--> Evidence for East African Orogenic Transpression and Cenozoic Tension in Neoproterozoic Basement Rocks, Northwestern Ethiopian Plateau

AAPG ACE 2018

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Evidence for East African Orogenic Transpression and Cenozoic Tension in Neoproterozoic Basement Rocks, Northwestern Ethiopian Plateau

Abstract

Ethiopian geology is well known for the Main Ethiopian Rift and the 2 km high Ethiopian Plateau that resulted from mantle plume dynamics. Although geologic studies in Ethiopia has experienced significant recent growth, basement studies are lacking and have been overshadowed by rift and plateau research. Ethiopian basement formed from the East African Orogen (EAO) during Gondwana formation, with U-Pb dates ranging from 850 to 500 Ma. Northern Ethiopia is composed of the low grade, greenschist facies of the Arabian-Nubian Shield (ANS), while southern Ethiopia’s backbone is made of high grade crystalline basement from the Mozambique Belt (MB). The junction between the ANS and the MB occurs in south-central Ethiopia, and forms the structurally complex Southern Ethiopian Shield (SES). Plutonic activity during and following the EAO emplaced granitic intrusions in the SES, creating a basement of folded gneiss, deformed banded granitoids, and undeformed granite intrusions. Within the last 45 Ma, uplift and crustal rifting in Ethiopia signaled the emplacement of the African Superswell below Ethiopia.

The study area encompasses parts of the SES within the northwestern Ethiopian Plateau. Basement samples were collected in a 450 m vertical transect from the Didessa River canyon wall. Both compressional and tensional strain from the last 850 Ma is recorded in outcrop, hand samples, and thin sections. Outcrop analysis has revealed north striking folds related to E-W compression, and wavelengths ranging from 10s of cm to 10s of m. Some folds have a transpressional component, resulting from right lateral movement during compression. Feldspathic strained microcline and augens in thin section agree with compressional stress directions observed at larger scales, displaying both phi-type and sigma-type transpressional porphyroclasts. Extensional features in outcrop analysis include normal faults, while thin sections show tensional gauges in a roughly NW-SE direction associated with the tensile stress regime during Cenozoic rifting. Rock samples collected along the Didessa canyon wall are banded granites with varying levels of deformation. Thin section analysis indicates the granites are I-type, and were crystallized prior to the EAO due to preserved transpressional deformation. This study presents a preliminary analysis of regional tectonic shear, compressional, and tensional basement deformation, and forms the foundation for future basement research in Ethiopia.