ALBERT

Description: The opening of the Gulf of Mexico was an important Mesozoic tectonic event that provides new insight in the role of magmatism and lithospheric stresses in the initiation of continental rifting. A new seismic velocity profile based on seismic refraction data in the northwestern Gulf of Mexico offshore Texas, where the basin started opening in the Early Jurassic, shows a rifted margin with strong lateral heterogeneity beneath the shelf and slope. The structure of the thinned crust is consistent with large-scale extensional faulting and moderate amounts of synrift magmatism before continental breakup. These new seismic constraints do not indicate the presence of a volcanic margin along the Texas coast, as has sometimes been proposed based on magnetic data. The Laurentian continental lithosphere of central Texas may have been too thick at the onset of rifting (〉100 km) to let magmatic diking control the extension. In contrast, the continental lithosphere of the northeastern Gulf of Mexico may have been thinner, such that magma-assisted rifting formed a volcanic margin there later in the Jurassic.

Description: During arc-continent collision, buoyant sections of sediments and rifted continental crust from a subducting plate will accrete to the forearc of the upper plate as long as this backstop remains intact. Deformation of the oceanic arc and forearc block may ultimately lead to accretion of these mafic rock units to the new orogen. The Taiwan mountain belt, which formed at ~6.5 Ma by oblique convergence between the Eurasian passive margin and the overriding Luzon arc in northern Taiwan, offers important insight in this process, since the collision is more advanced in the north than in the south. The incipient stage of arc-collision can be studied in southern Taiwan, while the northern portion of the orogen is presently undergoing collapse due to a flip in the subduction polarity between the Eurasian Plate and the Philippine Sea Plate. In this study, we seismically image the structure of the northern section of the mountain belt with a tomographic inversion. We present marine and land-based seismic refraction data, as well as local earthquake data, from transect T6 of the Taiwan Integrated Geodynamic Research (TAIGER) program across the Taiwan mountain belt and the adjacent Ryukyu arc. Our 2-D compressional seismic velocity model for this transect, which is based on a tomographic inversion of 10 213 P -wave arrival times, shows that the Eurasian crystalline continental crust thickens from ~24 km in the Taiwan Strait to ~40 km beneath the eastern Central Range of Taiwan. The detailed seismic velocity structure of the Taiwan mountain belt shows vertical continuity in the upper 15 km, which suggests that rocks are exhumed to the surface here from the middle crust in a near-vertical path. The continental crust of the westernmost Ryukyu arc is almost as thick (~40 km) as in the adjacent northern Central Range of Taiwan, and it appears to override the leading edge of the Philippine Sea Plate offshore northeastern Taiwan. If we assume that the western Ryukyu arc crust also thickened in the collision, then the mountain belt is wider and less thick in northern Taiwan than in central Taiwan (~50 km), which may be the result of post-collisional extension in the north.

Description: Recent multichannel seismic reflection data acquired offshore southwest Taiwan identify active extension along a deep-seated normal fault in the precollision setting of the Taiwan arc-continent collision. While ubiquitous minor flexural faulting may be observed in the Taiwan foreland, these new data image a listric, rift basin–bounding normal fault that penetrates deep into the crust and forms a significant fault scarp with ~850 m of relief near the continental shelf break southwest of the Taiwan collision zone. These observations, along with new geodynamic models of collision between a subduction zone and a young passive margin, indicate that the recent extension may be the expression of plate bending in continental crust as thin transitional crust subducts at the Manila Trench. A similar extensional episode prior to the onset of arc-continent collision ca. 6.7 Ma has been identified in rift basins of the southern Chinese margin near Taiwan, further suggesting that collision may be preceded by bending-related extension of the continental shelf. The Lishan fault, a major structural and morphologic boundary in the Taiwan orogen, may have been a similar rift basin–bounding fault before being reactivated during the Taiwan arc-continent collision. In this scenario, the Lishan fault divides Taiwan into a western domain representing collision of the thick crust of the continental shelf and an eastern domain representing subduction and collision of thin transitional crust along the continental slope with the Manila Trench.

Description: The collision of continental crust of the Eurasian Plate with the overriding Luzon arc in central Taiwan has led to compression, uplift and exhumation of rocks that were originally part of the Chinese rifted margin. Though the kinematics of the fold-thrust belt on the west side of the orogen has been described in detail, the style of deformation in the lower crust beneath Taiwan is still not well understood. In addition, the fate of the Luzon arc and forearc in the collision is also not clear. Compressional wave arrival times from active-source and earthquake seismic data from the Taiwan Integrated GEodynamic Research (TAIGER) program constrain the seismic velocity structure of the lithosphere along transect T5, an east-west corridor in central Taiwan. The results of our analysis indicate that the continental crust of the Eurasian margin forms a broad crustal root beneath central Taiwan, possibly with a thickness of 55 km. Compressional seismic velocities beneath the Central Range of Taiwan are as low as 5.5 km/s at 25 km depth, whereas P-wave seismic velocities in the middle crust on the eastern flank of the Taiwan mountain belt average 6.5-7.0 km/s. This suggests that the incoming sediments and upper crust of the Eurasian Plate are buried to mid-crustal depth in the western flank of the orogen before they are exhumed in the Central Range. To the east, the Luzon arc and forearc are deformed beneath the Coastal Range of central Taiwan. Fragments of the rifted margin of the South China Sea that were accreted in the early stages of the collision form a new backstop that controls the exhumation of Eurasian strata to the west in this evolving mountain belt.

Description: Seismic reflection and wide-angle data acquired across, south, and west of Taiwan show that extended to hyper-extended continental crust of the Chinese continental margin is present more than 200 km south of the shelf and is subducting at the Manila Trench. Furthermore, crustal-scale tomographic velocity models show that this crust is underthrusted to ~15 km depth below the accretionary prism, where it then is structurally underplated to the base of the prism. We document an increasing volume of accreted crust from south to north, and in our northern transect high-velocity material of the accretionary prism can be directly linked to outcrops of Central Range basement rocks. In map view the Central Range of Taiwan is clearly contiguous with the Hengchun Peninsula and Hengchun submarine ridge to the south. Accordingly, we propose a new model in which the Central Range forms directly from the accretionary prism, including the basement core, which originates from subducted, and then accreted, extended to hyper-extended continental crust.

Description: The Baranof Fan is one of three large deep-sea fans in the Gulf of Alaska, and is a key component in understanding large-scale erosion and sedimentation patterns for southeast Alaska and western Canada. We integrate new and existing seismic reflection profiles to provide new constraints on the Baranof Fan area, geometry, volume, and channel development. We estimate the fan’s area and total sediment volume to be ~323,000 km 2 and ~301,000 km 3 , respectively, making it among the largest deep-sea fans in the world. We show that the Baranof Fan consists of channel-levee deposits from at least three distinct aggradational channel systems: the currently active Horizon and Mukluk channels, and the waning system we call the Baranof channel. The oldest sedimentary deposits are in the northern fan, and the youngest deposits at the fan’s southern extent; in addition, the channels seem to avulse southward consistently through time. We suggest that Baranof Fan sediment is sourced from the Coast Mountains in southeastern Alaska, transported offshore most recently via fjord to glacial sea valley conduits. Because of the translation of the Pacific plate northwest past sediment sources on the North American plate along the Queen Charlotte strike-slip fault, we suggest that new channel formation, channel beheadings, and southward-migrating channel avulsions have been influenced by regional tectonics. Using a simplified tectonic reconstruction assuming a constant Pacific plate motion of 4.4 cm/yr, we estimate that Baranof Fan deposition initiated ca. 7 Ma.

Description: In southern Alaska, the Pacific plate and Yakutat terrane subduct beneath the North American plate along the Aleutian Trench and Pamplona zone, respectively, and are sliding past each other at minimal rates along the Transition fault. As the deformation front of the Pamplona zone stepped eastward during the Pliocene–Pleistocene, the Pacific–North American–Yakutat triple junction became unstable. Four recent seismic images reveal that the Transition fault changes from a single strike-slip boundary east of the deformation front to three strands that step increasingly seaward between the deformation front and the Aleutian Trench. The southern two strands deform the Pacific crust, and the outermost of these became increasingly convergent sometime since 1 Ma, as demonstrated by young growth strata. We propose that this internal deformation of the Pacific plate is an attempt to re-attain stability, which can only be reached by creating a tectonic boundary collinear with the Pamplona zone. The plate reorganization will result in initiation of subduction such that a portion of former Pacific crust will become accreted to the North American plate. Such accretion events caused by triple-junction instability may be an important mechanism for transferring oceanic crust to continental margins.

Description: 〈span〉〈div〉Abstract〈/div〉The eastern North American margin community seismic experiment (ENAM‐CSE) was conceived to target the ENAM Geodynamic Processes at Rifting and Subducting Margins (GeoPRISMS) primary site with a suite of both active‐ and passive‐source seismic data that would shed light on the processes associated with rift initiation and evolution. To fully understand the ENAM, it was necessary to acquire a seismic dataset that was both amphibious, spanning the passive margin from the continental interior onto the oceanic portion of the North American plate, and multiresolution, enabling imaging of the sediments, crust, and mantle lithosphere. The ENAM‐CSE datasets were collected on‐ and offshore of North Carolina and Virginia over a series of cruises and land‐based deployments between April 2014 and June 2015. The passive‐source component of the ENAM‐CSE included 30 broadband ocean‐bottom seismometers (OBSs) and 3 onshore broadband instruments. The broadband stations were deployed contemporaneously with those of the easternmost EarthScope Transportable Array creating a trans‐margin amphibious seismic dataset. The active‐source portion of the ENAM‐CSE included several components: (1) two onshore wide‐angle seismic profiles where explosive shots were recorded on closely spaced geophones; (2) four major offshore wide‐angle seismic profiles acquired with an airgun source and short‐period OBSs (SPOBSs), two of which were extended onland by deployments of short‐period seismometers; (3) marine multichannel seismic (MCS) data acquired along the four lines of SPOBSs and a series of other profiles along and across the margin. During the cruises, magnetic, gravity, and bathymetric data were also collected along all MCS profiles. All of the ENAM‐CSE products were made publicly available shortly after acquisition, ensuring unfettered community access to this unique dataset.〈/span〉

Description: SUMMARY The Swan Islands Transform Fault (SITF) marks the southern boundary of the Cayman Trough and the ocean–continent transition of the North American–Caribbean Plate boundary offshore Honduras. The CAYSEIS experiment acquired a 180-km-long seismic refraction and gravity profile across this transform margin, ∼70 km to the west of the Mid-Cayman Spreading Centre (MCSC). This profile shows the crustal structure across a transform fault system that juxtaposes Mesozoic-age continental crust to the south against the ∼10-Myr-old ultraslow spread oceanic crust to the north. Ocean-bottom seismographs were deployed along-profile, and inverse and forward traveltime modelling, supported by gravity analysis, reveals ∼23-km-thick continental crust that has been thinned over a distance of ∼70 km to ∼10 km-thick at the SITF, juxtaposed against ∼4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are not widely observed, the 7.0 km s−1 velocity contour is used to define the Moho along-profile. The apparent lack of reflections to the north of the SITF suggests that the Moho is more likely a transition zone between crust and mantle. Where the profile traverses bathymetric highs in the off-axis oceanic crust, higher P-wave velocity is observed at shallow crustal depths. S-wave arrival modelling also reveals elevated velocities at shallow depths, except for crust adjacent to the SITF that would have occupied the inside corner high of the ridge-transform intersection when on axis. We use a Vp/Vs ratio of 1.9 to mark where lithologies of the lower crust and uppermost mantle may be exhumed, and also to locate the upper-to-lower crustal transition, identify relict oceanic core complexes and regions of magmatically formed crust. An elevated Vp/Vs ratio suggests not only that serpentinized peridotite may be exposed at the seafloor in places, but also that seawater has been able to flow deep into the crust and upper mantle over 20–30-km-wide regions which may explain the lack of a distinct Moho. The SITF has higher velocities at shallower depths than observed in the oceanic crust to the north and, at the seabed, it is a relatively wide feature. However, the velocity–depth model subseabed suggests a fault zone no wider than ∼5–10 km, that is mirrored by a narrow seabed depression ∼7500 m deep. Gravity modelling shows that the SITF is also underlain, at 〉2 km subseabed, by a ∼20-km-wide region of density 〉3000 kg m−3 that may reflect a broad region of metamorphism. The residual mantle Bouguer anomaly across the survey region, when compared with the bathymetry, suggests that the transform may also have a component of left-lateral trans-tensional displacement that accounts for its apparently broad seabed appearance, and that the focus of magma supply may currently be displaced to the north of the MCSC segment centre. Our results suggest that Swan Islands margin development caused thinning of the adjacent continental crust, and that the adjacent oceanic crust formed in a cool ridge setting, either as a result of reduced mantle upwelling and/or due to fracture enhanced fluid flow.