Back-stripping

Back-stripping (or backstripping) is a geophysical analysis technique used on sedimentary rock sequences - the technique is used to quantitatively estimate the depth that the basement would be in the absence of sediment and water loading. This depth provides a measure of the unknown tectonic driving forces that are responsible for basin formation (otherwise know as tectonic subsidence or uplift). By comparing backstripped curves to theoretical curves for basin subsidence and uplift it is possible to deduce information on the basin forming mechanisms ^[1].

The technique developed by Watts & Ryan in 1976 ^[2] allows for the recovery of the basement subsidence and uplift history in the absence of sediment and water loading and, therefore isolate the contribution from the tectonic forces responsible for the formation of a rift basin ^[3]. It is a method by which successive layers of basin fill sediment are "stripped off" the total stratigraphy during analysis of that basin's history. In a typical scenario, a sedimentary basin deepens away from a marginal flexure, and the accompanying isochronous strata typically thicken basinward. By isolating the isochronous packages one-by-one, these can be "peeled off" or backstripped - and the lower bounding surface rotated upward to a datum. By successively backstripping isochrons, the basin's deepening history can be plotted in reverse, leading to clues as to its tectonic or isostatic origin. A more complete analysis uses decompaction of the remaining sequence following each stage of the back-stripping. This takes into account the amount of compaction caused by the loading of the later layers and allows a better estimation of the depositional thickness of the remaining layers and the variation of water depth with time.

Back-stripping Equation

The fundamental equation in back-stripping corrects the observed stratigraphic record for the effects of sediment and water loading and changes in water depth, and is given by:

$Y=S.\frac{(\rho_m-\rho_s)}{(\rho_m-\rho_w)}%2BW_d - \Delta_{SL} . \frac{\rho_m}{(\rho_m - \rho_w)}$

(1)

where Y is the tectonically driven subsidence, where $S$ is the decompacted sediment thickness, $\rho_s$ is the mean sediment density, $W_d$ is the average depth at which the sedimentary units were deposited, $\rho_w$ and $\rho_m$ are the densities of the water and mantle respectively, and $\Delta_{SL}$ the difference in sea-level height between the Present and the time at which the sediments were deposited. The three independent terms account for the contributions of sediment loading, water depth and sea-level oscillations to the subsidence of the basin ^[3] ^[1].

Derivation

To derive equation (1) one should first consider a 'loaded' column that represents a sedimentary unit accumulated over a certain geological time period, and a corresponding 'unloaded' column that represents the position of the underlying basement without the effects of the sediments. In the scenario, the pressure at the base of the loaded column, is given by:

$W_d\rho_wg%2BS\rho_sg%2BT_c\rho_cg$

(2)

where $W_d$ is the water depth of deposition, $T_c$ is the mean thickness of the crust, $S$ is the sediment thickness corrected for compaction, $g$ is the average gravity and $\rho_w$ , $\rho_s$ and $\rho_c$ are the densities of water, the sediment and the crust respectively. The pressure at the base of the unloaded column is given by:

$T\rho_wg%2BT_c\rho_cg%2Br\rho_mg$

(3)

where $Y$ is the tectonic or corrected subsidence, $\rho_m$ is the density of the mantle, and $r$ is the distance from the base of the unlaoded crust to the depth of compensation (which is assumed to be at the base of the loaded crust) and is given by:

$r=S%2BW_d-\Delta_{SL}-Y$

(4)

Substitution of (2),(3) and (4) after simplifying, we obtain (1).

Reference

^ ^a ^b www.gg.uwyo.edu/Geol4200-7/backstripping[1].pdf
^ Flexure of the lithosphere and continental margin basins, A.B. Watts, W.B.F. Ryan (1976), Techtonophysics]
^ ^a ^b Chapter 4: Well Backstripping and Subsidence Analysis in Gravity Anomalies, Flexure and the Thermo-Mechanical Evolution of the West Iberia Margin and its Conjugate of Newfoundland (2008), PhD Thesis by Tiago Cunha