Quantitative Constraints on Faulting and Fault Slip Rates in the Northern Main Ethiopian Rift

David J Sanderson,Bekele Abebe,Jonathan M Bull,Casey W Nixon,Thomas M Gernon,Melanie Siegburg,Atalay Ayele,Derek Keir,Giacomo Corti

doi:10.1029/2019tc006046

Abstract

AbstractThe Boset magmatic segment (BMS) of the northern Main Ethiopian Rift (MER) is an ideal natural laboratory to investigate the kinematics, interaction, and rates of activity within a fault network in a magma‐rich rift. In this paper we take advantage of the availability of (1) high‐resolution remote sensing data (LiDAR, ASTER); (2) absolute age chronology on offset reference surfaces; and (3) well‐exposed active normal fault arrays to place new constraints on rift kinematics and strain distribution, and to quantify the architecture and fault slip rates at different temporal scales within a magmatic segment. We found that the rift border faults strike approximately NE, while the younger faults in the rift segments strike NNE. Analyses of geometric rift parameters show that the axial active part of the rift is transtensional with an increase of the shear component northward. The fault displacement analyses and displacement:length ratios increase toward the segment tips, suggesting a significant contribution of fault growth by linkage. In contrast, magmatism is focused on the segment center and localized to a narrow zone. Estimated fault slip rates vary, with rates of up to ~0.37 mm/year in ~0.3 Ma old rift floor deposits, whereas higher rates of up to ~4.4 mm/year are observed for faults cutting through ~6 Ka lavas. The difference in slip rates indicates short‐term variability or a very active recent episode compared to long‐term low average slip rates.

Highlights

A quantitative understanding of fault networks and their kinematics including growth, reactivation, and interaction in magma‐assisted continental rifts has been hampered by a lack of chronological constraints, but studies worldwide provide a qualitative framework
In this paper we build on previous studies from Iceland (e.g., Bull et al, 2003; Grant & Kattenhorn, 2004; Tentler, 2005; Villemin & Bergerat, 2013); Hawaii (e.g., Bubeck et al, 2018; Holland et al, 2006; Kaven & Martel, 2007; Martel & Langley, 2006); the Taupo Rift Zone, New Zealand (e.g., Rowland et al, 2010; Rowland & Sibson, 2001, 2004); and the East African Rift (EAR) (Acocella et al, 2011; Casey et al, 2006; Corti, 2009; Rowland et al, 2007) by characterizing the structural style of the active Boset magmatic segment (BMS) of the northern Main Ethiopian Rift (MER)
We observe a small change in the obliquity of ~7° over several magmatic segments (Figures 2 and 12), suggesting the northern MER is associated with transtensional kinematics with an increasing shear component northwards (Figures 2 and 4)

Summary

Introduction

A quantitative understanding of fault networks and their kinematics including growth, reactivation, and interaction in magma‐assisted continental rifts has been hampered by a lack of chronological constraints, but studies worldwide provide a qualitative framework. Previous studies have mainly focused on characterizing large rift‐scale distribution of faults and their spatial relationship to volcanism (e.g., Abebe, Coltorti, & Pizzi, 2005; Agostini et al, 2011; Casey et al, 2006; Corti, 2009; Corti, Sani, et al, 2013; Ebinger & Casey, 2001; Wolfenden et al, 2004), with few placing quantitative constraints on fault displacement, geometries, and slip rates (Abebe, Manetti, et al, 2005; Casey et al, 2006; Kurz et al, 2007; Pizzi et al, 2006; Williams et al, 2004)

Objectives

Discussion

Conclusion