balanced section in thrust belts part 1 construction balanced geological cross sections are an important aid to understanding thrust belt structures and to estimating their hydrocarbon bearing potential two approaches to section balancing have been taken in the past construction of retrodeformable sections from raw data and modification of previous interpretations after an evaluation procedure these approaches may be unified by use of a langrangian grid projection of datum locations dips and stratigraphic thicknesses onto the section plane is performed directly by calculation or graphically using plunge lines spline interpolation and parallel fold modeling are the simplest ways of filling gaps in the sectional data but isogon interpolation yields rheologically more realistic results faults are interpolated using cutoff geometries as constraints fault tips are located from distance displacement plots whereas the depth to detachment is obtained by a modified chamberlin construction interpolations made on the section are directed back to the geologic map via plunge lines although these techniques of section construction do not guarantee a balance especially when complexities such as growth structures diapirs or strike slip faults are present they eliminate many potential errors

central costa rica deformed belt kinematics of diffuse faulting across the western panama block fault kinematics seismicity and geodetic data across central costa rica reveal a diffuse fault zone here named the central costa rica deformed belt ccrdb the ccrdb defines the western margin of the panama block and links the north panama deformed belt npdb along the caribbean coast with the middle america trench mat along the pacific coast the junction of the ccrdb and the mat coincides with an abrupt transition from smooth to rough crust on the subducting cocos plate rough smooth boundary shallow subduction of rough thickened oceanic crust associated with the cocos ridge shifts active shortening into the volcanic arc along faults of the ccrdb variable fault kinematics along this zone may reflect three combined deformation mechanisms horizontal shortening and shear from oceanic ridge indentation basal traction from shallow subduction and localized block uplift from subducting seamount roughness within the forearc domain 1 mesoscale faults express transtension where steep ne striking regional scale faults intersect the pacific coast across the volcanic arc domain 2 mesoscale faults exhibit mostly sinistral and dextral slip on ne and nw striking conjugate faults respectively approaching the npdb in the back arc domain 3 transcurrent faulting is modified by transpression and crustal thickening fault kinematics are consistent with earthquake focal mechanisms and global positioning system gps measurements radiometric age constraints confirm that faulting postdates the late neogene onset of shallow subduction the ensuing deformation front has propagated northward into the volcanic arc to its present position along the seismically active ccrdb within the forearc the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts

predicting patterns of strain from three dimensional fold geometries neutral surface folds and forced folds the geometries and densities of fractures associated with fold structures can be predicted by assuming that the strains accommodated by fractures mimic the bulk strains induced in the strata during folding this paper examines from a theoretical standpoint the distributions of bedding plane strains expected in folds formed by various folding mechanisms the relationship between the state of bedding plane strain and fold surface geometry is found to vary according to different fold types distinguished on the basis of their curvature properties the first type are developable fold surfaces which have gaussian curvature equal to zero folding mechanisms which are dominated by the mechanical strength of the layering such as buckling produce surfaces of this type folds of this type allow the possibility of estimating the bedding plane strains from the geometrical features of the folded layer neutral surface folds and flexural slip folds are discussed as examples the other main class of folds have non developable surfaces which have non zero gaussian curvature folded surfaces with this form arise predominantly from mechanisms that involve the passive deflection of the layering in response to displacement gradients originating outside of the layer e g drape folding although the geometry of these surfaces implies the presence of bedding plane strains the quantification of these strains cannot be made from the fold geometry but requires additional information on these displacement patterns

inner forearc response to subduction of the panama fracture zone southern central america abstract subduction of the right lateral panama fracture zone along the convergent margin of central america creates abrupt lateral variations in convergence rate obliquity and subducting crustal thickness at its intersection with the middle america trench this intersection known as the panama co nz ca triple junction is migrating to the southeast at a rate of 55 mm yr and currently coincides with the lateral termination of the fila costena thrust belt in the inner forearc of the overriding plate mapping in the inner forearc in the area that straddles the subducting panama fracture zone reveals that cocos caribbean convergence west of the triple junction leads to the development of an inner forearc thrust belt inboard of the colliding cocos ridge while little deformation is evident inboard of nazca caribbean convergence east of the triple junction this results in the lateral termination of the fila costena thrust belt in the region of the forearc that projects over the panama fracture zone where four out of five mapped thrust faults tip out and are buried by lahars three new balanced cross sections indicate a steep gradient in shortening from the center of the thrust belt to its southeastern termination the short term history of the inner forearc recorded in the landscape and topography of the fila costena is consistent with the southeastward migration of the thrust belt and the panama triple junction throughout the past 3 ma with evidence for the growth of a new topographic divide and reorganization of stream channel networks

morphology of the itapeva to tramandai transgressive dunefield barrier system and mid to late holocene sea level change the surface morphology of the transgressive dunefield barrier extending from itapeva to tramandai along the northern littoral of rio grande do sul is examined and an attempt is presented to link morphological changes across the barrier to holocene sea level changes the 4 5 km wide holocene barrier displays two typical morphologies an inner part dominated by large scale continuous alongshore overlapping dunefield phases comprising sand sheets dunes deflation plains and precipitation ridges and an outer part dominated by discontinuous medium to small scale triangular to lobate transgressive dunefield phases cut by both active and relict palaeo creeks or washouts holocene sea level in the region rose to c 1 to 3 m above present reaching a maximum around 5100 years bp and then progressively fell to the present level we argue that the effect on barrier development was to suppress the development of a drainage system during the rising and maximum stages and encourage the development of an organized drainage system in the form of regularly spaced washouts during the falling period and that this change in sea level from rising to falling therefore produced the large scale differences in barrier morphology copyright 2006 john wiley sons ltd

implications of the north and south panama thrust belts for the origin of the panama orocline high resolution imaging of the north nptb and south panama thrust belts using the seamarc ii seafloor mapping system and digital seismic reflection profiling provides an excellent data base with which to determine the kinematics of oroclinal deformation of panama remarkable continuity of structural orientations over 300 km along the nptb argues for widespread uniformity in direction of thrusting between panama and the caribbean plate westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence two major cross faults cut the wedge in its eastern part forming two minor blocks with different relative motions with respect to the caribbean frontal thrust orientations change across these faults major faults have been proposed to cut the isthmus of panama but simple cross isthmus faulting alone cannot explain the orientations of the major thrusts of the nptb bending of a flexible beam can explain much of the observed structure both directions and rates though the scale of the orocline precludes considering this as an elastic phenomenon plastic deformation of panama may be a useful mechanical explanation at least for eastern panama but consideration of plastic slip lines must include the northern andes as well changing from more rigid blocklike offsets on faults in western panama to a continuum of strain in eastern panama may be the best explanation for observations at present

middle miocene to present plate tectonic history of the southern central american volcanic arc abstract new mid miocene to present plate tectonic reconstructions of the southern central american volcanic arc cava reveal that the inception of cocos ridge subduction began no earlier than 3 ma and possibly as late as 2 ma the cocos ridge has been displaced from the malpelo ridge to the southeast since 9 ma along the panama fracture zone pfz system ambiguous pfz and coiba fracture zone cfz interaction since 9 ma precludes conclusively establishing the age of initial cocos ridge subduction detailed reconstructions based on magnetic anomalies offshore reveal several other variations in subduction parameters beneath southern central america that preceded subduction of the cocos ridge including southeastward migration of the nazca cocos caribbean triple junction along the middle america trench mat from 12 ma to present and subduction of 2 km high scarps both parallel and perpendicular to the trench from 6 to 1 ma the timing of changes in subduction processes has commonly been determined by and correlated with geologic changes in the upper plate however reliable 40 ar 39 ar dating of these events has become available only recently abstr programs geol soc am 2002 these new dates better constrain the magmatic and structural history of southern costa rica observations from this data set include a gap in the volcanic record from 11 to 6 ma which coincides temporally with emplacement of most plutons in southern costa rica normal arc volcanism ceased after 3 5 ma in southern costa rica and pliocene mostly 1 5 ma adakite volcanism was widely distributed from central panama to southern costa rica though volumetrically insignificant this new data reveals that many geologic phenomena commonly attributed to subduction and underplating of the buoyant cocos ridge in fact precede inception of cocos ridge subduction and seem to correlate more favorably in time with earlier tectonic events adakite volcanic activity corresponds in space and time with the subduction of a large scarp associated with a tectonic boundary off southern panama regional unconformities and an 11 6 ma gap in arc volcanism match temporally with oblique subduction of the nazca plate beneath central and southern costa rica cessation of volcanic activity low temperature cooling of plutons in the cordillera de talamanca ct and rapid increases in sedimentation in the fore arc and back arc basins coincide with passage of the nazca cocos caribbean triple junction and initiation of subduction of rough crust associated with cocos nazca rifting 3 5 ma closely followed by initial subduction of the cocos ridge 2 3 ma none of the aforementioned geologic events occurred at a time that would allow for underplating by the cocos ridge rather they are probably related to complex interactions with subduction of complicated plates offshore all of the aforementioned events indicate that the southern central american subduction system has been in flux since at least 12 ma

deformation of erosive and accretive forearcs during subduction of migrating and non migrating aseismic ridges results from 3 d finite element models and application to the central american peruvian and ryukyu margins subduction of aseismic oceanic ridges causes considerable uplift and deformation of the upper plate and may lead for example to the indentation of the forearc the formation of marine terraces or distinct fault patterns in the upper plate depending on the orientation of the ridge relative to the plate convergence direction the ridge may either be stationary or migrate along the margin here we use three dimensional numerical models to investigate the tectonic evolution of forearcs affected by ridge subduction in different experiments we distinguish between migrating non migrating ridges and accretive erosive margins respectively our results reveal that displacement and strain fields above migrating and non migrating ridges are asymmetric with respect to the ridge axis unless both ridge and plate convergence direction are perpendicular to the trench as the asymmetric deformation pattern shifts along the margin through time uplift caused by the underthrusting ridge is followed by subsidence when the ridge crest passed by and regions initially experiencing shortening may subsequently undergo extension and vice versa if the forearc comprises an accretionary prism the ridge induced reentrant is larger than those in models with erosive forearcs and strain localizes in the frontal part of the wedge additional models with a setup adjusted to the cocos and gagua ridges provide constraints on the onset of their subduction at the central american and ryukyu margins at 2 ma and 1 ma respectively displacement and strain fields from a model for the nazca ridge collision zone show good agreement with geological data from marine terraces and quaternary faulting

closure of the isthmus of panama the near shore marine record of costa rica and western panama the final closure of the isthmus of panama at 3 5 ma divided the american tropical ocean into two separate and different oceanographic regions consequences for the marine biota were profound but hitherto correlation of the pacific and caribbean coastal sections has not been precise enough to track biologic patterns we present here a correlation of 31 sections from the pacific and caribbean coasts of costa rica and western panama using calcareous nannofossils and planktonic foraminifera at both the tops and bottoms of each formation we estimate that the caribbean section ranges from 8 2 ma to 1 7 ma and the pacific sequence from 3 6 ma to 1 7 ma these intervals bracket postulated dates for final closure of the isthmus and provide the first well dated record of middle and late pliocene faunas from the region the caribbean and pacific sections include very different environments of deposition yet there is sufficient overlap and diversity of habitats to permit meaningful biological comparisons on the caribbean side formations tied together by the overlap of the upper pliocene markers sphenolithus abies and pseudoemiliana lacunosa 3 5 ma to 3 6 ma range from very shallow to shallow inner shelf 200 m and upper slope 200 800 m the pacific coast sections were mostly deposited in a trench slope environment which is absent on the caribbean side these sections fortuitously include abundant thick intra formational slumps containing shallow water fauna more appropriate for biological comparison with the caribbean biota similarly the 1 9 ma to 1 5 ma interval well constrained by various taxa includes middle to outer shelf and inner shelf to upper slope deposits on the caribbean side and marginal marine to inner shelf deposits on the pacific coast using our new biostratigraphic framework to correlate previously poorly constrained mollusc collections we show that evolutionary divergence of the pacific and caribbean near shore marine faunas had occurred by 3 5 ma this strongly suggests that the isthmus was effectively closed by this time

three dimensional retro deformation of the lechtal nappe northern calcareous alps we determine the amount of shortening depth of detachment and the style of deformation of the lechtal nappe the largest nappe within the northern calcareous alps german austrian border by three dimensional retro deformation analysis of ca 40 km2 area a three dimensional model was constructed by splining lines from eight n s cross sections spaced e w at ca 4 km intervals to define fault geometrics and seven stratigraphic layers within the permo triassic to cretaceous strata the modelled volume has four main thrusts which link to a common detachment at 2 5 km depth three dimensional fault displacements and heaves were determined using allan diagrams the algorithms fault parallel flow and flexual slip unfolding were used to restore northwards movement on the thrusts and folding of beds over thrust planes respectively minimum shortening estimates vary from east to west from 25 to 42 typical error 6 but additional shortening in the west is due only to folding

strain displacement and rotation associated with the formation of curvature in fold belts the example of the jura arc abstract a new simplified genetic classification scheme for arcuate fold thrust belts is proposed based on total strain patterns and displacement vector fields we distinguish three extreme end member models 1 oroclines pure bending of an initially straight belt 2 piedmont glacier with divergent transport directions and 3 primary arcs a simple geometric model set up for the simulation of strain patterns in primary arcs with uniform transport direction demonstrates that divergent strain trajectories and rotations of passive marker lines do not require any divergence in displacement directions these often quoted arguments are insufficient for the identification of oroclinal bending or piedmont glacier type of arc formation only three dimensional restorations of an arc provide the critical information about displacement directions in their absence arc parallel stretches and rotations in comparison with total strains provide the most useful criteria for the distinction of arc formation modes as an example the jura fold thrust belt of the external alps is discussed a large set of strain data includes total shortening estimates based on balanced cross sections local strain axes orientations from the inversion of fault populations homberg c 1996 unpublished phd thesis universite de paris vi france tectonic stylolites and micro strains from twinning in sparry calcite strain trajectories maximum shortening direction computed from these data define a strongly divergent fan with a 90 opening a complete displacement vector field for the entire jura has been determined from balanced cross sections augmented with three dimensional block mosaic restorations philippe y 1995 unpublished phd thesis universite de chambery france displacement vectors diverge by about 40 markedly less than strain trajectories the non parallelism between strain trajectories and transport directions indicates that considerable wrenching deformation did occur in both limbs of the jura arc paleomagnetically determined clockwise rotations of 0 13 from ten sites kempf o et al terra nova 10 6 10 behind the right hand half of the jura arc and two sites with a combined 23 anticlockwise rotation behind the left hand half of the arc are and additional argument in favor of such a wrenching deformation we conclude that the jura arc formed as a primary arc with a minor component of piedmont glacier type divergence in transport directions

thrust surface geometry implications for thrust belt evolution and section balancing techniques abstract the application of modern concepts of thrust tectonics to orogenic belts has yielded many detailed restorable sections thrusts generally restore with a staircase geometry onto the undeformed stratigraphic template since the work of rich 1934 it has been generally assumed that thrusts have propagated with a staircase trajectory and that hangingwall structures result from the movement of the thrust sheet up footwall ramps however thrusts often cut through previously folded strata with a smooth trajectory smooth trajectory thrusts can be identified from footwall geometry and strain state once a smooth trajectory thrust has been identified it is insufficient to produce a balanced deformed section and its restored counterpart sequential balanced sections must be constructed reversing the deformation history the restoration of thrusts to a staircase trajectory has no mechanical significance as the thrust is merely acting as a marker if restored back beyond the time at which it propagated thrust ramps are not necessarily actual steps in the thrust surface they are often just zones where the thrust cuts through bedding in a smoothly curving trajectory ductile deformation in thrust belts can reduce bed length at depth thus removing the need for structures which are necessary to preserve constant bed lengths ductile deformation in the internal zones of orogenic belts renders the construction of balanced sections in these zones highly problematical

forearc response to subduction of the cocos ridge panama costa rica stratigraphic paleontologic and structural data from two panama costa rica forearc peninsulas located landward of the aseismic cocos ridge along the southernmost middle america trench document rapid pliocene subsidence and basin infilling followed by quaternary deformation and uplift on the burica peninsula an approximately 3 000 m thick pliocene pleistocene clastic sedimentary sequence characterized by fine grained turbidite deposits and volumetrically minor coarser grained channel fill slump and debris flow deposits bears depth diagnostic foraminifera that document rapid pliocene subsidence and deposition on a south dipping paleoslope new paleobathymetric and age estimates from foraminifera in these rocks indicate shallowing of depositional paleodepths from 2 000 to 1 200 m during the late pliocene present day exposure of these rocks indicates an average quaternary uplift rate of 1 mm yr the turbidite section on the burica peninsula is interpreted to represent a trench slope deposit and is correlated to a similar marine sedimentary section on the adjacent osa peninsula of costa rica structures developed within pliocene pleistocene strata of the outer forearc burica and osa peninsulas record minor 5 subhorizontal margin perpendicular shortening and margin perpendicular extension bedding orientations on the osa and burica peninsulas generally record arcward tilting by 10 30 of these outer forearc blocks on the burica peninsula the medial fault zone a north striking high angle right lateral strike slip fault has truncated and reoriented the pliocene pleistocene sedimentary sequence by fault bend forced folding a linear margin parallel fault herein named the ballena celmira fault zone bounds the arcward side of the osa and burica peninsulas and has accommodated vertical displacements between the outer forearc burica osa area and inner forearc terraba area throughout the tertiary structural and seismicity observations from the forearc region suggest that neogene deformation of the leading edge of the arc in this area reflects the effects of isostatic uplift and possibly increased interplate shear stresses as a result of subduction of the cocos ridge beneath this part of the arc since 1 m y ago

geomorphic and exhumational response of the central american volcanic arc to cocos ridge subduction 1 the timing of collision of the cocos ridge at the middle america trench remains one of the outstanding questions in the tectonic evolution of the central american convergent margin new analyses of the tectonic geomorphology of the cordillera de talamanca the extinct volcanic arc inboard of the cocos ridge coupled with low temperature thermochronometry data provide insight into the cooling and erosional history of the arc from late miocene to present we identify a low relief surface at high elevation along the northeastern flanks of the range which represents a relict erosional landscape cut across shallow plutonic rocks of the arc edifice longitudinal profiles of rivers on this surface are isolated from steep downstream sections by prominent knickzones that are interpreted to reflect a migrating wave of transient incision generated during differential rock uplift of the range reconstruction of pre incision profiles suggests that rock uplift during the growth of the cordillera de talamanca is no greater than 2 km this inference is corroborated by results from our apatite u th he and apatite fission track analyses along an elevation transect on mt chirripo the highest mountain in the cordillera de talamanca low temperature cooling ages overlap significantly with published high temperature40ar 39ar ages the combined results imply that rapid cooling in the late miocene was related to secular cooling of a shallow pluton rather than exhumation our results imply that rapid incision along downstream channel segments differential rock uplift and growth of the talamanca as a bivergent orogenic wedge associated with the onset of cocos ridge subduction are relatively young characteristics of the range a review of previously published radiometric ages and revised plate reconstructions for the late miocene further suggest that the cessation of arc volcanism in both the cordillera de talamanca and the cordillera central of western panama was coeval with the initiation of oblique subduction of the nazca plate during the late miocene thus we conclude that the cessation of arc volcanism in the late miocene 5 8 ma and the onset of cocos ridge collision 3 ma are separate events that reflect recent changes in the configuration of the plate boundary system

kinematic 3 d retro modeling of an orogenic bend in the south limon fold and thrust belt eastern costa rica prediction of the incremental internal strain distribution the south limon fold and thrust belt in the back arc area of southern costa rica is characterized by a 90 curvature of the strike of the thrust planes and is therefore a natural laboratory for the analysis of curved orogens the analysis of curved fold and thrust belts is a challenge because of the varying structural orientations within the belt based on seismic reflection lines we created a 3 d subsurface model containing three major thrust faults and three stratigraphic horizons 3 d kinematic retro deformation modeling was carried out to analyze the spatial evolution of the fold and thrust belt the maximum amount of displacement on each of the faults is from hinterland to foreland thrust 1 800 m thrust 2 600 m thrust 3 250 m the model was restored sequentially to its pre deformational state the strain history of the stratigraphic horizons in the model was calculated at every step this shows that the internal strain pattern has an abrupt change at the orogenic bend contractional strain occurs in the forelimbs of the hanging wall anticlines while a zone of dilative strain spreads from the anticline crests to the backlimbs the modeling shows that a nne directed transport direction best explains the structural evolution of the bend this would require a left lateral strike slip zone in the north to compensate for the movement and thereby decoupling the south limon fold and thrust belt from northern costa rica therefore our modeling supports the presence of the trans isthmic fault system at least during the plio pleistocene

the seismic source parameters of the 1991 costa rica aftershock sequence evidence for a transcurrent plate boundary the april 22 1991 valle de la estrella costa rica earthquake ms 7 6 was a back arc thrusting event associated with the underthrusting of the caribbean plate beneath central america a network of three passcal type portable instruments was deployed to monitor the aftershock activity in southern costa rica 2 to 6 weeks after the main shock the waveforms recorded on three component midperiod seismometers were used to recover source information for 15 small aftershocks magnitudes between 3 2 and 4 4 with a linear moment tensor inversion method we conducted several tests to investigate the effects of unknown structure and event mislocation on source parameter recovery the longer period waveforms in general are less sensitive to the effects of the structural details so that the essential source information can be successfully extracted from the waveform data the earlier part of the seismic wave forms has proven to be the most important carrier of the source information a gross crustal model can be used to describe the structure for the source study the small changes in the waveform character resulting from the mislocation of the events or inexact green s functions generated from the oversimplified crustal model do not prohibit us from the recovery of the source orientation at local distances in contrast the determination of the focal depth is subject to uncertainty because of the lack of detailed structural information our focal mechanisms are generally in good agreement with p wave first motion fault plane solutions determined from a local short period network the aftershocks show a clear spatial segmentation based on focal mechanism type most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock in contrast a cluster of aftershocks northwest of the main shock showed dominantly left lateral strike slip motion on a northeasterly striking nodal plane this suggests that a diffuse deformation zone exists in central costa rica and is characterized by left lateral strike slip motion this diffuse transcurrent deformation zone coincides with several geologic and geophysical features and perhaps is a result of the slower subduction rate of the buoyant cocos ridge than its adjacent segments along the middle america trench mat the diffuse transcurrent boundary may intersect with the north panama deformed belt npdb near limon costa rica and is very likely a plate boundary for the proposed panama block

predicting the orientation of joints from fold shape results of pseudo three dimensional modeling and curvature analysis we treat layers of sedimentary rock as elastic plates and predict the orientations of joints by assuming that they open parallel to the maximum instantaneous stretch of a layer because the direction of maximum instantaneous stretch is parallel to the maximum curvature of a surface we hypothesize that joints will trend parallel to the minimum curvature of an elastically deformed layer after constructing pseudo three dimensional trishear models of laramide style uplifts that grow self similarly we calculated the direction of minimum curvature axes during the evolution of the fold our analysis of minimum curvature axes in evolving folds suggests several important characteristics for fold related joint sets 1 joints that are neither parallel nor perpendicular to the fold axis may be induced by local fold related strains 2 at any time during folding joint orientations may vary according to the structural position on a fold 3 at any location on a fold joint orientation may depend on when a joint forms during the evolution of the fold and 4 joint patterns in trishear folds may vary with stratigraphic position natural folds that evolve along simple geometric pathways may develop fold related joint sets the orientation dominance abutting relationships spacing and continuity of which will vary systematically throughout the structure this variation in joint system architecture may reflect the history of fold growth

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

deformation style and basin fill architecture of the offshore limon back arc basin costa rica abstract the limon back arc basin located at the eastern coast of costa rica is part of the southern central american arc trench system basin evolution started in late cretaceous times as response to the onset of the subduction of the farallon plate below the caribbean plate the limon basin can be subdivided into a northern and a southern sub basin separated by the e w trending trans isthmic fault system a regional grid of offshore seismic lines allows the comparative analysis of the basin fill architecture and the deformation style in both sub basins the northern sub basin shows exclusively normal faults the basin fill architecture resembles a passive continental margin setting showing a distinct wedge shaped geometry and seaward propagating depositional units the area of the rio san juan delta is characterized by gravity driven deltaic deformation exhibiting a series of listric growth faults both on the shelf and in slope areas in contrast to the northern basin the southern sub basin is characterized by the development of thin skinned fold and thrust belt tectonics best recorded in concentric or asymmetric hangingwall anticlines separated by listric or planar southwestward dipping thrust faults due to the pronounced ne propagating of folding the shelf is broader and the slopes are steeper in the southern sub basin compared with the northern sub basin thus both sub basins show a very different tectonic style in the north limon basin an extensional regime established whereas in the south limon basin compression dominated however in both sub basins the basal detachment is probably controlled by a lithological change from limestone to shale

a plate tectonic model for the mesozoic and early cenozoic history of the caribbean plate abstract we present a model in which the caribbean plate is an intra american feature formed along the caribbean spreading center as opposed to the current model that considers the caribbean plate as a far travelled crustal segment that formed in the pacific region paleomagnetic data which cover an age range from jurassic through paleocene indicate the ophiolite complexes in costa rica and panama formed in an equatorial position the ophiolites did not fundamentally change their position relative to south america since their origin basaltic rocks of the lower part of the ophiolites are of mid ocean ridge type suggesting formation at a spreading center they are interpreted to have formed as part of the proto caribbean crust at a spreading axis in an intra american position during jurassic and early cretaceous times the upper part of the ophiolites is mainly built up by island arc and intraplate basalts the island arc basalts evolved at the central american landbridge which started in the middle cretaceous the intraplate basalts are related to the caribbean plateau basalt which thickened and stiffened the caribbean crust in middle cretaceous to probably campanian times changes in relative plate motions along the middle american convergent plate margin are reflected by orientations and time sequences of paleostress tensors calculated from fault slip data in southern mexico and costa rica kinematic indicators from mylonite zones display sinistral movement along secondary shear zones within the southern mexican crustal complexes and along the northern boundary of the chortis block in the motagua polochic fault system this reflects the eastward drift of the chortis block since the beginning of the cenozoic at this time the chortis block became part of the caribbean plate which moved more than 1000 km towards the east relative to the north and south american plates during the cenozoic

kinematic model for the curvature of the northern subalpine chain france abstract analyses of cross sectional shortening mapped faults and calcite twin strain are used in conjunction with geometric kinematic modeling to interpret the mechanism of curvature formation in the northern subalpine chain of france there is a 45 change in strike around the chain that is convex towards the foreland northwest five models for fold thrust belt curvature are applied to the northern subalpine chain they can reproduce the geometry and the kinematic history of a curved fold thrust belt and can be used to predict the magnitude and orientations of strains and the three dimensional geometry of the curvature the models are pure bending radial thrusting curve parallel simple shear uniform displacement uniform shortening and transport parallel simple shear two principal criteria for distinguishing among the models geometrically are the horizontal tangential strain extension or contraction parallel to strike and cross sectional shortening the transport parallel simple shear model predicts no tangential extension in the central domain and greater radial shortening and significant tangential extension in the northeastern and southern domains this is the pattern observed in the northern subalpine chain therefore the curvature of the northern subalpine chain is attributed to a transport parallel simple shear mechanism

a hierarchical tectonic model of the gulf of mexico and caribbean region abstract a quantitative computer graphic methodology hierarchical tectonic analysis has been used to model the tectonic evolution of the gulf of mexico and caribbean region this method employs a highly structured hierarchical description of the relative motions of pairs of tectonic elements in order to quantify the timing and style of plate interactions the main features described in this model are 1 1 a revised fit of pangea 2 2 the opening of the gulf of mexico in the middle late jurassic 3 3 the formation of a proto caribbean ocean due to the separation of north and south america during the early cretaceous 4 4 the formation of the greater antilles arc along the western margin of the proto caribbean ocean 5 5 the insertion of the farallon plate between north and south america during the middle cretaceous resulting in the northeastward advance of the greater antilles arc and the subduction of proto caribbean oceanic crust 6 6 the complete subduction of the proto caribbean ocean and the collision of the greater antilles arc with the bahamas platform in the late cretaceous 7 7 the truncation of the caribbean section of the farallon plate from its pacific counterpart and the formation of the panama arc latest cretaceous and 8 8 the westward movement of north and south america with respect to the caribbean plate resulting in the opening of the yucatan basin paleocene and subsequently in the evolution of the cayman trough and e w zone of strike slip

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

map view retrodeformation of an arcuate fold and thrust belt the jura case 1 the mechanism of arcuate mountain range formation is a matter of debate here we perform a map view restoration of a detailed three dimensional model of the jura arc a typical arcuate mountain range in the foothills of the swiss and french alps this retrodeformation is performed using the unfold surface balanced program based on a block mosaic method it results in a displacement field divergent toward the deformation front this displacement field suggests counterclockwise rigid rotations in the horizontal plane up to 30 in the southern internal jura and a corresponding vertical axis rotation or shear strain of the savoie molasse basin hinterlandward we also predict a 10 clockwise vertical axis rotation of the molasse basin in switzerland all these rotations agree with those documented by paleomagnetic data the former are taken to result from a substantial decrease in shortening at the southern jura end while the divergence of displacements is interpreted to result from variations in the detachment level distribution

costa rica petroleum geology of the caribbean margin the existing database for the caribbean margin of costa rica figure 1 in cludes 1 information from wells drilled in the past by sinclair union gulf elf and pemex and more recently at the end of the 1980s with the aid of petrocanada international assistance pciac table 1 2 regional to semidetailed marine and land seismic data acquired in the 1970s and 1980s by gsi elf the university of texas sea gap gulf comesa and petrocanada figure 2 3 complete coverage of aeromagnetic and gravimetric high resolution data 4 geochemical analysis from outcrop samples oil and gas seeps as well as from well cuttings of the majority of the wells drilled in the caribbean and 5 a comprehensive sedimentological analysis of the cretaceous tertiaty sedimentary cover of the basin

fault related folding a review of kinematic models and their application abstract folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts accretionary wedges fold and thrust belts and intra plate settings in either strike slip compressional or extensional regimes due to their widespread occurrence knowledge about the development of these structures is important to a broad spectrum of geoscience sub disciplines such as structural geology seismology geomorphology petroleum geology and quaternary geology fault related folding has been analyzed intensively over the last 140 years for the sake of this review we divide the folds according to the way the faults and the folds form that is into detachment fault bend and fault propagation folds all fault related folds are caused by changes in fault parameters the simplest method to produce folds is to transport material along faults that have stepped flat ramp flat geometries fault bend fold alternatively the slip can decrease along the length of the fault and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy either a detachment fold or a fault propagation fold is formed respectively detachment folding was first investigated in the early 20th century whereas the full significance of fault propagation folds was recognized quite late in the 1980s seminal work on fault related folding was carried out in the 1930s but quantitative kinematic models have only been available in the last 30 years these models are extremely valuable because they allow a comprehensive understanding of the evolution of fault related folds and lead to more accurate predictions of the sub surface structure from the mid 1990s onwards numerical simulations have been used to identify how fault parameters such as dip and fault bend angle propagation to slip ratio and shape of the trishear zone influence the geometry of the related folding this is directly applicable to the analysis of the shape of anticlines produced however this does not mean that fold geometry is uniquely related to fault geometry on the contrary different kinematic approaches can lead to a similar fold shape

the crustal structure of the cocos ridge off costa rica 1 the submarine cocos ridge in the northwestern panama basin a bathymetric feature more than 1000 km long and 250 500 km broad is about 2 km shallower than the adjacent basin it is generally interpreted as the trace of the galapagos hot spot two 127 and 260 km long seismic wide angle sections were recorded along and across this ridge offshore the osa peninsula costa rica crustal thickening is seen everywhere along the sections on the northwestern outer ridge flank increased thickness is exclusively attributed to the upper crust and expressed by 2 km thick flow basalts the quepos plateau caps the upper crust in this area toward the center of the cocos ridge the moho deepens from 11 12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80 of the crust and is three times the thickness of normal oceanic lower crust it is homogeneously structured and the velocities which range from 6 5 km s at the top to 7 35 km s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition similarities to the crustal velocity structure of iceland central kerguelen plateau and broken ridge are consistent with a formation of this 13 15 ma old cocos ridge segment by excessive magmatism in a near plate boundary setting

arc continent collision and orocline formation closing of the central american seaway closure of the central american seaway was a local tectonic event with potentially global biotic and environmental repercussions we report geochronological six u pb la icp ms zircon ages and geochemical 19 xrf and icp ms analyses data from the isthmus of panama that allow definition of a distinctive succession of plateau sequences to subduction related protoarc to arc volcaniclastic rocks intruded by late cretaceous to middle eocene intermediate plutonic rocks 67 6 1 4 ma to 41 1 0 7 ma paleomagnetic analyses 24 sites 192 cores in this same belt reveal large counterclockwise vertical axis rotations 70 9 6 7 and moderate clockwise rotations between 40 4 1 and 56 2 11 1 on either side of an east west trending fault at the apex of the isthmus rio gatun fault consistent with isthmus curvature an oligocene miocene arc crosscuts the older deformed and segmented arc sequences and shows no significant vertical axis rotation or deformation there are three main stages of deformation 1 left lateral strike slip offset of the arc 100 km and counterclockwise vertical axis rotation of western arc segments between 38 and 28 ma 2 clockwise rotation of central arc segments between 28 and 25 ma and 3 orocline tightening after 25 ma when this reconstruction is placed in a global plate tectonic framework and published exhumation data is added the central american seaway disappears at 15 ma suggesting that by the time of northern hemisphere glaciation deep water circulation had long been severed in central america

constraints on inner forearc deformation from balanced cross sections fila costena thrust belt costa rica 1 the fila costena thrust belt in the forearc of costa rica is accommodating a significant portion of the convergence of the cocos plate and panama microplate geologic mapping of the thrust belt depicts a duplex with three horses that incorporate eocene limestones and oligocene to early miocene clastics inboard of the subducting cocos ridge axis by constructing a cross section at this location along a ne sw trending transect perpendicular to the thrust belt we constrain a shortening rate of approximately 40 mm a and propose that as much as 50 of the total plate convergence rate is taken up in the inner forearc the eocene limestones at the base of the thrust sheets pinch out in both directions away from the onland projection of the cocos ridge axis owing to decrease in slip on faults and a lateral ramp in the basal decollement the thrust belt terminates near the panama border at the onland projection of the subducting panama fracture zone these observations suggest that shortening is propagating to the east with the migration of the panama triple junction and the onset of shallow subduction of the thickened edge of the cocos plate the absence of similar features in the nicaraguan forearc where the subducting crust is older subducts more steeply and lacks incoming ridges and seamounts indicates that deformation of the forearc basin in costa rica reflects greater coupling between the converging plates inboard of the cocos ridge

observations of rayleigh wave phase velocity and coseismic deformation using an optical fiber interferometric vertical strainmeter at the safod borehole california we present observations from a vertical optical fiber interferometric strainmeter in the san andreas fault observatory at depth borehole near parkfield california the sensor detects both teleseismic earthquakes and local events along with coseismic strain steps consistent with theoretical dislocation models for teleseismic events we investigate the possibility of determining local rayleigh wave phase velocities beneath the borehole by comparing the ratio of vertical ground acceleration from a nearby seismometer to vertical strain while similar studies have used horizontal components and rotations this is the first such attempt utilizing vertical measurements we show that at periods from around 16 40 seconds we can recover general dispersion characteristics that are within a few percent of models of realistic local structure