Tectonics

Résumé

Tectonics (; ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. These processes include those of mountain-building, the growth and behavior of the strong, old cores of continents known as cratons, and the ways in which the relatively rigid plates that constitute the Earth's outer shell interact with each other. Principles of tectonics also provide a framework for understanding the earthquake and volcanic belts that directly affect much of the global population. Tectonic studies are important as guides for economic geologists searching for fossil fuels and ore deposits of metallic and nonmetallic resources. An understanding of tectonic principles can help geomorphologists to explain erosion patterns and other Earth-surface features. Main types of tectonic regime Extensional tectonics Extensional tectonics Extensional tectonics is associated with the stretching and thinning of the crust or

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https://en.wikipedia.org/wiki/Tectonics

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Long-term tectonic control on Holocene shelf sedimentation offshore La Jolla, California

Nicolas Le Dantec

A high-resolution Compressed High-Intensity Radar Pulse (CHIRP) survey reveals shore-parallel variations in the Holocene sediment thickness offshore La Jolla, California. Sediment thicknesses decrease from >20 m in the south near Scripps Canyon to zero in the north approaching Torrey Pines. In addition to the south-to-north variation in sediment thickness, the transgressive surface observed in seismic lines shoals from Scripps Canyon to the north. Despite these dramatic shore-parallel subsurface changes, the nearshore bathymetry exhibits little to no change along strike. A left jog (i.e., a constraining bend) along the Rose Canyon fault causes local uplift in the region and appears to explain the northward shoaling of the transgressive surface, the decrease in relief on the transgressive surface away from the left jog, and the Holocene sediment thickness variation. This tectonic deformation is shore parallel, and thus the accommodation can be separated into its tectonic and eustatic components.

2007

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Tectonic controls on nearshore sediment accumulation and submarine canyon morphology offshore La Jolla, Southern California

Nicolas Le Dantec

CHIRP seismic and swath bathymetry data acquired offshore La Jolla, California provide an unprecedented three-dimensional view of the La Jolla and Scripps submarine canyons. Shore-parallel patterns of tectonic deformation appear to control nearshore sediment thickness and distribution around the canyons. These shore-parallel patterns allow the impact of local tectonic deformation to be separated from the influence of eustatic sea-level fluctuations. Based on stratal geometry and acoustic character, we identify a prominent angular unconformity inferred to be the transgressive surface and three sedimentary sequences: an acoustically laminated estuarine unit deposited during early transgression, an infilling or “healing-phase” unit formed during the transgression, and an upper transparent unit. Beneath the transgressive surface, steeply dipping reflectors with several dip reversals record faulting and folding along the La Jolla margin. Scripps Canyon is located at the crest of an antiform, where the rocks are fractured and more susceptible to erosion. La Jolla Canyon is located along the northern strand of the Rose Canyon Fault Zone, which separates Cretaceous lithified rocks to the south from poorly cemented Eocene sands and gravels to the north. Isopach and structure contour maps of the three sedimentary units reveal how their thicknesses and spatial distributions relate to regional tectonic deformation. For example, the estuarine unit is predominantly deposited along the edges of the canyons in paleotopographic lows that may have been inlets along barrier beaches during the Holocene sea-level rise. The distribution of the infilling unit is controlled by pre-existing relief that records tectonic deformation and erosional processes. The thickness and distribution of the upper transparent unit are controlled by long-wavelength, tectonically induced relief on the transgressive surface and hydrodynamics.

Elsevier2010

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The subject of small-scale marine sediment mechanics is an area of active research, motivated by its key role in beach and shelf processes. Much of the existing small-scale work has focused on two-phase flows and particle behavior at the sediment interface, and their role in sediment transport and shaping seabed morphology. The study presented here considers the behavior of unconsolidated sediments below the seabed, 100 to 1000 grain diameters deep. Motivated by field observations off La Jolla Shores Beach that suggest dynamical behavior at such depths, a series of laboratory experiments were conducted to study the behavior of unconsolidated, saturated granular assemblies. Careful measurements of the apparent mass at the base of dry and saturated, glass beads pilings were acquired to examine stress redirection effects. The experiment protocol included slowly lowering the piling base to achieve reproducible piling configurations through wall friction mobilization. Results are analyzed using the Janssen and Oriented Stress Linearity (OSL) models, the latter allowing interpretation of the arching behavior in terms of arch direction and friction angle along arches. The data show qualitative similarities between dry and saturated granular assemblies, suggesting a commonality in their underlying physics, but quantitative differences in the intensity of the arching mechanism, probably associated with lubrication effects. Analysis of the piling quasistatic descent shows evidence of a progressive upward decompression punctuated with dynamic piling reconfigurations interpreted as relaxation oscillations. Both features are associated with spatial and temporal stress heterogeneities and have implications for modeling the mechanics of granular assemblies. Granular matter theories are expected to reproduce such observations occurring under minute deformations in these laboratory experiments. This work also describes a study of nearshore sediment accumulation and submarine canyon morphology offshore La Jolla, using new seismic data and bathymetry. Isopach maps of the sedimentary sequences overlying the bedrock reveal the critical role of the tectonic landscape on the spatial distribution of modern stratigraphic units. A detailed examination of the morphological characteristics of the Scripps and La Jolla canyons refines the relationship between canyon location and development and regional tectonic deformation, and provides insight on the stability of unconsolidated, saturated marine sediments.

University of California San Diego2009

Chargement

Tectonic controls on nearshore sediment accumulation and submarine canyon morphology offshore La Jolla, Southern California

Nicolas Le Dantec

Elsevier2010

University of California San Diego2009