Field School 11 Field Guide - Pofadder Shear Zone

1 Introduction

During the Field School we will have a special opportunity of investigating the major Pofadder Shear Zone within the Namaqua-Natal Belt. During this investigation, you will have the opportunity of examining the transition from brittle-ductile shearing and also practice measuring kinematic indicators in the field.

The c. 1000 Ma NW-SE upper Greenschist-lower Amphibolite facies Pofadder Shear Zone is a c. 500 km long right-lateral strike-slip shear zone that extends across South Africa and Namibia with c. 30 km offset (Lambert, 2013). This feature is of especial interest because it has been exhumed to levels which highlight the brittle-ductile transition and highlights effects of a Paleo-Earthquake (Melosh et al., 2016).

Geological overview of the Pofadder core zone (Melosh et al., 2014)

2 Namaqua-Natal Belt

The c. 1800 - 1000 Ma Namaqua-Natal Belt forms the western boundary of the Kalahari Shield and is associated with the formation of the Rodinia Supercontinent. During the field school we will be focusing on the Namaqualand sector of this Belt. The Namaqualand sector can be divided into four domains, each separated by major crustal shear zones:

Overview of the Namaqua-sector of the Namaqua-Natal Belt (Bilal et al., 2016)

The lithologies of these different domains can be summarised as follows:

Lithological subdivision across the various Namaqua-Natal domains (Edlington et al., 2006)

Deformation of the Namaqualand sector can is defined within four major events (Groenewald per coms):

D1: c. 1905-1885 Ma Orange River Orogeny, which formed the Richtersveld magmatic arc and intrusion of the Vioolsdrift Suite granitoids. Evidence of this deformation is only preserved in the low-grade Vioolsdrift Domain, within the Richtersveld. By the end of this field school, you will be experts on this geology, because it will form the basement geology throughout the Gariep Belt.

D2: c. 1220-1100 Ma Namaqua-Natal Orogeny, which is linked to continental collision during the main phase of Rodinia assembly, this event, together with the next formed most of the large thrust zones that separate the various domains. This event resulted in the formation of the gneissic and schistose fabric commonly observed along our Pofadder transect.

D3: c. 1100-1005 Ma Peak deformation during the Namaqua-Natal Orogeny. This event also resulted in the development of mega-scale refolding of D2-linked folds into upright and sheath folds.

D4: c. 1005-980 Ma development of dextral ductile shear zones, e.g. the Pofadder Shear Zone. This event can be further subdivided into six intermediate deformational phases (a-d):

D4a: Early stages of shear zone development with shear-linked reorientation and drag of earlier fabrics, with fabrics becoming steeper toward the core of the shear zone.

D4b: Formation of mylonitic and cataclasite units within the shear zone

D4c: Formation of the Pofadder very fine-grained ultramylonitic units, including the core, which furthermore crosscuts the D4b structures.

D4d: Brecciation of the ultramylonites and cataclasites, this represents a large-scale Paleoearthquake

D4e: Major steep faulting and the development of sub-vertical lineations

D4f: Late epidote-filled cross-cutting fractures

3 Kinematics

There are several features which may be used in a ductile shear zone to establish sense of movement, but firstly, an answer to the age old question: "which plane should I be looking at?"

Kinematic evidence of shearing can be seen at high-angles to the lineation (ETH, 2016)

Now that you know which plane to look at, there are a number of features which may assist you in establishing the sense of shearing; here are few listed below:

You may use S-C fabrics to establish the sense of shear (ETH, 2016)

You may also use C-C fabrics to establish the sense of shear (ETH, 2016)

And, the most common rotated porphyroclasts (ETH, 2016)