Isostasy - Airy & Pratt

Isostasy

• describes the state of equilibrium of a lithospheric plate floating on the asthenosphere
• the weight of columns of rock, at some depth called the depth of compensation, is everywhere equal

 Airy Isostasy

mountains have a crustal root that compensates for the additional relief (variation in thickness)

Pratt Isostasy

density varies laterally, so that mountains have a lower density than higher density, thinner portions of crust

Possible Mechanisms of Crustal Subsidence

Mechanisms that can generate sufficient subsidence to create basins:

Crustal thinning    extensional stretching, erosion during uplift, and magmatic withdrawal
Mantle-lithospheric thickening    cooling of lithosphere following either cessation of stretching or heating due to adiabatic melting or rise of asthenospheric melts
Sedimentary and volcanic loading    local isostatic compensation of crust and regional lithospheric flexure, dependent on flexural rigidity of lithosphere, during sedimentation and volcanism
Tectonic loading    local isostatic compensation of crust and regional lithospheric flexure, dependent on flexural rigidity of underlying lithosphere, during overthrusting and/or underpulling
Subcrustal loading    lithospheric flexure during underthrusting of dense lithosphere
Asthenospheric flow    dynamic effects of asthenospheric flow, commonly due to descent or delamination of subducted lithosphere
Crustal densification    increased density of crust due to changing pressure/temperature conditions and/or emplacement of higher-density melts into lower-density crust



Principles of Sedimentology and Stratigraphy (5th ed.), Sam Boggs, Jr.

Principle of Superposition

The principle of superposition states that in a sequence of undisturbed sedimentary layers, the oldest layers are on the bottom and the youngest layers are on the top.

This principle was formulated by Nicolas Steno in the 17th century.

Thrust faults can alter the understanding of the principle of superposition because they occur parallel to bedding and can be hard to detect. They can create situations where older layers overly younger layers. Scientists of Nicolas Steno's time were unaware of thrust faults.