Cenozoic tectonic and thermal history of the Nenana basin, central interior Alaska: new constraints from seismic reflection data, fracture history, and apatite fission-track analyses

The Wagwater Trough, Jamaica, tectonically and sedimentologically represents a failed rift arm of the Cayman Trough, a Tertiary extensional plate boundary zone between the North American and Caribbean plates. Integration of regional geologic-geophysical data with the geometrical relationships of the northern Wagwater Belt sediments and volcanics demonstrates that the trough has proceeded through the four major stages of development for failed rift arms outlined by Burke in 1977. Initial basement swelling, rifting, rupture (sedimentation/subsidence), and later convergent deformation stages were correlated to the Tertiary depositional and deformational history within the Wagwater Trough. The transgressive depositional package tectonically recorded these events and can be di ided into 4 lithogenetic units: (1) upper Paleocene-lower Eocene Wagwater alluvial fan conglomerates, (2) lower Eocene Nutfield pillow basalts, (3) lower Eocene Richmond fan delta/submarine fan conglomerates to mudstones, and (4) middle Eocene Font Hill deep marine limestones. The thermal doming and swelling stages occurred during the late Cretaceous-early Paleocene as evidenced by concordant ages of granodiorites (65 m.y.B.P.) in the Cayman and Wagwater Troughs and by the Paleocene unconformity at the base of the Wagwater Formation. This stage resulted in the eventual formation of an active triple junction within the Cayman Trough by late Paleocene. Initial rifting and deposition are recorded by the basal Wagwater Formation. The graded, boulder-cobble conglomeratic litharenites with granodiorite clasts were deposited in an alluvial fan environment adjacent to the fault scarps bounding the recently created Wagwater Trough. The volcanic activity associated with the triple junction is represented by the Nutfield spilitic pillow basalts, extruded in deep marine waters following the deposition of the terrestrial Wagwater Formation. These events together with the following indicate rapid deposition, subsidence, and ocean encroachment into the Wagwater Trough. Active rifting ensued between the North American and Caribbean plates along the east-west-trending Cayman Trough. This rupture stage is characterized by thick transgressive sedimentation and differential subsidence as the nonmarine Wagwater Formation grades into the marine Richmond Formation. This depositional transition is further manifested by both slope and shelf facies of litharenites within the Richmond. The slope facies consists of thick boulder conglomerates fining up into turbidite sequences and volcanic slide conglomerates overlying the Nutfield flows. The shelf facies include: (1) a pebble-cobble conglomeratic fan delta complex, (2) several sandstone feeder channel deposits radiating north from the fan delta to the shelf break, and (3) progradational mudstones and siltstones The interfingering of the Richmond and Wagwater lithogenetic units along a major east-west contact zone in the northern end of the belt is in direct contrast to the abrupt yet conformable slope facies contact between the two formations. These events indicate a much slower subsidence on the shelf in contrast to the rapid subsidence on the slope. By middle Eocene, clastic input was shut off, subsidence and marine transgression ensured, and a deep marine biomicrite, the Font Hill Limestone, capped the Richmond Formation within the Wagwater Trough. The final stage of aulacogen development began during middle Miocene associated with a change from north-south rifting to left lateral motion along the Cayman Trough. As a consequence of this change in plate motion, the Wagwater Trough was uplifted and wrenched by left lateral transcompression. Today, fluvial sedimentation along the Wagwater River, the largest in Jamaica, progresses down the ancient axis of the Wagwater Trough. End_of_Article - Last_Page 532------------

Structural data combined with analysis of satellite images and seismic profiles show that a major left-lateral strike-slip fault affects the Veracruz basin and post-5 Ma volcanic rocks of the Los Tuxtlas volcanic field (LTVF). The main volcanic alignment of the LTVF is located along this fault. Additional structural data collected in the Trans-Mexican volcanic belt (areas of Xalapa, Teziutlán and Huauchinango) show that the shear zone affects Pliocene Trans-Mexican volcanic rocks. Low seismicity associated to faulted Quaternary markers such as alluvial fans, alluvial terraces and volcanoes argue for active faulting in this area. Plio-Quaternary strike-slip faulting in the Veracruz basin and in the eastern Trans-Mexican volcanic belt is important because it connects two important structural provinces: the left-lateral strike-slip faults province to the south and the left-lateral transtensive faulting that affects the central part of the Trans-Mexican volcanic belt. These three active deformation zones constitute the boundary between the southern Mexico block and the North American plate. It is generally assumed that strike-slip faulting along the Trans-Mexican and Central America volcanic arcs is the result of oblique subduction of the Cocos plate under the North American and Caribbean plates. However slip vectors along the Middle America trench are almost perpendicular to the trench. This Neogene sinistral strike-slip motion could be partially driven by the eastward motion of the Caribbean plate rather than by strain partitioning along the oblique Middle America trench subduction zone.

Seismicity, fault-plane solutions, and Cenozoic geology are used to infer contemporary west to northwest extension and components of lateral slip between subplates of the western North American plate. Seismicity of the western interior of the Cordillera is characterized by earthquakes that occur in broad zones, up to 150 km wide, in the Nevada and the Intermountain seismic zones. Epicenters are scattered and when accurately located many do not coincide with mapped faults. Focal depths are shallow and seldom exceed 20 km. Secondary seismicity occurs around the margins of the Colorado Plateau, in central Idaho, and in eastern Oregon and Washington. The contemporary strain pattern of the Western United States, as interpreted from fault-plane solutions, suggests that three intraplate lithospheric blocks—the Sierra Nevada, the Great Basin-High Lava Plains, and the Northern Rocky Mountains-Columbia Plateau are moving west to northwest as “slivers” between the obliquely converging Pacific and North American plates. Intraplate extension within the Great Basin is primarily accommodated by north-south normal faulting in north-central Nevada and along the Wasatch Front, but significant components of strike-slip faulting occur along active seismic zones in southwestern Nevada and along the northern Intermountain seismic belt in Montana. A thin crust, ~25 km, characterizes the east margin of the Great Basin, with average Pn-velocities of ~7.5 km /s. A crustal low-velocity layer, at 5 to 15 km depth, coincides with the eastern margin of the Great Basin. A thin crust, with an apparent Pn-velocity of ~7.7 km /s, occurs on the northwest margin of the Great Basin and beneath parts of the Oregon-High Lava Plains. The central part of the Great Basin has a thicker crust, ~30 km, and higher Pn-velocities, 7.7 to 7.9 km /s. The Colorado Plateau and the Rocky Mountains have thicker crusts, 40 to 50 km, and Pn-velocities of ~7.8 km/s. An upper-mantle diapir or an upwelling thermal mechanism, facilitated by stress relaxation above the now-truncated Farallon subducting plate, is postulated to have uplifted, extended, and heated the crust of the Great Basin beginning at ~20 m.y. Late Cenozoic centers of volcanism appear to have progressed outward from a northern Great Basin thermal center in two divergent directions—northwesterly along a combined extensional-strike-slip zone of deformation in the High Lava Plains of southeastern Oregon and northeasterly along an extensional zone marked by the Snake River Plain. Lithospheric heating is thought to have produced a broad uplift of the Great Basin with concomitant crustal thinning and diminishing of upper-mantle Pn-velocities. The thermal mechanisms are hypothesized to have produced laterally divergent mantle flow to form symmetric zones of crustal thinning and low Pn-velocities that presently mark the east and west margins of the Great Basin.

The orientation of the principal stress axes within deformation zones between two mobile plates is modeled here analytically, using a thin-plate theory. The simple analytical approach helps to explain why plates cease to move after collision. Orogenic periods last only several tens of million years because the stress associated with a particular constant driving force (causing a constant strain rate) is no longer able to maintain a significant horizontal displacement. In contrast, the uplift rate increases rapidly as the horizontal velocity decreases, and this may explain why the termination of orogenic epochs are usually heralded by the rapid deposition of thick sequences of immature sedimentary rocks or flysch. The analytical model also elaborates the relationship between homogeneous bulk deformations driven by a constant stress orientation and those due to a fixed displacement direction of physical boundaries of the deformation zone. Two major kinematic possibilities are considered: (1) plates converging either orthogonally or obliquely, making the deformation zone an analogue for orogenic collision; and (2) plates diverging either orthogonally or obliquely, so that the deformation zone is dynamically similar to initiating rift basins. The theoretical investigation led to the formulation of the following rules. Deformation zones between converging plates have the major axis of bulk deviatoric stress coinciding with the bisector of the acute angle between the relative plate velocity vector and the normal to the deformation- zone boundary. In case of extension within a deformation or rift zone separating diverging plates, the bisector will outline the minor axis of the bulk deviatoric stress. The deformation tensor of the analytical model yields a new method for estimating the orientation of paleostress in natural examples, here applied to the deformed wall rock of the Moroccan Border fault. The marker used is a competent sequence of Devonian sandstone and limestone asymmetrically folded adjacent to the dextral Border fault. The steeply plunging Z-folds of the marker beds suggest that the principal deviatoric paleostress, τ 1 , was oriented 37°-44° to the fault trace. The age of the Moroccan Border fault is poorly constrained and may be Variscan or younger. The τ 1 orientation implies a major component of simple shear parallel to the strike-slip fault and a minor component of extension perpendicular to the fault trace. The implied crustal movement is compatible with the modern tectonics of the Eurasian-African collision zone where part of the differential motion between the Eurasian and African plates is accommodated by strike-slip faults.

Most rifted margins involve a certain degree of obliquity. Oblique extension has been well-studied in a magma-poor continental rift setting. However, few studies have addressed the structure and evolution of oblique rifting in a magma-rich continental rift setting. In this study, we used remote sensing and field measurements combined with geophysical data in order to study the tectono-magmatic evolution of seaward-dipping basalt sequences, known also as seaward dipping reflectors (SDRs), along the western Nuussuaq oblique volcanic passive margin. We calculated the directions and dips of lavas within onshore SDRs based on remote sensing and field data and analyzed the offshore SDRs through seismic reflection profiles to obtain a precise 3D structure of the inner SDRs along this margin. Our results show that the development of the inner SDRs can be divided into two stages. During the first stage (the late Paleocene), the oblique extension was partitioned into strike-slip and dip-slip components, and the basalt sequences were mainly bounded by N-S-trending continentward-dipping faults. During the second stage (the early Eocene), a local stress reorientation occurred in the western Nuussuaq and the basalt sequences were mainly bounded by NE-SW-trending continentward-dipping faults. The obliquity of the margin originated from both the reactivation of the inherited Itilli Fault and the magma intrusion into the crust, which weakened the crust and produced a pressure-induced extension orthogonal to the margin.

Biostratigraphy of the Sinjar Formation is investigated in two sections (Dokan and Sinjar)from northeastern and northwestern Iraq, respectively. Two hundred samples from all thelimestones and marl that form the main lithological components of the studied sections werecollected. The studied limestones and marl are rich in microfossils. Through thin sections, wewere able to identify thirty species of benthic foraminifera and fifteen species of othermicrofossils (coral, algae, mollusca, bryozoa, and echinoids) at Dokan section, and fifty-onespecies of benthic foraminifera and thirty species of other microfossils at Sinjar section. 3biozones were distinguished from both sections 1-Biozone A: Kathina sp.- Lockhartia huntiAssemblage zone (SBZ 5) (Dokan section); (Kathina pemavuti - Lockhartia hunti Assemblagezone (Sinjar section), 2- Biozone B: Idalina sinjarica Total Range zone (SBZ 6-7) and 3-Biozone C: Alveolina globosa- Alveolina pasitisilata Concurrent Range Zone (SBZ8-10).These zones indicate the Late Paleocene –Early Eocene age of the Sinjar formation. Thebiostratigraphic correlations in the studied sections are based on benthic foraminiferalzonations. Showed the correlation comparison between the biostratigraphic zones of thecommonly used benthic zonal scheme around the Late Paleocene -Early Eocene in and outsideof Iraq. paleoecological studies suggest that the carbonate sedimentation of the SinjarFormation thrived in 18-25oC, with mesophotic to oligophobic light, under an oligotrophic tomesotrophic middle ramp environment with normal marine to slightly saline and at waterdepths from 40 - 80 m. Stable isotopic carbon (δ13C) and oxygen (δ18O) data revealed generallyhot conditions with high productivity during the deposition of the Sinjar Formationaccompanied by an abrupt change in paleoenvironmental conditions across the Paleocene-Eocene contact.

Central Alaska is a broad zone of crustal deformation that is produced by collision and flat slab subduction. Within central Alaska, there are large‐scale right‐lateral strike‐slip faults, such as the Denali fault (2002 M w 7.9), as well as smaller‐scale fold‐and‐thrust belts and a set of left‐lateral strike‐slip fault zones, one of which is the Minto Flats fault zone (MFFZ). We use seismological evidence to document a pair of overlapping left‐lateral faults that define the MFFZ. Microseismicity delineates this 180‐km‐long fault zone. Using body waves and surface waves, we perform moment tensor inversions for the 11 best‐recorded earthquakes in the fault zone. Moment tensors reveal consistent left‐lateral faulting throughout the fault zone. A finite‐source model for the 1995 M w 6.0 earthquake is consistent with left‐lateral faulting and provides rupture details for the largest known event in the fault zone. The two main faults are separated by 10 km and overlap by 67 km, forming a releasing stepover geometry within a local transtensional setting. Between the faults is a 90‐km‐long, 12‐km‐wide, and 8‐km‐deep sedimentary basin (the Nenana basin). We interpret the transtensional faulting to be responsible for the development of the basin over the past 6 Ma. The distances of fault overlap and fault separation are key parameters for determining (1) the 3D morphology of the sedimentary basin and (2) the likelihood of earthquake ruptures jumping from one fault to the next. The structure of the Nenana basin is consistent with shear motion accommodated by the identified faults. The 10 km fault separation suggests that ruptures are not likely to span the entire fault zone. Earthquakes as large as M w 7.0–7.5 could occur on the faults. The transtensional fault zone provides an important constraint for understanding the larger‐scale intraplate tectonic setting of central Alaska. Online Material: Figures of waveform fits, depth estimation for moment tensors, variation in seismicity, and fault‐plane identification.

Sedimentary architecture, structural setting, and Late Cenozoic depocentre migration of an asymmetric transtensional basin: Lake Izabal, eastern Guatemala

The Biluoxueshan transpressive deformation zone monitored by synkinematic plutons, around the Eastern Himalayan Syntaxis

To a first order, the Caribbean plate converges obliquely at ~2 cm/yr toward the North American plate. This transpression is partly accommodated across the island of Hispaniola by the partitioning of motion between a fold-and-thrust belt trending NW-SE, and two E-W left-lateral fault systems located 150 km apart. The southern fault, the Enriquillo-Plantain Garden Fault (EPGF), is morphologically well expressed in western Haiti but its precise geometry in eastern Haiti is debatable. There, Lake Azuei stretches over 20 km in a direction parallel to the fold-and-thrust belt while its southern shoreline strikes EW, parallel to the expected trend of the EPGF. Because of a high sedimentation rate, the history of transpressional deformation should be captured in the lake stratigraphy and, accordingly, we acquired 220 km of multichannel seismic reflection (MCS) profiles across its surface. The survey followed a grid pattern with a spacing of 1.2 km and achieved a penetration of up to 300 m beneath lakebed. Interpretation of the dataset documents two major structures. First, the western side of the lake is occupied by a broad NW-trending monoclinal fold. This fold is cross-cut by a few NW-striking vertical (strike-slip) faults. We propose that this monocline is the surface expression of a SW-dipping blind thrust fault. The progressive steepening of the seismic horizons with depth suggests that it has been continuously active during the deposition of at least 300 m of sediments. The other major structure consists of a ~2 km-wide deformation zone that borders the EW-trending southern shore. This deformation zone is faintly imaged below a shallow gas front. We tentatively propose that it corresponds to a set of fault-propagation folds that are developing ahead of an EW, S-dipping oblique-slip fault. Such a model has been proposed already from three other independent studies involving GPS monitoring, seismological monitoring, and detailed field mapping. It is also supported by CHIRP profiles acquired concurrently with our MCS data and that document folding of the topmost turbidites but a lack of evidence for any stratigraphic offset across faults. Furthermore, a set of en echelon folds in that area are trending EW, while WNW-ESE fold axes would be expected instead above an EW vertical strike-slip fault.

Coral reefs are particularly sensitive to environmental disturbances, such as rapid shifts in temperature or carbonate saturation. Work on modern reefs has suggested that some corals will fare better than others in times of stress and that their life history traits might correlate with species survival. These same traits can be applied to fossil taxa to assess whether life history traits correspond with coral survival through past intervals of stress similar to future climate predictions. This study aims to identify whether ecological selection (based on physiology, behavior, habitat, etc.) plays a role in the long‐term survival of corals during the late Paleocene and early Eocene. The late Paleocene‐early Eocene interval is associated with multiple hyperthermal events that correspond to rises in atmospheric pCO2 and sea surface temperature, ocean acidification, and increases in weathering and turbidity. Coral reefs are rare during the late Paleocene and early Eocene, but despite the lack of reef habitat, corals do not experience an extinction at the generic level and there is little extinction at the species level. In fact, generic and species richness increases throughout the late Paleocene and early Eocene. We show that corals with certain traits (coloniality, carnivorous, or suspension feeding diet, hermaphroditic brooding reproduction, living in clastic settings) are more likely to survive climate change in the early Eocene. These findings have important implications for modern coral ecology and allow us to make more nuanced predictions about which taxa will have higher extinction risk in present‐day climate change.

Fracture zone trends and magnetic anomalies in the Atlantic Ocean indicate that the North American plate must have moved with respect to the South American plate during the opening of the Atlantic. A comparison of plate tectonic flow lines with fracture zones identified from Geosat and Seasat altimeter data suggests that the North American‐South American plate boundary migrated northward from die Guinea‐Demarara shear margin to the Vema Fracture Zone before chron 34 (84 Ma), to north of the Doldrums Fracture Zone before chron 22 (51.9 Ma), and to north of the Mercurius Fracture Zone between chron 32 (72.5 Ma) and chron 13 (35.5 Ma). The paleoridge offset through time identified from magnetic anomalies and the computed cumulative strike slip motion in the plate boundary area, indicate that the triple junction may have been located between the Mercurius and the Fifteen‐Twenty fracture zones after 67 Ma (chron 30). Plate reconstructions indicate a Late Cretaceous phase of transtension, followed by transpression in the Tertiary for the Tiburon/Barracuda Ridge area south of the Fifteen‐Twenty Fracture Zone. The ocean floor in this area is characterized by a series of ridges and troughs with large Bouguer gravity anomalies (up to ∼135 mGal). We use smoothing spline estimation to invert Bouguer anomalies for crustal layer structure. Our model results suggest that the Moho is uplifted 2–4 km over short wavelengths (∼70 km) at the Barracuda and Tiburon ridges and imply large anelastic strains. The severely thinned crust at the two ridges implies that crustal extension must have taken place before they were uplifted. We propose that the North‐South American plate boundary migrated to the latitude of the Tiburon Ridge, bounded by the Vema and Marathon fracture zones, before chron 34 (84 Ma). Post‐chron 34 crustal thinning during a transtensional tectonic regime may have been localized at preexisting structural weaknesses such as the Vema, Marathon, Mercurius, and Fifteen‐Twenty fracture zone troughs, but reaching the Fifteen‐Twenty Fracture Zone and future Barracuda Ridge area only after chron 32 (72.5 Ma). This interpretation concurs with our crustal structural model, which shows stronger crustal thinning underneath the Tiburon Ridge than at the Barracuda Ridge. Subsequent transpression may have continued along the existing zones of weakness in the Tertiary, creating the presently observed crustal deformation and uplift of the Moho, accompanied by anelastic failure of the crust. Middle‐Eocene‐Upper Oligocene turbidites on the slope of the Tiburon Ridge, now located 800 m above the abyssal plain, suggest that most of its uplift occurred at post‐Oligocene times. The unusually shallow Moho underneath the Tiburon and Barracuda ridges represents an unstable density distribution, which may indicate that compressive stresses are still present to maintain these anomalies, and that the North American‐South American plate boundary may still be located in this area.

The Oriental Cordillera of Dominican Republic is a mildly deformed area next to the north-eastern Caribbean plate boundary. Recent geological mapping of the cordillera reject the older ‘terrane’ subdivision, showing instead a coherent antiformal structure cored by Early Cretaceous island-arc rocks and covered by Late Cretaceous layered fore-arc sediments. The antiform is cut as a positive flower structure by late NW-SE strike-slip faults with sinistral movement (the main is the Yabón fault), that splay from the principal displacement zone of Samaná. Shear bands adjacent to the Yabón fault contains en echelon folds with sigmoidal deformation and a fan arrangement of the hinge lines recording the progressive rotation of the folds towards the fault. The superposition of these folds to the previous antiformal folding produce dome and basin interference patterns. The structure at the easternmost part of the cordillera is a linked system of strikeslip faults, folds and pop-ups, with changes of the transverse profile along-strike, some folds are cut as half-anticlines and half-synclines by Riedel shears. Five small trastensional basins formed in the Paleogene by crustal thinning at the left stepovers and bends of the strike- slip faults, favouring the protrusion of ultrabasic rocks to the surface along the Yabón fault. The structural pattern of the cordillera has been used qualitatively to interpret the orientation of the principal strain axes in different times and the modes of deformation. We conclude that homogeneous contractional deformation in the upper Cretaceous changed to partitioned transpression in the Paleocene, in consonance with a diminution in the convergence angle between the Caribbean and North American plates. These results are in agreement with the decrease of arc volcanism in La Hispaniola and the age of the first structures formed in the Caiman trough.

Planktonic foraminiferal biostratigraphical and paleoecological studies were carried out on the Paleocene-Eocene successions exposed at the Sagamu quarry of the West African Portland Cement Company (WAPCO) southwestern Nigeria. The exposed section from the base to the top includes the Ewekoro Formation (Paleocene) which is essentially limestones and the Oshosun Formation (Late Paleocene – Early Eocene) mainly of shales. Biostratigraphy and paleoecology of foraminifera species was carried out. Twenty species of planktic and sixteen species of benthic forms were identified from the Oshosun shale interval of the section. In general, benthic species of infaunal and epifaunal habitat dominate the population. Based on the abundance and stratigraphical distribution of the planktonic foraminiferal species, two planktonic foraminiferal biostratigraphic zones were recognized, within the Oshosun Formation: a Globanomalina pseudomenardii Zone of Late Paleocene and Morosovella subbotinae Zone of Early Eocene age. Benthonic foraminifers are generally shallow marine (mostly muddy bottom dwellers). The estimated paleo-depth ranges between 50m and 150m. The environment of deposition was relatively stable during the Late Paleocene – Early Eocene times.

As exploratory work in Alaska moves beyond the known petroleum basins, new remote areas will be explored, where little sub surface data is available. This paper examines the overburden of Alaska, and develops a general relationship for determining overburden pressures based on the general geographic location in this region.To develop such relationships, well logs available to the Public are used. To characterize the overburden on a large scale, three major sedimentary basins of the Alaska, the North Slope, Nenana Basin, and Cook Inlet Basin, are studied. Overburden is estimated by integrating density of the deposits, from using density log data, and using MATLAB to filter false readings. From this data, a regionalized relationship is developed for pressure vs depth, based on geographical location.The collision of the Pacific Plate and North American Plate has resulted in thrust tectonics, associated with shortening and thickening of the crust at southern part of the Alaska Microplate. The studied basins are located along different locations in this deformation zone and evident with different lithological patterns across a north-south direction. Depending on tectonics and diagenesis of sediments, rocks undergo different compaction processes which make overburden various across this region. As a result of plate tectonics and variation in depositional environments, an increasing trend is observed across the entire Alaska region. Such trend can be used in further exploration work in this region to approximate overburden stress at any location in the state.