neogene caribbean plate rotation and associated central american tectonic evolution during the neogene the caribbean plate moved eastward with respect to the north and south american plates reconstructing this motion by closing the cayman trough along its bounding transform faults creates a gore of ocean between northern central america and southern mexico we present instead a reconstruction which juxtaposes these two continental areas by strike slip motion using geological evidence from around the cayman trough the strike slip fault zones of northern central america and the continental margin of southern mexico as our constraints we propose that seafloor spreading began in the cayman trough at 30 5 ma and that subsequently there was 1100 km of relative plate displacement at an average rate of 3 7 0 6 cm yr this neogene plate motion cannot be achieved by rigid plate rotation about a single pole which implies that central america must have rotated anticlockwise relative to the rest of the plate there is evidence for internal plate deformation in central america consistent with such a sense of rotation our suggestion of an originally easterly trending arc in southern central america is more consistent with the timing and geometry of the collision of panama with northern colombia and the resultant deformation than were earlier models that depicted collision of a northerly trending arc

rotation of the chortis block causes dextral slip on the guayape fault the chortis block is undergoing deformation as a direct result of large magnitude sinistral strike slip faulting on the north america caribbean plate boundary we have studied the chortis block using geologic mapping remote imagery and earthquake seismology to evaluate the effect that large magnitude strike slip faulting has on an adjacent microcontinental block in the eastern part of the chortis block recent geologic mapping has shown that the northeast striking guayape fault is dextral instead of sinistral as previously assumed because this is a major fault that cuts most of the central america isthmus the new evidence for dextral slip necessitates a reinterpretation of the active tectonics of the chortis block the presence of several north trending rifts in the western part of the chortis block has led previous workers to propose that the chortis block is divided into smaller independently rotating blocks we use earthquake seismology and remote imagery data to extend this interpretation to the central portion of the chortis block however we have modified their model to account for dextral slip on the guayape fault we believe that rotation of the chortis block causes dextral slip on the guayape fault even though central america is undergoing large sinistral displacement with respect to north america

time of sinistral slip along the polochic fault of guatemala abstract the polochic fault of guatemala and chiapas mexico was a neogene transform segment of the north american caribbean plate boundary with a total sinistral slip of 130 km principal activity was within the interval from about 10 3 to 6 6 m y b p or less a time bracket based upon k ar ages from volcanic rock clasts in the colotenango beds of northwest guatemala earliest possible movement was established from ages of volcanic rock constituents of the older colotenango conglomerates deposited by streams that flowed north from a terrane now in central guatemala and onto terrane that was subsequently offset along the polochic fault it is believed that the selegua river was connected to the chixoy river of central guatemala this river system deposited the older part of the colotenango conglomerate now in western guatemala about 130 km west of its source area two volcanic clasts from the conglomerate yield ages of 12 3 1 0 and 10 3 1 0 m y the younger age is believed to set the maximum age of sedimentation of this older part of the conglomerate and is probably close in time to initiation of fault activity distinctive brown serpentinite clasts in the conglomerate have been shown to be petrographically mineralogically and geochemically similar to serpentinite from the presumed source area north of salama guatemala a youngest age of major offset is established by the younger part of the colotenango beds a volcaniclastic deposit which fills much of the ancient polochic fault controlled cuilco river valley our studies of volcanic clasts of the younger colotenango conglomerate reveal pristine glass attesting to its relatively young age k ar determination establishes an age of 6 6 0 3 m y b p for these clasts the youngest age of movement has not been established with certainty it lies between the 6 6 m y age and the age of the uppermost volcaniclastic valley fill the synchroneity of spreading in the cayman trough macdonald and holcombe 1978 with transform activity along the polochic suggests strongly that the two were genetically related the earliest data in the bracketed time interval correlates with the age of ongoing spreading for the mid cayman rise the mid miocene to pliocene volcanic arc 9 19 m y b p of mexico and its probable continuation in guatemala 8 19 m y b p sinistrally offset across the polochic fits well with the chronology discussed above for the polochic

geologic evaluation of plate kinematic models for the north american caribbean plate boundary zone heubeck ch and mann p 1991 geologic evaluation of plate kinematic models for the north american caribbean plate boundary zone tectonophysics 191 l 26 four published plate kinematic models for the present day north american caribbean plate boundary predict significantly different fault behavior along this dominantly strike slip plate boundary zone pbz using a computer graphics system we compare the deformational pattern predicted by the four kinematic models with geologic and seismic observations along the entire length of the pbz from central america to the northern lesser antilles our results indicate that none of the kinematic models predict all the geologic and seismic observations across the entire pbz however all four models predict mapped fault character and observed earthquake focal mechanisms along certain segments of the pbz we conclude that the present day deformation within the north american caribbean plate boundary zone cannot be described by rotation about a single pole instead the caribbean plate appears to be segmented into at least three major sub plates or blocks each of which requires an individual pole of rotation to describe its motion relative to the north american plate sub plate boundaries correspond to two poorly studied zones of diffuse deformation within the caribbean plate honduras depression of central america and beata ridge of the caribbean sea

neotectonic evolution of the isthmus of tehuantepec southeastern mexico abstract the isthmus of tehuantepec of southeastern mexico is located near the triple junction of the north american cocos and caribbean plates a neotectonic study including fault tectonic analysis and study of sub surface data was performed in order to understand the tectonic evolution of this complex zone the plio quaternary fault pattern is described and the brittle structures are interpreted in terms of palaeostress orientations we propose a model of tectonic evolution of the isthmus of tehuantepec since the late miocene 6 ma characterized by extensional tectonics the present structure of the isthmus results from the superimposition of three distinct types of tectonism 1 a tilting of the eastern isthmus during the late miocene early pliocene along a major n s trending normal fault zone the isthmus fault zone as a consequence of the deformation of the subducting slab of the cocos plate along the subducted part of the tehuantepec fracture zone 2 the subsidence of the southern isthmus in relation to n s extension associated with the eastward displacement of the western caribbean plate with respect to the southern north american plate along the polochic motagua fault system and 3 extensions that develop in the northern half of the isthmus related to the evolution of the passive margin of the gulf of mexico

beyond the motagua and polochic faults active strike slip faulting along the western north america caribbean plate boundary zone abstract i investigate the role of two strike slip faults in the tectonics of the western north america caribbean plate margin the ixcan fault located in guatemala north of the polochic fault is seismically active with earthquakes of magnitude up to 5 7 reported recently fault plane solutions along this curvilinear but generally e w trending fault indicate left lateral strike slip displacement several historic earthquakes appear to have taken place along the ixcan fault since 1728 the largest one being the 1816 event m 7 5 the nw se trending concordia fault in southeastern mexico appears to be the site of the great m 7 6 earthquake of 1902 isoseismals for this event suggest shallow left lateral strike slip faulting i propose a seismotectonic model in which both faults are part of the deformation associated to the north america caribbean plate boundary zone transpressive structures are found in the fault steps between strike slip fault systems

depth and age in the cayman trough abstract caribbean plate boundaries are almost everywhere either strike slip or convergent so oceanic crust accreted at the spreading axis within the cayman trough provides information critical to determining the relative motions and tectonic interactions among the caribbean and surrounding plates the tectonic evolution of the trough as inferred from magnetic anomaly identifications is not entirely compatible with either its crustal subsidence or the geological evidence for time of opening and rate of offset movement along the north american caribbean plate boundary a close examination of basement topography and depth within the trough suggests that the total opening of the cayman trough is about 1100 km and that this opening occurred in two stages crust generated by the present ongoing stage of very slow spreading has extensive relief and shows a distinct increase of crustal depth with increasing distance from the spreading center crust generated by an earlier episode of faster spreading is characterized by reduced topographic relief and no appreciable differential subsidence and is separated from the younger crust by distinct topographic breaks the rate of spreading of the current slow spreading episode is15 5mm yr based upon a new crustal subsidence curve which accommodates lateral as well as vertical cooling of the accreting slab the trough has opened at this rate for about 30 ma the rate of opening prior to 30 ma cannot be determined with any reliability from crustal subsidence a reexamination of magnetic anomalies in the central part of the trough shows that these anomalies can be generated at a much slower spreading rate than previously thought this revised interpretation of cayman trough evolution brings magnetic subsidence and geological evidence into much better agreement

tectonic wedges and offset laramide structures along the polochic fault of guatemala and chiapas mexico reaffirmation of large neogene displacement fault wedges along the polochic fault of northern guatemala and chiapas mexico record left lateral slip of from 20 to 65 km that accompanied documented neogene slip of 130 km across this north american caribbean plate boundary fault wedges can be correlated to source areas consistent with left lateral displacement along the polochic fault they occur adjacent to restraining bends in three areas where changes in strike of the fault have been mapped these wedges which have been previously interpreted as allochthons emplaced along low angle reverse faults are bounded by high angle faults folds that terminate against the polochic fault are part of a regional fold belt that originated during the campanian through paleocene laramide orogenic event a previous contention that some of these major folds are secondary and related to fault movement is shown to be incorrect the 130 km model of left lateral offset across the polochic fault is reviewed along with evidence for and against a neogene time of major displacement additional evidence for the model is presented 1 late cretaceous granitoid rocks from near motozintla chiapas mexico northern fault block are correlated with granitic rocks of the santa maria batholith in the region of huehuetenango guatemala southern block 2 mineralization at the contacts of the granitoid intrusives of the two areas is correlated 3 points of deflection of laramide structural axes are correlated across the fault 4 a major thrust sheet similar to that of the sierra de santa cruz in eastern guatemala is reassembled by reversing 130 km of left lateral slip which has offset serpentinites north of the fault in western guatemala and those south of the fault in central guatemala

neogene north american caribbean plate boundary across northern central america offset along the polochic fault abstract the polochic fault was a segment of the north american caribbean plate boundary across central america in the neogene its 130 km of left slip was previously determined by matching structures and stratigraphie outcrop patterns of northwest and central guatemala across the fault additional support for the model and the youthfulness of the recorded offset comes from an essentially perfect match of major geomorphic features across the fault a reconstruction process which eliminates 123 km of left slip brings together rivers and drainage divides that existed before the polochic became active with the reconstruction carried across the isthmus on an east west fault the regional structural geology assumes the coherent pattern of a continuous orogenic belt whose geometry is compatible with the model of collisional tectonics centered on the motagua suture zone confined within this belt narrowed to some 60 km by the reconstruction lie the major laramide thrusts folds and tectonically emplaced serpentinites of guatemala crystalline rocks of guatemala re join the chiapas massif and paleozoic sedimentary rocks exposed in the core of an almost continuous anticlinorium extend from southern chiapas to lake izabal the polochic does not bend in eastern guatemala but continues eastward to the motagua fault where it dies westward drift of the northern block resulted in rifting which extended from eastern guatemala into the caribbean along the cayman trough the honduras depression may represent an element of a triple junction along with the polochic and izabal cayman rift the polochic continues westward into the pacific ocean and offsets the middle america trench the polochic has offset the miocene volcanic belt of northern central america confirming the previous estimate of a neogene time of movement about 300 km of relative east west neogene displacement has been recorded on the mid cayman rise only 130 km of which can be accounted for across the polochic it is suggested that cumulative extension on north south faults south of the motagua fault zone between the trench and the honduras depression might make up that difference

an airborne gravity study of eastern north carolina the naval research laboratory nrl has developed a prototype airborne gravity measurement system the core of the system is a lacoste and romberg air sea gravity meter mounted on a three axis stable platform corrections to the gravimeter data for altitude and variations in altitude are determined from a combination of highly precise radar and pressure altimeters the original prototype system was designed for use over oceanic areas we recently incorporated the pressure measurement to extend use of the airborne system to terrestrial regions where occasional radar altitudes over points of known topographic height can be obtained the radar heights are used to relate the pressure altitudes to absolute altitudes and to determine the slopes of the isobaric surfaces vertical accelerations due to horizontal velocity over a curved rotating earth the eotvos correction and precise two dimensional positions are determined from a texas instrument p code global positioning system the updated system was tested

reply to comment on late cretaceous allochthons and post cretaceous strike slip displacement along the cuilco chixoy polochic fault guatemala

the chixoy polochic fault and its associated fractures in western guatemala in western guatemala post cretaceous volcanic and sedimentary rocks locally cover the trace of the chixoy polochic fault no single throughgoing fault cuts these units however a complicated series of en echelon lineations identified from aerial photographs and field mapping represent the accommodations of displacements along the underlying fault comparison of fracture trends in the chixoy polochic fault zone with models from experimental studies and field examples of strike slip faults suggest that left lateral displacement along the chixoy polochic fault in this area is probably no more than a few kilometres since the accumulation of the overlying beds of probable post mid tertiary age earlier lateral displacements of large magnitude are not precluded although none have been documented by field study

age and spreading history of the cayman trough as determined from depth heat flow and magnetic anomalies the opening of the cayman trough records aspects of relative motion between the north american and caribbean plates essential to accurate reconstruction of caribbean evolution assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates depths to basement show that the crust has subsided away from the present axis in two stages a younger stage characterized by substantial and asymmetric greater to the east subsidence preceded by an older stage of slight increases of depth with distance depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm yr since 25 30 ma and at 20 30 mm yr prior to 30 ma marine magnetic anomalies show lineations perpendicular to the trough including a distinct central anomaly but the pattern is neither well defined nor symmetrical about the present axis however partial anomaly sequences can be identified on both sides of the axis and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps mostly eastward the overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm yr since and prior to 26 ma both subsidence and magnetics indicate that the trough opened by at least 45 50 ma

sedimentary architecture structural setting and late cenozoic depocentre migration of an asymmetric transtensional basin lake izabal eastern guatemala abstract lake izabal located in eastern guatemala lies in an ew trending basin located along the transform margin between the north american and caribbean plates this plate boundary consists of two main left lateral strike slip faults known as the polochic and motagua fault system pmfs the basin is a 100 20 km lens shaped depression in which the lake occupies its eastern half to the north the basin is confined by a principal deformation zone pdz which is recognized in this area as the most important upper crustal branch of the polochic fault the eastern part of the basin has been uplifted and dissected exposing the basin infill along the slopes of the montana del mico analysis of a set of multichannel seismic reflection profiles most of them acquired within the lake combined with well and outcrop information allowed to outline the structure and evolution of the basin and its stratigraphic architecture the basin is strongly asymmetrical both in cross and along strike directions and is filled up by neogene to quaternary sediments with the deepest side of the basin being to the north close and parallel to the pdz the base of the infill is a mid tertiary unconformity whose differential subsidence through time brought the basin to its present structural setting five main evolutionary stages are recognized based on seismo stratigraphic and structural features which are related to the development and activity of the bordering faults development of the basin began in its easternmost sector a small half graben tilted to the south developed in response to the activity of an oligocene or early middle miocene growth fault in the second stage late miocene the basin tilts to the north causing the sedimentary sequences to thicken towards the northern pdz this stage is dominated by oblique extension and by progressive migration of the basin depocenter towards the west the third stage early pliocene records a strong acceleration of the strike slip activity which causes a rapid westward migration of the depocenter the fourth stage late pliocene although still dominated by regional transtension is related to the development of a transpressional event along the eastern side of the basin as evidenced by the occurrence of folds and reverse faults parallel to the northern bend of the polochic fault during the fifth stage quaternary the eastern part of the basin remains relatively quiescent while the western part undergoes subsidence with localized transtension still related to the activity in this sector of the polochic fault left lateral movement along the polochic fault caused the depocenter to migrate 75 80 km at different velocities with slip rates ranging between 2 1 and 16 3 mm yr this study suggests that the polochic fault developed in the early middle miocene in connection to the deposition of the early sediments of the izabal basin even though an older age oligocene could also be possible to our knowledge this is one of the best examples of sedimentary architecture documenting depocentre migration due to fault movement along a transform system

quaternary seismo tectonic activity of the polochic fault guatemala 1 the polochic motagua fault system is part of the sinistral transform boundary between the north american and caribbean plates in guatemala and the associated seismic activity poses a threat to 70 of the country s population the aim of this study is to constrain the late quaternary activity of the polochic fault by determining the active structure geometry and quantifying recent displacement rates as well as paleo seismic events slip rates have been estimated from offsets of quaternary volcanic markers and alluvial fan using in situ cosmogenic 36cl exposure dating holocene left lateral slip rate and mid pleistocene vertical slip rate have been estimated to 4 8 2 3 mm y and 0 3 0 06 mm y respectively on the central part of the polochic fault the horizontal slip rate is within the range of longer term geological slip rates and short term gps based estimates in addition the non negligible vertical motion participates in the uplift of the block north of the fault and seems to be a manifestation of the regional far field stress regime we excavated the first trench for paleo seismological study on the polochic fault in which we distinguish four large paleo seismic events since 17 ky during which the polochic fault ruptured the ground surface

airborne gravity is here the achievement of practical and useful airborne gravity exploration for petroleum is a major culmination of more than twenty years of research and development commercial surveys have now been conducted for three years the precision and anomaly resolution attainable are twice as good as that of current shipborne gravity data this paper presents data to demonstrate the precision of airborne gravity and discusses its usefulness with simultaneous magnetics and topography industrial acceptance of the method portends a new era in geophysical exploration for oil and gas which gives promise to double the effectiveness of explorations on land in both time and cost

geochemical evidence for hydroclimatic variability over the last 2460 years from crevice lake in yellowstone national park usa abstract a 2460 year long hydroclimatic record for crevice lake yellowstone national park montana was constructed from the 18 o values of endogenic carbonates the 18 o record is compared to the palmer hydrologic drought index phdi and pacific decadal oscillation pdo indices as well as inferred discharge of the yellowstone river during the last century high 18 o values coincide with drought conditions and the warm phase of the pdo index low 18 o values coincide with wet years and a negative pdo index comparison of tree ring inferred discharge of the yellowstone river with the 18 o record over the last 300 years indicates that periods of high discharge i e wet winters with significant snow pack correspond with low 18 o values extrapolating this relationship we infer wet winters and high river discharge for the periods of 1090 1030 970 870 670 620 and 500 430 cal years bp the wet intervals at 670 and 500 cal bp are synchronous with similar events in banff canada and walker lake nevada the wet intervals at 970 and 670 cal bp overlap with wet intervals at walker lake and major drought events identified in the western great basin these results suggest that the northern border of yellowstone national park straddles the boundary between northern rocky mountains and great basin climate regimes

preservation of contrasting geothermal gradients across the caribbean north america plate boundary motagua fault guatemala 1 strike slip plate boundaries juxtapose crustal blocks that may have different geodynamic origins and therefore different thermal structures thermo kinematic modeling of this type of strike slip plate boundary predicts an asymmetric signature in the low temperature thermochronologic record across the fault age elevation profiles of zircon u th he ages across the motagua fault a 500 km long segment of the transform boundary between the north american and caribbean plates document a sharp cooling age discontinuity across the fault this discontinuity could be interpreted as a difference in denudation history on each side of the fault however a low relief miocene erosional surface extends across the fault this surface has been uplifted and incised and provides a geomorphic argument against differential denudation across the fault by integrating magmatic volcanic and heat flow data age elevation profiles and thermo kinematic modeling we propose that large horizontal displacement along the motagua fault has offset a persistent geothermal asymmetry across the fault and explains both the age discontinuities and the age elevation patterns this study illustrates how thermochronology can be used to detect large strike slip displacements and more generally opens new perspectives in understanding the impact of nonuniform thermal structures on thermochronologic results

geostatistical simulation for geophysical applications part i simulation geostatistical simulation is used to create a synthetic three dimensional geologic environment such that randomly generated values of a physical property have a required covariance and mean we create a spatially stationary multivariate gaussian simulation with a gaussian covariance this type of simulation permits modeling of natural variations which may be an important source of noise in geologic investigations whereas interpolation techniques such as kriging tend to smooth out those variations computationally efficient frequency domain methods allow the rapid creation of multiple scenarios each having the required statistical distribution for the particular problem considered a given porosity distribution is simulated and then functionally related to physical properties needed for input into geophysical models additionally megascopic cavities having random size shape and orientation similar to those observed in karst settings are emplaced in the physical model geostatistical simulation plays

late cretaceous allochthons and post cretaceous strike slip displacement along the cuilco chixoy polochic fault guatemala the disposition of allochthonous masses emplaced in late cretaceous to early tertiary time along the cuilco chixoy polochic strike slip fault zone in western guatemala constrains the amount of tertiary displacement the absence of major disruption of the allochthons argues against lateral displacements of hundreds of kilometers during the tertiary in westernmost guatemala along the flank of the cuilco chixoy polochic fault granite volcaniclastic and volcanic rocks minor carbonate and phyllite comprise a sequence of south dipping allochthons these tilted slabs lie against almost horizontal jurassic and cretaceous beds to the north field relations indicate that the slides were emplaced across the cuilco chixoy polochic fault from south to north probably during late cretaceous or early tertiary time other fault bounded masses which crop out tens of kilometers eastward near san sebastian huehuetenango probably were emplaced contemporaneously with those to the west locally these allochthons lie astride the active trace of the cuilco chixoy polochic fault along which offset streams record about 1 km of left lateral motion total displacement recorded by the apparent offset of allochthonous sheets is no more than several kilometers farther east along much of the cuilco chixoy polochic fault zone a throughgoing strike slip trace has not been identified we postulate that the pre tertiary tectonic record is obscured by an extensive series of allochthons composed of regional stratigraphic suites emplaced during late cretaceous time in western guatemala stratigraphic relationships between serpentinized peridotite and carbonate beds of late cretaceous age indicate turonian as a maximum age for initial emplacement of allochthonous material regional relationships indicate that deformation culminated during latest cretaceous earliest tertiary time the emplacement of thrust sheets along the cuilco chixoy polochic fault is a likely product of contemporaneous cretaceous convergence along the motagua zone to the south collision of the chortis terrane against southern yucatan resulted in crustal shortening uplift and consequent northward directed emplacement of slides allochthons composed of ultramafic rocks sedimentary cover and less commonly granitic rocks were probably derived from the suture zone and the uplifted region between the cuilco chixoy polochic and motagua faults

earthquakes of the holocene geologic studies of earthquakes involve mapping of coseismic surface faulting and secondary deformation from historical events trenching and geomorphic analysis to define the timing and size of past earthquakes and investigations of fault zone structure and geometry in both unconsolidated sediments and bedrock this research is now being referred to as paleoseismology seismic geology and earthquake geology since the mid 1970s it has led to some of the most exciting and important contributions to the understanding of earthquake behavior in space and time hanks 1985 allen 1986 the present report is the first of what is hoped will become a regularly contributed summary of the geologic aspects of the study of earthquakes