Abstract

Studies that attempt to simulate continental rifting and subsequent breakup require detailed knowledge of crustal stresses, however observational constraints from continental rifts are lacking. In addition, a knowledge of the stress field around active volcanoes can be used to detect sub-surface changes to the volcanic system. Here we use shear wave splitting to measure the seismic anisotropy of the crust in Northern Afar, a region of active, magma-rich continental breakup. We combine shear wave splitting tomography with modelling of gravitational and magmatic induced stresses to propose a model for crustal stress and strain across the rift. Results show that at the Ethiopian Plateau, seismic anisotropy is consistently oriented N–S. Seismic anisotropy within the rift is generally oriented NNW–SSE, with the exception of regions north and south of the Danakil Depression where seismic anisotropy is rift-perpendicular. These results suggest that the crust at the rift axis is characterized by rift-aligned structures and melt inclusions, consistent with a focusing of tectonic extension at the rift axis. In contrast, we show that at regions within the rift where extension rate is minimal the seismic anisotropy is best explained by the gravitationally induced stress field originating from variations in crustal thickness. Seismic anisotropy away from the rift is controlled by a combination of inherited crustal structures and gravitationally induced extension whereas at the Dabbahu region we show that the stress field changes orientation in response to magmatic intrusions. Our proposed model provides a benchmark of crustal stress in Northern Afar which will aid the monitoring of volcanic hazard. In addition we show that gravitational forces play a key role in measurements of seismic anisotropy, and must be considered in future studies. We demonstrate that during the final stages of continental rifting the stress field at the rift axis is primarily controlled by tectonic extension, but that gravitational forces and magmatic intrusions can play a key role in the orientation of the stress field.

Highlights

  • The transition from continental rifting to seafloor spreading is a fundamental stage of plate tectonics; yet the mechanisms behind it have remained poorly constrained

  • Seismic anisotropy north of the Danakil Depression is oriented perpendicular to the rift, NE–SW

  • We present the first detailed study of the seismic anisotropy of the Danakil Depression of Northern Afar through automated shear wave splitting, shear wave splitting tomography and forward modelling of both the gravitational and magmatically induced stress fields

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Summary

Introduction

The transition from continental rifting to seafloor spreading is a fundamental stage of plate tectonics; yet the mechanisms behind it have remained poorly constrained. Constructions of rifted passive margins (e.g., Pindell et al, 2014) Such studies require an understanding of how extension and stress are oriented across a continental rift. The orientation of extension from rift margin to rift axis in the final stages of continental breakup remains poorly constrained. An understanding of extension and the associated stress field during continental rifting is crucial for volcanic hazard assessments. In this study we use shear-wave splitting of local earthquakes to measure strain and interpret the stress field at a late stage continental rift. This allows us to gain a deeper understanding of continental rifting, providing observational constraints for future studies of rifting and volcanic hazards

Tectonic background
Sources of crustal seismic anisotropy
Seismic data
Shear wave splitting
Shear wave splitting tomography
Gravitational modelling
Results and discussion
The western rift flank and margin
The Danakil depression
The Dabbahu region
Summary
Conclusions
Full Text
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