study offers an alternative explanation of thin black shale beds hosted within organic deficient successions beyond models involving marked changes in productivity on the one hand and the establishment of a stratified water column on the other

contour mapping of relic structures in the precambrian basement of the reelfoot rift north american midcontinent a new contour map of the basement of the reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features the major features are two asymmetric intrarift basins bounded by three structural highs and the rift margins the basins are oriented normal to the northeast trend of the rift two of the highs appear to be ridges of undetermined width that extend across the rift the third high is an isolated dome or platform located between the basins the minor features are three linear structures of low relief oriented subparallel to the trend of the rift two of these located within the rift basins may divide the rift basins into paired subbasins these mapped features may be the remnants of initial extensional rifting half graben faulting and basement subsidence the rift basins are interpreted as having formed as opposing half graben and the structural highs are interpreted as having formed as associated accommodation zones some of these features appear to be reactivated seismogenic structures within the modern midcontinent compressional stress regime a detailed knowledge of the geometries of the reelfoot rift s basement features therefore is essential when evaluating their seismic risk potential

basinwide stratigraphic synthesis and sequence stratigraphy upper pragian emsian and eifelian stages lower to middle devonian appalachian basin

geochemistry of silurian devonian volcanic rocks in the coastal volcanic belt machias eastport area maine evidence for a pre acadian arc the coastal volcanic belt in easternmost maine consists of silurian devonian volcanic rocks related to the convergence of avalonia with the leading edge of composite laurentia just before and during early stages of the acadian orogeny new major and trace element analyses from the machias eastport area indicate that the early to late silurian volcanic rocks in the dennys edmunds and leighton formations formed in a continental arc environment in contrast to some earlier interpretations these rocks have a continuous compositional range from basalt through andesite to rhyolite the mafic rocks in these formations are calc alkaline and show subduction related geochemical signatures on various discrimination diagrams whereas the felsic rocks have geochemical characteristics of volcanic arc granites in contrast the thick volcanic deposits of the younger late silurian eastport formation are bimodal and tholeiitic and have transitional to within plate geochemical signatures thus the transition over time as the acadian seaway closed from subduction and arc settings to continental extension can been seen in this rock sequence and it constrains the time of avalonia s docking regionally volcanic rocks in the kingston terrane of southern new brunswick are likely correlatives of the earlier arc volcanic rocks dennys edmunds and leighton formations while eastport formation analogues are found in the coastal maine magmatic province cranberry island series and probably the passamaquoddy bay volcanic series which similarly appear to have formed after the transition to an extensional environment occurred although other extensional settings cannot be entirely ruled out basalts in the post acadian perry formation indicate continental extension was also active in the late devonian

stratiform tourmalinites in metamorphic terranes and their geologic significance stratiform tourmalinites are significant minor rock types in many regional metamorphic terranes of the world tourmalinites are more widespread than previously recognized and are especially common in proterozoic and early paleozoic sequences dominated by clastic metasedimentary rocks they consist of conformable layers made up primarily of quartz and abundant tourmaline the latter typically exceeding 15 to 20 of rock volumes a few tourmalinites display striking sedimentary structures such as graded bedding cross laminations slump and flame structures and rip up clasts these and other geologic features provide important constraints for assessing the origin of these rocks and they suggest that tourmalinites form by the early diagenetic modification of a primary boron rich chemical precipitate tourmalinites are significant in preserving a valuable record of unusual chemical and paleogeographic conditions in clastic sedimentary basins and in their close association with a variety of stratabound mineral deposits

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

southeastern margin of the middle paleozoic shelf southwesternmost appalachians regional stability bracketed by acadian and alleghanian tectonism lower devonian lower mississippian metasedimentary rocks in the southernmost appalachian orogen near the southeast corner of laurentia record a period of continental margin stabilization bracketed by orogenic events best assigned to the early acadian and earliest alleghanian orogenies these strata are in the talladega belt a far traveled allochthon forming the southwestward extent of the western blue ridge in the allochthon lower paleozoic trailing margin carbonate shelf rocks are unconformably overlain by a 2 5 km thick early acadian silurian lower devonian clastic wedge derived from basement cored uplifts to the south and southeast deposition of this clastic wedge was followed by continental margin stabilization and deposition of a starved basin sequence containing widespread lower devonian lower mississippian chert near the outer shelf margin the chert sequence grades northeastward through a shelf slope basin plain transition into a thick black shale basinal facies these units were deposited during the classic acadian orogeny in new england and the catskill delta s progradation across the central appalachian foreland the major clastic response to acadian uplift the talladega sequence subsequently was subjected to low grade regional metamorphism and pervasive deformation possibly as late as middle mississippian during what were probably the initial stages of the alleghanian orogeny deposition of the lower devonian lower mississippian sequence of the talladega belt thus represents a significant hiatus in deformation along southeast laurentia bracketed first by orogenic activity related to early acadian silurian early devonian foundering of the early paleozoic shelf and deposition of a clastic wedge and later by earliest alleghanian regional dynamothermal events

ambient noise level and site characterization in northern egypt the characterization of the background seismic noise in any seismological network is an essential task for checking the performance of the recording stations hence a funded project from science and technology funding authority stdf was awarded by the national research institute of astronomy and geophysics nriag in 2017 to detect the recording capabilities of the egyptian national seismological network ensn established after the cairo earthquake which occurred in egypt on october 12 1992 this article covers the first output of the project which shows the results of noise assessment for nine broadband stations located in northern egypt for a proper assessment of seismic data quality for a part of the ensn network the passcal quick look extended pqlx software was used which is based on the calculation of power spectral density psd and probability density function pdf for the background noise the analysis covered almost one year of continuously recorded seismic data the results show that some stations have very low detectability and higher noise level in some frequency bands which affects the recording of seismic events while others show good performance complete site analysis for all stations was done to illustrate the present site response to be able to recommend whether to keep a station or to relocate it to achieve this the seismic noise data extracted from all stations of the considered zone was studied to determine the horizontal to vertical h v spectral ratio method to study the amplification parameters of the recording site the output of this study is useful for detecting operational problems in the northern part of the egyptian seismological network and could be relevant for the reconfiguration and optimization of ensn stations

the burnsville fault evidence for the timing and kinematics of southern appalachian acadian dextral transform tectonics there has been a conspicuous absence of documented acadian structures in the blue ridge province of the southern appalachians even though a growing body of evidence suggests that the middle paleozoic acadian orogeny affected the rocks of this region new mapping along with structural petrologic and geochronologic data from western north carolina show that the contact between the metasedimentary and metavolcanic rocks of the ashe metamorphic suite and the grenville basement rocks of the western blue ridge corresponds to a devonian high grade dextral strike slip fault zone the burnsville fault timing of motion on the burnsville fault is constrained by field relationships metamorphic fabrics and assemblages and radiometric ages a u pb zircon crystallization age for a pegmatite sheared by the burnsville fault shows that the last movement on the fault must he younger than 377 ma previously published 4 0 ar 3 9 ar cooling ages show that the burnsville fault must be older than ca 360 ma we have mapped the burnsville fault for 100 km in northwest ern north carolina the gossan lead fault of northwesternmost north carolina and southwestern virginia is the likely continuation of the burnsville fault to the north east southwest of asheville north carolina the burnsville fault may connect to the devonian dahlonega shear zone or may be cut by post devonian thrust faults associated with the alleghanian orogeny diachroneity of acadian elastic wedge the presence of silurian devonian pull apart basins the presence of devonian high grade dextral shearing in the inner piedmont and recent plate reconstructions for the middle paleozoic support the interpretation that the late phase of the acadian orogeny in the southern appalachians was primarily a dextral transpressional event the burnsville fault and the inner piedmont formed the boundaries of a dextral transform margin that may have extended the length of the appalachian orogen this interpretation requires that models for the paleozoic tectonics of the appalachians incorporate major pre alleghanian dextral displacement

thickness trends and sequence stratigraphy of the middle devonian marcellus formation appalachian basin implications for acadian foreland basin evolution analysis of more than 900 wireline logs indicates that the middle devonian marcellus formation encompasses two third order transgressive regressive t r sequences mss1 and mss2 in ascending order compositional elements of the marcellus formation crucial to the successful development of this emerging shale gas play including quartz clay carbonate pyrite and organic carbon vary predictably within the proposed sequence stratigraphic framework thickness trends of marcellus t r sequences and lithostratigraphic units reflect the interplay of acadian thrust load induced subsidence short term base level fluctuations and recurrent basement structures rapid thickening of both t r sequences especially mss2 toward the northeastern region of the basin preserves a record of greater accommodation space and proximity to clastic sources early in the acadian orogeny however local variations in t r sequence thickness in the western more distal area of the basin may reflect the reactivation of inherited eocambrian basement structures including the rome trough and northwest striking cross structural discontinuities induced by acadian plate convergence episodes of block displacement locally warped the basin into northeast southwest trending regions of starved sedimentation and or erosion adjacent to depocenters in which regressive systems tract deposits were ponded block movement appears to have initiated in late early devonian time resulting first in thinning and local erosion of the oriskany sandstone in northwest pennsylvania this study in addition to providing the basis for a predictive sequence stratigraphic model that can be used to further marcellus exploration tells of a foreland basin more tectonically complex than accounted for by simple flexural models

taconian and acadian transpression between the internal humber zone and the gaspe belt in the gaspe peninsula tectonic history of the shickshock sud fault zone the shickshock sud fault has a history of ordovician taconian silurian salinic and devonian acadian movements taconian deformation involving ductile dextral oblique slip faulting is recorded in cambrian rocks in the footwall of the shickshock sud fault metabasalt and metaarkose at amphibolite grade are converted into phyllonite and mylonitic schist shear bands asymmetric garnet porphyroclasts c s fabrics and mica fish textures indicate dextral shearing the regional sense of shear is top to west and southwest on generally southeast dipping shear zones hornblende of metabasalt yielded an 40ar 39ar age of 455 9 2 ma and muscovite from the mylonitic schist yielded an 40ar 39ar age of 454 3 0 9 ma which indicate metamorphism and deformation during the taconian orogeny evidence for silurian activity is indicated by the salinic unconformity to the south related to normal block faulting deformation features in the ordovician and silurian devonian rocks in the hanging wall were predominantl

along strike acadian structural variations in the quebec appalachians consequence of a collision along an irregular margin the quebec appalachians have been shaped by two major orogenies the middle to late ordovician taconian and the middle devonian acadian thrust faults and nappe structures characterized the taconian deformation throughout the quebec appalachians whereas structural styles pertaining to the acadian orogeny differ from southern quebec to the gaspe peninsula structural analysis of late ordovician to middle devonian supracrustal rocks shows two different acadian deformational regimes strike slip tectonics in the gaspe peninsula and thrust or dip slip tectonics in southern quebec in southern quebec acadian regional deformation produced folds which vary from upright to overturned and tight to isoclinal from the nw to the se major faults e g la guadeloupe are northwestward directed thrust faults marked by highly ductile shear zones outlined by mylonite in the gaspe peninsula structural trend is ne and major e w dextral strike slip faults transect this trend folds are open and upright inclined and tighter near the major faults where they have a clockwise rotation like the regional cleavage major faults e g grand pabos follow corridors delimiting high strain zones where the fabrics developed are indicative of a ductilebrittle regime of deformation in the temiscouata region major structural features of both southern quebec and the gaspe peninsula are recognized structures of this region consist of ene trending dextral strike slip faults and high angle wnw verging reverse faults parallel to major nne folds these structures reflect the transition from purely horizontal movement along major strike slip faults of the gaspe peninsula to vertical movement along thrust faults in southern quebec acadian structural variations within the quebec appalachians are interpreted in terms of a continental collision of gondwana along the irregular margin of laurentian and its taconian accreted terranes