The Turkana magmatic rift (Northern Kenya) initiated at 45 Ma as one of the nucleation zones of rifting in the East African Rift. It forms an anomalously broad-rifted zone (c. 200 km) striking with a north-south trend and lying within a NW-SE topographic depression, floored on both sides of the Turkana area by Cretaceous rifts in the Sudan and Anza plains. From a compilation of available data, combined with newly acquired remote sensing and DEM dataset, we propose a five-stage tectono-magmatic model for the Turkana rift evolution (45–23 Ma; 23–15 Ma; 15–6 Ma; 6–2.6 Ma and 2.6 Ma-Present). The corresponding ‘restored’ maps clearly show the changing spatial distribution of magmatism and fault/basin network with time, hence supplying some clues about dynamics of continental extension. First-order basement-rooted transverse faults zones are identified and their influence on nucleation and propagation of strain is demonstrated, whereas the role of magmatic ‘soft-spots’ as concentrating strain is minimized. Blocking of deformation as well as rift jump and lateral transfer of strain are discussed in relation to various types of fault interaction (dip direction, strikes and acute/obtuse angle of the intersecting faults). The causal links between rift nucleation ‘cells’ and inherited transverse weakness zones in the Turkana rift might also exist elsewhere along the eastern branch of the East African Rift, hence suggesting a complex and discontinuous mode of rift propagation.

No abstract

No abstract

The Panoche Giant Injection Complex (PGIC; California) constitutes the most complete sandstone intrusion network yet described, and is an excellent analogue for subsurface hydrocarbon reservoirs modified by sand remobilisation. Sandstone dykes and sills were intruded during the Late Palaeocene into slope mudstones of the Great Valley forearc basin, and are exposed for more than 300 km2. The PGIC consists of dykes and sills and represents upwards infilling of natural hydraulic fractures sourced from highly overpressured Cretaceous sand bodies. Over 1300 orientation measurements show that dykes are almost randomly oriented with only a slight orientation bias trending NE–SW, N–S or NW–SE, suggesting either a horizontally isotropic state of stress during intrusion or modification of stress by newly‐formed fractures that override the remote stress. Dykes are segmented in a pattern consistent with radial propagation with fingering towards tips similar to that observed for other mixed mode fractures. Kinematic indicators reveal there was no systematic sense of opening for the intrusions. This is interpreted as the result of short‐range mechanical interactions. Cross‐cutting relationships between injections imply a diachronous timing and a fluid pressure in the source units that was in excess of the lithostatic load. Finally we document a suite of minor structures within the host section that allowed the strain of the forcefully intruded sand to be accommodated.