Faults and fissures

In the 1980s De Beers stumbled upon the Leopard Fissure that was later to be developed at the turn of the century as part of the Klipspringer Joint Venture. This fault here links up the western Leopard fissure to the south west with an inferred small kimberlite pipe situated in excess 15km along trend from the Klipspringer mine plant. A nearby tension fracture emits a green clay material. The fault goes undercover a fair distance along its path, but the dry pans do not tolerate vegetation easily (and thankfully cattle too), so the surface inference of its structure is readily unravelled. It dips steeply towards the south, and displays great depth extent, as characterized by historical mine workings deep within the Klipspringer mine shaft that lie underwater. The other end of it terminates in a well exposed, and weathered aphanitic kimberlite dyke, but not before it crossed paths with dolerite. It is a special master fault associated with dextral R shears, forming a series of en echelon folds and extensional fractures along its strike. The accompanying R shears define the hummocky topography of these dry pans, evidently a well preserved example of simple dextral shear. The Bushveld Complex aged failed rifting event of the Archaean granite landscape left diabase sills and dykes (nearly 2000 million year old marker), to be later deformed by Jurassic rifting and precursory dolerite intrusion to Gondwana’s breakup.

Fault or a farm wall

You would not think this is a fault, or even a conjugate shear. It could be an ancient farmer’s wall, but the history would not support this. Calcite veins at low angles (synthetic) to its strike, beyond the fault tip, reveal it’s true provenance. These faults are common in the Precambrian landscape of the Kaapvaal craton, they make surface expression for perhaps 100 metres before forming fault tips as they disappear beneath the ground to depth. Not much anthropological activity has disturbed them through the course of time (100’s million years time scale). Deep seated conduits form different types of volcanism and utilize these faults to emplace special stones of interest to mankind.

Precambrian tears

A real fault in the field, defined by its scarp face, debris field and trace continuity. This one cuts through the Archaean granite landscape and is associated with a handful of other indicators that demonstrate its importance. Like all old faults, this one has likely been reactivated several times, and not always in the same sense. The views below show the fault in exposure, its debris and a view of the scarp face and relation to the dry pan in the distance beyond the veldt. This fault also has connections to deep within the Earth’s crust. The Precambrian landscape set on Archaean craton is pervasively faulted and fractured, and the surface expression is complicated by the nature of the tear faults that pivot, hinge and disappear along strike


Simple example of sinistral movement along an ‘R’ shear fault with emphasis on associated extensional fractures displaying various phases of calcrete evolution. One such fracture displays nodular and the other dendritic calcrete. These have locally evolved into calcrete horizons where they are exposed across the face of the fractures. The penultimate image shows the fault contact forms the margin of the river, across which extension has occurred parallel to the shear fault; there are two horizons of calcrete visible in the facing soil profile. In each of these cases the calcrete forms in consequence to biochemical processes (pedogenic carbonate origins) beneath the roots of vegetation, aided by the rising water table. Nodular calcrete has found great application as hard coarse in road construction. The last image shows an exhumed calcrete nodule bank (right) in an entirely different location masking the Precambrian outcrop in the middle of a large dry pan.
#Precambrian #calcrete #biochemistry #exploration

Shear zones

Field example of another tear fault that has deformed 3000 Ma Archaean granites of southern Africa. This one is a hinge variety on the margins of a shear zone. Upstream it forms a structurally controlled river bank. There is no fault debris due to the palaeo-transport downstream. It forms the margin of a transtensional shear zone that transfers into a transpressional zone further downstream as can be seen from the mounds in the relevant views. The folds are parallel to the primary and secondary conventional shear components, and antithetic tension fractures have formed across them, filled by calcite veins and hydrothermal alteration products. In the vicinity conjugate cleavage sets are well developed in an outcrop of metasediments. The cleavage planes are parallel to the conjugate components of shear. The master fault extends to much greater depths. Four hundred metres on trend with an inferred kimberlite blow.