Rift initiation within cold, thick, strong lithosphere and the evolving linkage to form a contiguous plate boundary remains debated in part owing to the lack of time–space constraints on kinematics of basement‐involved faults. Different rift sectors initiate diachronously and may eventually link to produce a jigsaw spatial pattern, as in the East African rift, and along the Atlantic Ocean margins. The space–time distribution of earthquakes illuminates the geometry and kinematics of fault zones within the crystalline crust, as well as areas with pressurized magma bodies. We use seismicity and Global Navigation System Satellites (GNSS) data from the Turkana Rift Array Investigating Lithospheric Structure (TRAILS) project in East Africa and a new digital compilation of faults and eruptive centres to evaluate models for the kinematic linkage of two initially separate rift sectors: the Main Ethiopian Rift (MER) and the Eastern rift (ER). The ca. 300 km wide zone of linkage includes failed basins and linkage zones; seismicity outlines active structures. Models of GNSS data indicate that the ca. 250 km‐wide zone of seismically active en echelon basins north of the Turkana Depression is a zone, or block, of distributed strain with small counterclockwise rotation that serves to connect the Main Ethiopian and Eastern rifts. Its western boundary is poorly defined owing to data gaps in South Sudan. Strain across the northern and southern boundaries of this block, and an ca. 50 km‐wide kink in the southern Turkana rift is accommodated by en echelon normal faults linked by short strike‐slip faults in crystalline basement, and relay ramps at the surface. Short segments of obliquely oriented basement structures facilitate across‐rift linkage of faults, but basement shear zones and Mesozoic rift faults are not actively straining. This configuration has existed for at least 2–5 My without the development of localized shear zones or transform faults, documenting the importance of distributed deformation in continental rift tectonics.
Strongly lineated terrain outside of Iceland's active plate boundary zones is created by faults and dikes aligned with the rift zones where they formed, similar to the spreading fabric defined by abyssal hills generated at mid‐ocean ridge spreading centers. As expected, rift‐parallel normal faults and fissures dominate in the active rift zones, but in older crust to the east and west, faults with strike‐slip and oblique‐slip displacements dominate. Some areas have widespread, small‐scale, strike‐slip, and oblique‐slip faults, while others have more widely spaced, larger, strike‐slip fault zones. In most cases, the strike‐slip and oblique‐slip faults strike subparallel to nearby older dikes and normal faults assumed to indicate the orientation of the rift zones where they formed. Strike‐slip displacements overprinting normal faults and along dike margins suggest reactivation of spreading‐related zones of weakness. More complicated fault geometries and kinematics occur near the oblique rifts and the major transform fault zones. The sense of movement on the strike‐slip and oblique‐slip faults is broadly systematic with respect to the active Northern and Eastern Rift Zones supporting the interpretation that they are the result of crustal block rotations on either side of rift zones that propagate to the north and south away from the center of the Iceland hot spot. Similar fault kinematics may occur along mid‐ocean ridges and other magmatic rifts where rift propagation occurs on a range of scales.
more » « less- NSF-PAR ID:
- 10448950
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Tectonics
- Volume:
- 37
- Issue:
- 12
- ISSN:
- 0278-7407
- Page Range / eLocation ID:
- p. 4567-4594
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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