Sedimentary Basin Analysis http://eqsun.geo.arizona.edu/geo5xx/geos517/

Sedimentary basins can be classified based on the type of plate motions (divergent, convergent), type of the lithosphere, distance from plate margins. Sedimentary Basin Analysis http://eqsun.geo.arizona.edu/geo5xx/geos517/

From divergent to convergent margins

Convergent margins: collisional and non collisional Basins related to convergent margins Peripheral foreland system Fore arc Back arc Retroarc

After Dickinson (1974): Oceanic basins Rifted continental margins Arc trench system Suture belts Intracontinental basins

TO FORM A SEDIMENTARY BASIN YOU NEED ACCOMODATION SPACE Subsidence of the crust is caused by: 1) Attenuation of the crust due to stretching and erosion (divergent margins); 2) Contraction of lithosphere due to cooling (divergent margins); 3) Depression of both crust and lithosphere h by sedimentary or tectonic loading (convergent margins).

Primary mechanisms of basin subsidence Isostasy (changes in lithospheric thickness and/or density) Flexure (loading) Thermal effects

Isostasy Based on Archemides principle: A body immersed in a fluid is buoyed up by a force equal to the weight (volume) of the displaced fluid. Isostasy assumes local compensation: i.e. the earth is composed of free floating pistons

Depth of compensation Following the theory of isostasy, a mass above sea level is supported below sea level, and there is thus a certain depth at which the total weight per unit area is equal all around the Earth: depth of compensation (ca. 113 km).

Models for local isostasy Airy: where different in topographic highs are accommodated by changes in thickness Pratt:.by changes in density Adjacent blocks with different densities or thickness will have different relative relief

After Angevine et al., 1990

Typical lithospheric structure under the continents and the oceans

After Angevine et al., 1990

Thermal effects Thermal effects change the density of the lithosphere so that the isostatic balance is changed Initial cooling of the lithosphere by conduction: t1/2 After 10s the lithosphere will cool exponentially ill (subsidence: because colder rocks are denser.)

A. If everything else is equal the total amount of uplift during heating (e.g. intrusion doming) is equal to the amount of subsidence during cooling. B. Subsidence can occur in case of erosion; thickening of the mantle lid during cooling or stretching. After Angevine et al., 1990

Following McKenzie ( In time 2 the lithosphere h will subside because of isostatic compensation (you replace denser mantle lid with less dense asthenosphere. In time 3 the lithosphere cools and thickens as warm asthenosphere converts in cool lithosphere (thermal subsidence)

Examples of divergent margins

Examples of convergent margins Peripheral forelands Retroarc forelands

Pro wedge Foreland Alps The Alpine orogeny Foreland Retro wedge After Stampfli et al. (2002) The Alpine orogeny is the result of the collision between Europe and Adria ~50 My ago, following closure of the Alpine Tethys.

The Himalayan orogeny Foreland Himalaya N Tibetean Plateau The Himalayan orogeny is the result of the collision i between India and Asia ~50 My ago, following closure of the NeoTethys.

Example of ocean continent collision and retroarc foreland: The Andes Altiplano Puna Plateau Cordillera Foreland The Andes are the result of Tertiary crustal thickening and magmatic processes related to the subduction of the Nazca plate lt under the South American Plate. Courtesy Isabelle Coutand

Mechanisms of foreland basin formation Peripheral basins: Modified after DeCelles and Giles (1996)

Mechanisms of foreland basin formation Retroarc basins: Modified after DeCelles and Giles (1996)

Slide 28 c1 dynamic slab load is caused by viscous coupling between the subducting slab, overlying mantle-wedge materil and the base of the overidding continental plate carrapa, 1/5/2006

Sedimentary facies and depositional environment in pheripheral p basins Shallowing upward trend up section

GEODYNAMICS OF FORELANDS D=flexural strength The shape of a FB and the magnitude of subsidence largely depend on the age and thickness of the underlying crust (i.e. closing young rift=low D; old continental margin=high D).

Thin versus thick sckinned deformation Typical (continuous) foreland basins develop in association with thin skinned thrust belts: i e where sedimentary cover rocks are shortened i.e. where sedimentary cover rocks are shortened by folding and thrusting above undeformed basement.

Thin versus thick sckinned deformation Broken foreland basins (Sierras Pampeanas and Laramide style) develop associated with thick skinnedbasement upliftwhere crystalline basement rocks translate along high angle reverse faults.

Preconditions to form a continuous foreland dbasin The existance of topographic highs and horizontal shortening; The pre existance existance of a thick pile of layered strata.

Inferred preconditions to form a broken foreland dbasin The existance of topographic highs and horizontal shortening; The presence of a subducting plate with very low angle of descent into the mantle (i.e. flat slab or flat subduction).

Component of tectonic subsidence in broken foreland dbasins In a broken foreland the crust is subdivided into short beams that can rotate about horizontal axes relative to one another. This rotation is a principal component of tectonic susbidence (in addition to flexural loading)

What are the controlling factors for the angle of subduction? Th iti t t i hi t f The pre existing tectonic history of an orogenic belt and the shape of its plate margin

Example of flat slab and broken foreland: Sierra Pampeanas (Andes) I S ha i l ik In South America along strike variabilities in the structure of the Andes generates along strike variations in geometry of foreland basins

Example of broken foreland: the Rocky Mountains and the Laramide foreland system The depocenter of the upper Cretaceous (A) in eastern Wyoming is too far from the thrust belt to have been simply caused by loading and flexure. B) Eocene basin and ranges; uplift and subsidence are contemporaneous; C) thickness of the Green River Basin reflects tilting and bending of the crustal blocks.

Laramide and Sierra Pampeanas comparison

Seismic exercise: thin skinned or thick skinned?

Seismic example: thin skinned or thick skinned? After Ramos et al. (2002)