Browsing by Author "Sawyer, Dale S"

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    Evolution of North Malé Atoll Rim during the Last Full Glacial Cycle (Malé Island, Republic of Maldives)
    (2014-07-31) Koksal, Tugba; Droxler, André W; Sawyer, Dale S; Nittrouer, Jeffrey A; Gischler, Eberhard
    My study focuses on the evolution of Malé Island to elucidate the late Pleistocene-Holocene evolution of the discontinuous North Malé Atoll rim. Sample analyses from two boreholes, published information from additional boreholes, and a multi-beam bathymetric map for Malé Island deep surroundings, are available for this study. Facies analyses of the lower sedimentary unit reveal an overall deepening coralgal reef that accumulated probably during the previous interglacial and subsequently was altered by meteoric diagenesis during a 100 ky-long exposure. The upper Holocene unconsolidated coralgal reef, overlying the karstified coralgal MIS 5e lower unit, was initiated at ~8200 kyr BP and vertically grew 25 m high until 6510 kyr BP, protected behind a karstified late Pleistocene reef. A small, though 30-35 m deep, newly formed lagoon started to fill up only at ~5500 kyr BP, when a reef initiated on top of the highest elevated Pleistocene karstified reef and sea level rise stalled.
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    Late Quaternary Mixed Carbonate-Siliciclastic Sediment Slope Accumulation: Unexpected Responses of Australia and Papua New Guinea Reefs to Glacio-Eustatic Sea Level Fluctuations
    (2014-06-24) Harper, Brandon Babbel; Droxler, Andre W; Dickens, Gerald R; Anderson, John B; Sawyer, Dale S; Rudolf, Volker H. W.
    Overtime, the north Queensland (Australia) and Gulf of Papua (Papua New Guinea) margin has been constructed by the biogenic carbonate growth and development of large barrier reefs on the mid-to-outer shelves and offshore platform/atoll tops, in conjunction with latitudinally variable riverine delivery of terrigenous siliciclastic material to the coast and inner shelf. This association forms Earth’s longest tropical mixed carbonate-siliciclastic system. Spatial and temporal variations of neritic carbonate (chiefly aragonite) production and siliciclastic sediment supply are controlled by late Quaternary high amplitude sea level fluctuations and climate changes. Sediment accumulation on the upper slopes adjacent to mixed margins is an important indicator of reef establishment/demise and siliciclastic shelf bypass during a time when glacio-eustatic sea level fluctuations are well understood and sediment dating techniques well developed. Sedimentation variability since the last deglaciation and, to some degree, during the last glacial cycle, is well understood and described by the transgressive and highstand shedding, reciprocal and coeval sedimentation concepts; based on low resolution records. My Ph.D. research results enhance the understanding of reef initiation, accretion, and demise along the Great Barrier Reef and Gulf of Papua margin by adding new cores and elemental (Sr, Ca, Si, K, Al) data from high resolution x-ray fluorescence core scanning. Sea level related reef evolution along this margin is more complex than previously thought. Contrary to established models, the central Great Barrier Reef exited the photic zone during peak interglacial, Marine Isotope Stage (MIS) 5e, resulting in the drowning of the reef and extremely low aragonite mass accumulation on the upper slopes. When sea level fell, reef-tops re-entered the photic zone increased production and export of aragonite to the upper slopes. The precise timing and elevation of sea level during MIS-5a is defined by the presence and absence of aragonite within slope sediments from the flooding and exposure of reef-tops. Additionally, along the Papuan Peninsula, a barrier reef established on a glacial shelf edge delta during re-flooding at 19 ka, followed by the subsequent drowning and back-stepping of the reef, synchronous to Termination I melt water pulses, away from siliciclastics.
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    Late stages of breakup of the Galicia rifted margin
    (2017-07-18) Alexanian, Ara; Sawyer, Dale S
    The Galicia margin offshore of Spain represents a magma poor end member of a rifted margin. The margin exhibits varied fault structures and complex seismic stratigraphy. The R/V Marcus G. Langseth acquired three dimensional seismic data in the Deep Galicia Margin in 2013. These data are the main focus of this thesis and are used to describe the geology at this margin. Seismic reflection and refraction data have been acquired along the Galicia margin to understand the process of rifting. This study presents previously unpublished pre-stack time migrated seismic data to the west of and integrated with a previously published swath line collected along the margin. The swath line is also reprocessed to pre-stack depth migration using an interval velocity model. Reflection tomography was performed to obtain a refined velocity model in the sedimentary units in the upper portion of the seismic record, which was combined with refraction velocity models for the deeper parts of the survey. The composite seismic line reveals structures that relate to the late stages of rifting at the Deep Galicia Margin, including what appears to be the boundary between exhumed mantle from the Galicia domain and oceanic crust, denoting the ocean-continent transition. Correlated structures and horizons between the conjugate Galicia and Newfoundland margins indicate a “breakup fault” responsible for the final detachment of the two continents. This mantle-penetrating fault likely continued to be active during the onset of oceanic spreading. Thus, a double asymmetry model predicts the formation of the Galicia and Newfoundland rifted margins. The continental crust thins until breakup in the Deep Galicia Margin. Many previous rift models have been used to explain the sequence of faulting that accompanies the breakup of continental crust and exhumation of mantle there. The interplay of fault structure with stratigraphy is interpreted in the Galicia 3D data. There are 55 major Cretaceous aged faults interpreted in separate groups. Sediment basin formation may be related to the transitions between the fault groups. Four widespread horizons are seen in the volume, including two within fault blocks of continental crust. The final breakup of the continental crust may have been caused by a detachment fault that also truncates the S detachment fault. The later detachment fault is similar to faults seen in oceanic core complexes.
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    Mechanisms of late-stage rift deformation: evidence from new 3D seismic data over the West Iberia margin
    (2016-04-21) Jordan, Brian E; Sawyer, Dale S
    I describe a new 3D seismic dataset over the West Iberia margin that demonstrates the complex 3D architecture of hyperextended continental crust and exhumed continental mantle at the distal edge of the margin. Faulted continental crustal blocks show higher degrees of extension in the north than the south. Similarly, pre-tectonic sediments are cut by more small-scale normal faults in the north. I hypothesize that extension within the frontal basin was constrained by basement topography, specifically a small mantle ridge identified at the southern extent of the basin that may have obstructed the extension of the continental crustal blocks and overlying sediments. Several thrust faults also cut the pre-tectonic sediments and are interpreted to be the result of gravity-driven sliding during late-stage extension. After large-scale extension of continental crustal blocks, further extensional creep caused small-scale normal faulting of the pre-tectonic sediment in the north.
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    On the Evolution of Planets: From Convective Bi-stability to Volcanic Edifice Instability
    (2015-11-24) Weller, Matt; Lenardic, Adrian; Morgan, Julia K; Sawyer, Dale S; McGovern, Patrick J; Johns-Krull, Christopher M
    The Eastern Olympus Mons Basal Scarp (EOMBS) is conditionally stable when the edifice contains pore fluid, and critically stable, or in failure, when the edifice contains a dipping-overpressured-confined aquifer and mechanical sublayer at depth. Failure of the fault bounded portion of the flank results in estimated volumes of material ranging from 5600–6900km3, or 32–39% of the estimated volume of the “East” Olympus Mons aureole lobe. We suggest that the EOMBS faults may be an expression of early stage flank collapse and aureole lobe formation. Ages of deformed volcano adjacent plains indicate that this portion of the edifice may have been tectonically active at <50Ma, and may be coeval with estimated ages of adjacent outflow channels, 25–40Ma. This observation suggests that conditions that favor flank failure, such as water at depth below the edifice, existed in the relatively recent past and potentially could drive deformation to the present day. Coupled 3D mantle convection and planetary tectonics models are used to explore the links between tectonic-regimes, the level of internal heating (Q) within the mantle, planetary surface-temperature, and planetary lithospheric-strength. At high and low values of Q, for moderate to high yield, hot and cold single-plate planets prevail. For intermediate Q, multiple stable tectonic-states exist. In this parameter space, the specific evolutionary path of the system has a dominant role in determining its tectonic state. For low to moderate lithospheric yield strength, mobile-lid behavior (a plate tectonic-like mode of convection) is attainable for high degrees of internal heating (i.e., early in a planet’s thermal evolution). This state is sensitive to climate driven changes in surface-temperatures. Relatively small increases in surface-temperature can be sufficient to usher in a transition from a mobile- to a stagnant-lid regime. Once stagnant, a return to mobile-lid is not attainable by a reduction of surface-temperatures alone. For lower levels of Q, the tectonic regime becomes less sensitive to surface-temperature changes. These results indicate that terrestrial planets can alternate between multiple tectonic-states over giga-year timescales. Within parameter space regions that allow for bi-stable behavior, any model-based prediction as to the current mode of tectonics is inherently non-unique in the absence of constraints on the geologic and climatic histories of a planet.
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    Phases of evolution of the Post-rift of the Galicia Margin. Evidence from the Galicia 3D reflection seismic survey
    (2016-09-06) Tesi Sanjurjo, Mari Anna; Sawyer, Dale S
    Understanding the evolution of the Galicia Margin and magma-poor rifted margins in general have been done by studying different perspectives and disciplines. This study focuses on analyzing the interplay of three crucial components that hold the record of the margin’s history: stratigraphy, depositional environments and structural setting. We renewed the seismic stratigraphic framework of the Galicia Margin by interpreting and correlating depositional sequences using the Galicia 3D reflection seismic survey; and combine it with the existing knowledge of the structural, depositional environment and sedimentary disciplines. By studying the interplay, we found evidence on the seismic to redefine the post-rift and include phases of the evolution (breakup, post-breakup and margin maturation) that had not been discussed before and that are part of the margin’s history.
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    The Absolute Motion of Trenches and Age of the Subducting Slab
    (2014-12-05) Mathews, David Christopher; Gordon, Richard G; Sawyer, Dale S; Niu, Fenglin
    Previous work proposes that many trenches advance, but inferred motions vary considerably between different estimates of plate motion relative to hotspots. We remedy this by using recent estimates of absolute plate motions inferred from seismic anisotropy, incorporating the MORVEL relative plate angular velocities, and fully propagating the uncertainties. Nearly half the trench velocities differ significantly from prior results, with the greatest differences at the Kermadec, New Hebrides, and Marianas trenches. Trench velocity ranges from retreat of 126 ± 20 mm a-1 to advance of 52 ± 14 mm a-1 with a median of 9 mm a-1 of retreat. Out of 57 locations, trench advance is significant at only five locations (along the Hikurangi, Marianas, and Izu-Bonin trenches), retreat is significant at 23 locations, and trench motion differs insignificantly from zero at 29 locations. Trench advance increases with age and absolute velocity of subducting lithosphere and with angle of subduction.
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    Transform Faults, Fracture Zones, and the Kinematics of Horizontal Thermal Contraction of Oceanic Lithosphere
    (2015-12-04) Mishra, Jay Kumar; Gordon, Richard G; Lenardic, Adrian; Sawyer, Dale S; Akin, John E
    Plate rigidity is a key assumption of the plate tectonics theory. The assumption allows us to study plate motion on Earth with great simplicity. However, recent developments in understanding plate cooling have put the assumption of plate rigidity to test. Kumar and Gordon [2009] advance the theory of horizontal thermal contraction of oceanic lithosphere as the ultimate test of plate rigidity and predict that relative intraplate velocities due to horizontal thermal contraction within the oceanic lithosphere of a tectonic plate can be as high as 3 – 10 mm a-1. Contractional strains have been predicted to vary inversely with age and hence most of the strains related with horizontal thermal contraction should be accumulated and dissipated by young oceanic lithosphere. Transform faults provide the necessary free boundaries across which most of the contractional stresses can be dissipated by ridge-parallel contraction of the oceanic lithosphere. The strike of the transform faults, earlier predicted to be parallel to the relative plate motion direction, thus, should be biased as a result of transform fault perpendicular contraction of oceanic lithosphere. I first improve the global transform fault dataset used in MORVEL [DeMets et al., 2010] and then calculate residuals between the observed transform fault azimuths and those predicted for the rigid oceanic plates. I find that on an average, for the six plate pairs with both left-lateral and right-lateral slipping transform faults, azimuths are biased by about 0.75°±0.36° clockwise for left-lateral slipping and by −0.73°±0.22° (= ±1 standard error) counter-clockwise for the right-lateral slipping. I, then investigate if this observed bias can be caused by horizontal thermal contraction. For the six selected pairs with both right-lateral and left-lateral slipping transform faults, the mean bias is predicted to be only 0.38° for only right-lateral slipping faults and −0.46° for only left-slipping transform faults. Thus the magnitudes of bias predicted by horizontal thermal contraction are ≈60% as large as the observed residual between the observed strikes of transform faults and the strikes observed for the assumption of plate rigidity. Thus we cannot exclude some normal faulting in transform fault valleys. A global analysis of plate motion based on the transform fault azimuths corrected for the predicted bias due to horizontal thermal contraction shows that the hypothesis of no horizontal thermal contraction can be rejected with at least 40% of the contractional strains causing the rotation of strikes of transform faults and the remaining being accommodated by normal faulting in transform fault valleys. Thus we conclude that plates are not rigid. Based on the predictions of horizontal thermal contraction, I further test its effect on intraplate velocities and build a 2D kinematic model of the Pacific lithosphere between the Rivera and the Heezen fracture zones to calculate the predicted intraplate relative velocity field due to horizontal thermal contraction. For the kinematic assumption that balances net contraction and extension across the fracture zones, young oceanic lithosphere along the Rivera fracture zone is predicted to have a contractional velocity of 2.6 mm a-1 in the South-Southeast direction. This is big enough to account for the misfit of 5±3 mm a-1 in the PA-NA-NB-AN plate circuit and not big enough to explain the misfit of 14±5 mm a-1 for the CO-PA-NZ plate circuit.