The Mount Hay block is a ~12-km-thick, deep continental crustal section exposed in the Arunta inlier in central Australia. The ~4-km-wide, granulite-facies (770–776 ± 38 °C) Capricorn ridge shear zone cross-cuts the dominant granulite-facies fabric of the Mount Hay block. In its present geometry, the Capricorn ridge shear zone contains a steeply south-southeast-dipping foliation, steeply east-southeast-plunging lineation, and south-side-up shear-sense indicators. When post-granulite-facies tilting is removed, the shear zone restores to a shallowly to moderately (30–50°) dipping, normal shear zone in which the lineation is oblique to the inferred Proterozoic plate boundary, suggesting oblique divergence. The field observations and reconstruction indicate that strain can be localized in the high-temperature, deep-crustal roots of extensional fault systems. This geometry of a discrete, moderately dipping, deep-crustal shear zone is consistent with simple-shear conceptual models of crustal extension.

Abstract We review previous models for the Paleogene tectonic evolution of the Arabian and Eastern Somali basins and present a model based on a new compilation of magnetic and gravity data. Using plate reconstructions, we derive a self-consistent set of isochrons for Chron 27 to Chron 21 (61-46 Ma). The new isochrons account for the development of successive ridge propagation events along the Carlsberg Ridge, leading to an important spreading asymmetry between the conjugate basins. Our model predicts the growth of the outer and inner pseudo-faults associated with the ridge propagation events. The location of outer pseudo-faults appears to remain very stable despite a drastic change in the direction of ridge propagation before Chron 24 (c. 54 Ma). The motion of the Indian plate relative to the Somalian plate is stable in direction through Paleogene time; spreading velocities decrease from 6 to 3 cm a -1 . Our reconstructions also confirm that the Arabia-India plate boundary was located west of the Owen Ridge along the Oman margin during Paleogene time; some compression is predicted at about Chron 21 (47 Ma) between the Indian and Arabian plates.

Abstract The Itremo region of central Madagascar has an importance in the evolution of the East African Orogen (EAO) that belies its size. Unusually for the southern EAO (Mozambique Belt), it is made up of low-grade metasedimentary rocks and therefore preserves an almost unique window into upper crustal deformation during this key period of the Gondwana supercontinent cycle. In this paper new field mapping of three linked regions in the Itremo Sheet and in the upper part of the underlying mid-crustal Antananarivo Block are presented. From these a complete structural section through the eastern Itremo Sheet is produced and the complex deformation record preserved there is then discussed. An early deformation (D1) consists of 10 km scale recumbent isoclinal folds that predate intrusion of a c. 780–800 ma igneous suite. Metamorphic aureoles around these plutonic bodies overprint D1-related fabrics. Local deformation accompanies intrusion of the c. 780–800 Ma, suite (D2). Extensive E–W contractional deformation occurs between 780 and c. 570 Ma, that is here amalgamated as a composite D3 event, which includes thrusts and at least two phases of upright folds. Post-551 Ma, normal shearing (D4) marks the boundary between the Itremo Sheet and the underlying Antananarivo Block (the Betsileo Shear Zone), and may have also been responsible for formation of the Saronara Shear Zone. Finally, E–W open folding and dextral shear zone development marks a late N–S contractional event that is interpreted as a far-field response to collision between the northern Bemarivo Belt and central Madagascar.