Nature of a structural discontinuity between lower and upper crustal domains: constraints from field and anisotropy of magnetic susceptibility studies in central-eastern Eritrea

In the Neoproterozoic East African Orogen
(EAO) of Eritrea, lower to middle crustal high-grade
metamorphic rocks are juxtaposed against low-grade
upper crustal rocks along diffuse tectonic contact
zones or discontinuities. In the central eastern part of
Eritrea, such a tectonic zone is exposed as a low-angle
shear zone separating two distinct high- and low-grade
domains, the Ghedem and Bizen, respectively.
Integrated field, microfabric, and anisotropy of
magnetic susceptibility (AMS) studies show that this
low-angle shear zone formed during late deformation,
D2, with top-to-the-E/SE sense of motion. The
hanging wall upper crustal volcanosedimentary
schists are mainly paramagnetic and the footwall
middle crustal mylonitized orthogneisses are mainly
ferrimagnetic. Magnetic fabric studies revealed a good
agreement between metamorphic/mylonitic and
magnetic foliations (Kmin) and helped to explain
fabric development in the shear zone. The magnetic
lineations (Kmax) reflect stretching lineations where
stretched mineral aggregates dominate fine-grained
mylonitic matrices and intersection lineations where
microstructural studies revealed two fabric elements.
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AMS directional plots indicate that the orientations of
the magnetic lineation and of the pole to the magnetic
foliation vary systematically across the shear zone.
While Kmax axes form two broad maxima oriented
approximately N–S and E–W, the Kmin axes change
from subhorizontal, generally westward inclination in
the west to moderate to steep inclination in the
direction of tectonic movement to the east. Because
there is a systematic change in inclination of Kmin for
individual samples, all samples together form a fairly
well defined cluster distribution. The distribution of
Kmin in combination with the E–W scattered plot
of the Kmax is in accordance with the E/SE flow of
mylonites over exhumed Damas core complex in the
late Neoproterozoic. During the Cenozoic, the Red Sea
rift-related detachments exploited the late orogenic
shear zone, indicating that the discontinuities between
ductile middle and brittle upper crustal layers in the
region are reactivated low-angle shear zones and
possible sites of core complexes.