The evolution of the intra-Carpathian basins and their relationship to the Carpathian mountain system

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1982.Microfiche copy available in Archives and ScienceVita.Includes bibliographies.by Leigh Handy Royden.Ph.D

Dynamic topography produced by lower crustal flow against rheological strength heterogeneities bordering the Tibetan Plateau

Dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins. With lubrication equations commonly used to describe viscous flow in a thin-gap geometry, we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity. Dynamic stresses depend on the mean velocity (Ū), viscosity (µ) and channel thickness (h), uniquely through the term µŪ/h^2. These stresses are then applied to the base of an elastic upper crust and the deflection of the elastic layer is computed to yield the predicted dynamic topography. We compare model calculations with observed topography of the eastern Tibetan Plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid Sichuan Basin. Model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 × 10^(18) Pa s flowing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm yr^(−1)

Report of the panel on plate motion and deformation, section 2

Given here is a panel report on the goals and objectives, requirements and recommendations for the investigation of plate motion and deformation. The goals are to refine our knowledge of plate motions, study regional and local deformation, and contribute to the solution of important societal problems. The requirements include basic space-positioning measurements, the use of global and regional data sets obtained with space-based techniques, topographic and geoid data to help characterize the internal processes that shape the planet, gravity data to study the density structure at depth and help determine the driving mechanisms for plate tectonics, and satellite images to map lithology, structure and morphology. The most important recommendation of the panel is for the implementation of a world-wide space-geodetic fiducial network to provide a systematic and uniform measure of global strain

Hotspot swells and the lifespan of volcanic ocean islands

Volcanic ocean islands generally form on swells—seafloor that is shallower than expected for its age over areas hundreds to more than a thousand kilometers wide—and ultimately subside to form atolls and guyots (flat-topped seamounts). The mechanisms of island drowning remain enigmatic, however, and the subaerial lifespan of volcanic islands varies widely. We examine swell bathymetry and island drowning at 14 hotspots and find a correspondence between island lifespan and residence time atop swell bathymetry, implying that islands drown as tectonic plate motion transports them past mantle sources of swell uplift. This correspondence argues strongly for dynamic uplift of the lithosphere at ocean hotspots. Our results also explain global variations in island lifespan, which influence island topography, biodiversity, and climate.United States. National Aeronautics and Space Administration (Grant NNX13AN67H

Subduction with variations in slab buoyancy: Models and application to the Banda and Apennine systems

Temporal variations in the buoyancy of subducting lithosphere exert a fi rst-order control on subduction rate, slab dip and the position of the associated volcanic arc. We use a semi-analytic, three-dimensional subduction model to simulate unforced subduction, in which trench motion is driven solely by slab buoyancy. Model rates of subduction and model slab dip respond almost immediately to changes in the buoyancy of the subducting lithosphere entering the trench; as more buoyant slab segments correlate with slower subduction rates and steeper slab dip. The results are largely consistent with observations from the Banda and southern Apennine subduction systems, where subduction slowed and ended shortly after the entry of continental lithosphere into the trench. Over a 2 m.y. period, model subduction rates decrease from ̃70 mm/year to ̃30 mm/year for the Banda Arc, and from ̃40 mm/ year to ̃20 mm/year for the Apennine Arc. Increases in model slab dip and decreases in arc-trench distance are likewise consistent with hypocenter locations and volcanic arc position along the Banda and Sunda arcs. In contrast, a time period of ̃10 m.y. is needed for model subduction rates to slow to near zero, much longer than the ̃3 m.y. upper bound on the observed slowing and cessation of trench motion in the Apennine and Banda systems. One possible explanation is that slab break-off, or the formation of large slab windows, occurred during the last stages of subduction, eliminating toroidal fl ow around the slab and allowing the slab to steepen rapidly into its final positio

Bending and Unbending of an Elastic Lithosphere: The Cenozoic History of the Apennine and Dinaride Foredeep Basins

The Adriatic region forms an intermediate continental foreland overthrust along its northeastern margin by the southwest vergent Dinaric thrust belt in Eocene-Oligocene time and along its southwestern margin by the northeast vergent Apennine thrust belt in Pliocene-Quaternary time. Orogenic activity within these thrust belts was accompanied by the development of two superposed foredeep basin systems of opposite polarity and different ages. Using well log, biostratigraphic, and seismostratigraphic data, the geometry of this composite basin system was reconstructed along three profiles at beginning of Quaternary, middle Pliocene, beginning Pliocene, and beginning Eocene time. Modeling of reconstructed geometries using a thin elastic sheet approximation yields a range of acceptable effective elastic plate thicknesses for the central Adriatic region of Te = 5–10 km for Eocene-Oligocene flexure and Te = 10–15 km for Pliocene-Quaternary flexure (although an upper bound for Te could not be established on one of the three profiles). These results are consistent with a constant effective elastic plate thickness of Te = 10 km for the Adriatic lithosphere and preclude the possibility that significant weakening of the Adriatic plate occurred between flexural events. Modeling of incremental deflections between Pliocene and Quaternary time gives results consistent with constant values of Te = 10 km in the central Adriatic and Te = 15 km in the northern Adriatic and Po Plain and shows little evidence for weakening of the plate during Pliocene-Quaternary time. Thus within the resolution of the data presented in this paper, there is little evidence for viscous relaxation of the lithosphere on time scales between about 2 and 50 m.y. Analysis of bending of an idealized lithosphere with a simple brittle-elastic-ductile rheology, and a low to moderate thermal gradient suggests that the small values of Te observed within the Adriatic region can be readily understood as the result of bending of the lithosphere to unusually high curvature (4 × 10−6 m−1) and do not require unusually high temperatures within the foreland lithosphere. The same rheological model is also consistent with the absence of significant inelastic yielding for at least 50 m.y. after the cessation of loading. An apparent unbending of the Adriatic lithosphere began in early Quaternary time, approximately coeval with the cessation of major thrusting within the Apennine thrust belt. The three-dimensional pattern of Quaternary deflection makes it difficult to attribute this phenomenon to local depositional processes and suggests that unbending reflects a fundamental change in the subduction process in early Quaternary time. Our preferred interpretation is that unbending is the result of a diminution of forces acting on the subducted Adriatic lithosphere at mantle depths

Subduction Dynamics and Mantle Pressure: 2. Towards a Global Understanding of Slab Dip and Upper Mantle Circulation

©2020. American Geophysical Union. All Rights Reserved. We investigate the relationship between the global distribution of deep slab dips (at 250- to 300-km depth) and pressure and circulation in the upper mantle. Using an analytic method to compute dynamic pressure in a 3-D global upper mantle domain, and a force balance between slab dip, slab buoyancy, and pressure, we model dips for all major subduction zones. Overall, our models suggest that global-scale mantle flow, as dictated by the shapes and velocities of Earth's plates and slabs, plays a fundamental role in creating the global pattern of slab dips. The dip trends of the South American and western Pacific subduction zones are controlled, in our models, by spatial variations in the dynamic pressure associated with flow. Our best fitting models produce global root mean square dip misfits of less than 10° for asthenospheric viscosities of 2.5–4.0 × 1020 Pas. This result is only obtained with a large flux of asthenosphere from upper to lower mantle at subduction boundaries, occurring on the overriding plate side of slabs, without which dips are significantly steeper than observed. This effect cannot be resolved by processes that affect only certain subduction systems and requires flux of asthenosphere into the lower mantle at subduction systems globally (or an alternative mechanism that produces more negative pressures on the overriding plate side of slabs). Upper mantle pressure fields that fit global slab dips yield negative dynamic pressure on the upper plate side of slabs, positive pressure on the subducting plate side, and an east-to-west pressure increase beneath the Pacific Plate

Slab segmentation and late Cenozoic disruption of the Hellenic arc

International audienceThe Hellenic subduction zone displays well-defined temporal and spatial variations in subduction rate and offers an excellent natural laboratory for studying the interaction among slab buoyancy, subduction rate, and tectonic deformation. In space, the active Hellenic subduction front is dextrally offset by 100–120 km across the Kephalonia Transform Zone, coinciding with the junction of a slowly subducting Adriatic continental lithosphere in the north (5–10 mm/yr) and a rapidly subducting Ionian oceanic lithosphere in the south (∼35 mm/yr). Subduction rates can be shown to have decreased from late Eocene time onward, reaching 5–12 mm/yr by late Miocene time, before increasing again along the southern portion of the subduction system. Geodynamic modeling demonstrates that the differing rates of subduction and the resultant trench offset arise naturally from subduction of oceanic (Pindos) lithosphere until late Eocene time, followed by subduction of a broad tract of continental or transitional lithosphere (Hellenic external carbonate platform) and then by Miocene entry of high-density oceanic (Ionian) lithosphere into the southern Hellenic trench. Model results yield an initiation age for the Kephalonia Transform of 6–8 Ma, in good agreement with observations. Consistency between geodynamic model results and geologic observations suggest that the middle Miocene and younger deformation of the Hellenic upper plate, including formation of the Central Hellenic Shear Zone, can be quantitatively understood as the result of spatial variations in the buoyancy of the subducting slab. Using this assumption, we make late Eocene, middle Miocene, and Pliocene reconstructions of the Hellenic system that include quantitative constraints from subduction modeling and geologic constraints on the timing and mode of upper plate deformation