Tuesday, June 7, 2016

Salt Diapirism on Axel Heibger Island, Nunavut

So, what is the present objective for Elise studying radar?

By Connormah (Based off) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Axel Heiberg Island in red.
As briefly mentioned in a previous post, I will be contributing towards a project on using radar to study geological features in the Canadian Arctic.  Specifically, my part of the project will be monitoring salt diapir evolution on Axel Heiberg Island using radar interferometry (InSAR).  The techniques and applications of InSAR will be discussed in a following post.  Today, we will be taking a look at the geology of Axel Heiberg Island, and the characteristics of the salt diapirs that have been found there as described and mapped by Jackson and Harrison (2006).

First, what are salt diapirs?  Salt, and other evapouratitic deposits, tend to be less dense than the surrounding strata.  Like oil rising through water, tectonic activity can mobilize evapourite deposits, causing large mounds of salt to rise upward through the overlying rock units.  The result is dome-like deformation structures surrounding the diapirs of salt.  

©2012 Geotrack International Pty Ltd
Extent of Sverdrup Basin in the Canadian Archipelago
Diapirism is not particularly common, making Axel Heiberg Island a unique place to study.  With 46 exposed outcrops, Axel Heiberg has one of the highest concentrations of evapouritic diapirs in the world, second only to Iran.  Because of its remote location in the Arctic, the Axel Heiberg salt diapris remain largely understudied, being discovered by seismic exploration between 1969 and 1985.  They were only mapped by field traverses as recently as 2004 in a mapping exercise in the mid-western portion of the island.  Folded salt layers have been found within the same basin (Sverdrup Basin, expanding across the western Canadian Archipelago) that contain expansive volumes of petroleum resources, potentially making Axel Heiberg Island a site for economic development.  Additionally, studying evapourite diapirs on Earth may give us clues as to the behaviours and characteristics of salt masses on other planetary bodies, such as the newly found salt masses on Ceres.  Studying the movements of the Axel Heiberg diapirs may prove to be immensely beneficial to the Canadian economy, as well as the advancement of planetary science exploration.

Axel Heiberg Island almost exclusively part of the Sverdrup Basin.  The Sverdrup Basin is a 12-15 km thick succession of strata dated from the Carboniferous up until the Eocene.  The section of strata on Axel Heiberg is approximately 10 km thick; the thickest section of Mesozoic strata in the Sverdrup Basin.  Approximately 100 diapirs have been found within the basin, of which 60 are exposed at surface.  Three quarters of exposed diapris within the basin are located on Axel Heiberg Island.  The diapirs composed of anhydrite, which is weathering to gypusum, and contain interbeds of brecciated carbonates.  The basin has been subjected to numerous tectonic events, contributing to the rise of the diapirs.  The basin was subject to rifting until the mid-Permian, and the basin thermally subsided between the late-Permian though the Early Jurassic.  The older diapirs began their ascend during the Late Triassic, and were greatly accelerated by the onset of Arctic ocean seafloor spreading in the Cretaceous.  During this time, rifting was accompanied by mafic dike swarms and flood basalt volcanism.  The presence of minibasins surrounding the diapris provide evidence of regional shortening post-Late Cretaceous.

Cross-section of evaporutic diaprism on western Axel Heiberg Island.  Note the younger evapourite canopy in the WABS region, where two generations of diapirism has occurred (from Harrison and Jackson, 2006). 

Post-flood volcanism and other tectonic activies were more recent folding events.  The deformation structures present are not very straight forward.  Most of the region is characterized by sinusoidal folds, with anticlines and synclines equally wide.  These trend to the north.  However, Jackson and Harrison have found a 60 km wide area of deformation where shorter folds that are out of line with the surrounding region.  This area, termed a wall-and-basin structure, contains a high abundance of diapirs.  They hypothesize that the wall-and-basin structure represents a canopy, where rising diapirs came together, joining to form an stratiform sheet.  The style of deformation indicates a possible detachment of the strata along the salt canopy.  The abundant minibasins around the wall-and-basin structure is analogous to the Gulf of Mexico, where units have also subsided over large salt canopies.

Despite being well exposed in the polar desert, the rising evapourites on Axel Heiberg Island have not been well studied.  Radar analysis and remote sensing are key tools in learning more about the geological behaviour of remote sites that are difficult to field check.  In measuring the rate of movement within the Axel Heiberg diapirs, we can apply this knowledge to salt diapirs in other inaccessible areas, such as politically unstable countries, and other bodies in our solar system.  Salt diapirs have both economic significances, as well as geological relevance in determine the stratigraphic relationships within a basin.

This is why we are doing what we are doing, time to learn some more radar! (After convocation this week.)

Jackson, M.P.A., and Harrison, J.C., 2006, An allochthonous salt canopy on Axel Heiberg Island, Sverdrup Basin, Arctic Canada: Geology, v. 34, no. 12, p. 1045–1048, doi: 10.1130/G22798A.1.

1 comment:

  1. A very thorough summary, especially considering that you're on vacation this week!