Australia: The Land Where Time Began

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Narrow Rifts - General Characteristics

East Africa has some of the best examples of narrow intracontinental rifts that are tectonically active. The Nubian Plate and the Somalia Plate, to the southwest of the Afar triple junction, are separating at about 6-7 mm/year (Fernandes et al., 2004). The motion of the diverging plates has resulted in extensional deformation, that is localised into a series of discrete rift segments that vary in age, such as the Western Rift, the Eastern Rift, the Main Ethiopian Rift and the Afar Depression. Characteristics common to rifts forming in relatively strong, cool continental lithosphere.

Normal faults flanking Asymmetric rift basins
Regional tensional stress with shallow seismicity
Local thinning of the crust, modified by magmatic activity
Upper mantle that is of low velocity and high density with high heat flow

Normal faults flanking Asymmetric rift basins. Continental rifts, associated with sedimentary basin formation, are bounded by normal faults. Asymmetric half graben morphology is displayed by most rifts that are tectonically active, the majority of the strain being accommodated along border faults bounding the deep side of the basins. Segmentation of the rift valley may result from half graben polarity changes occurring along the rift axis strike. Within each individual basin, the longest faults are typically the border faults, as seen in plan view. Uplift of the flanks of the rift, producing an asymmetric topographic profile that is characteristic, results from the combination of flexural isostatic compensation of the lithosphere, and slip on these faults. Faults may occur on the lower relief side of the basin, exhibiting a monocline dipping towards the centre of the basin. Sedimentary and volcanic units, thickening towards the fault plane are produced, deposition occurring during slip on the bounding normal fault. Control of the timing of normal faulting and volcanism is provided by the syn-rift unit age, as well as units pre-dating the rifting. Towards the tips of the border faults, where they interact with other faults bounding adjacent basins, displacements decrease, as seen in plan view. Faults within these transfer zones may accommodate horizontal (including strike-slip) and vertical displacements between adjacent basins.

Regional tensional stress with shallow seismicity. Earthquakes are mostly confined to depths of 12-15 km in the uppermost levels of the crust beneath most continental rift axes, the seismogenic layer that is defined is thinner than other regions of continental crust, earthquakes occurring at depths of 30 km or more away from the axis of the rift. The implication of these patterns is that the crust is weakened  locally by rifting and thinning, which affects its mechanical behaviour.

The majority of large earthquakes in Ethiopia occur between the Afar Triangle and the Red Sea, as indicated by the seismicity record from 1960 to 2005. It has been found that across the Main Ethiopian Rift more than 50 % of extension during the period from 1960 to 2005 has been accommodated aseismically, based on seismic moment release analysis (Hofetetter & Beth, 2003). Combinations of movement types, normal, oblique & strike-slip, has been shown by earthquakes. To the north of the Afar Depression the horizontal component of most axes of minimum compressive stress strike to the north and northeast at high angles to the trend of the segments.

The seismicity patterns within the Ethiopian Rift and its flanks have been determined by using almost 2000 earthquakes (Keir et al., 2006). In a series of right-stepping magmatism zones, each 20 km wide, inside the rift, earthquake clusters occur parallel to faults and volcanic centres. Of the total extensional strain, up to 80 % is localised within these magmatic segments (Bilham et al., 1999; Ebinger & Casey, 2001). Major border faults are the sites of the largest earthquakes that typically occur along or near them, though according to seismic data the border faults are mostly aseismic. Earthquakes are concentrated around volcanoes and fissures to depths of less than 14 km, which the authors suggest, probably reflects the movement of magma in dikes. Seismic activity in the flanks of the rift has been suggested to possibly reflect crustal flexure, as well as movement along faults. As determined from earthquake focal mechanisms, the orientation of minimum compressive stress is approximately horizontal, parallel to an azimuth of 103o. As in Afar, the direction of stress has bee found to be consistent with determinations of the direction of extension from tension fractures in young lava, less than 7000 years old, geodetic measurements, and global plate kinematic data.

Local thinning of the crust, modified by magmatic activity. A characteristic of continental rifts has been found to be crustal thinning beneath the axis of rifts, based on geophysical data. As with fault geometries in rift basins, the thickness of crust is variable, and possibly asymmetric. Magmatic intrusions can result in the occurrence of thick crust beneath the flanks of a rift, which indicates that the thinning of the crust is mostly a local phenomenon (Mackenzie et al., 2005; Tiberi et al., 2005). Inherited, pre-rift, differences in the structure may be reflected in the variations in the thickness of the crust.

The crustal velocity structure beneath the Adama Rift Basin, in the northern section of the Main Ethiopian Rift, was determined by the use of seismic controlled-source reflection and seismic refraction studies (Macke nzie et al., 2005). An asymmetric crustal structure, in which crustal thinning occurred a bit west of the rift valley, was shown by the velocity model they constructed. Within the rift valley a thin layer with low velocity, 3.3 km/s, occurs that thickens towards the east from 1 to 2.5 km. Below the low velocity layer, and extending along the length of the profile, there is a sequence of sedimentary and volcanic rock of intermediate velocity, that is 2-5 km thick. Apart from a narrow region, that is 20-30 kilometres wide, in the upper crust, beneath the central part of the rift valley, where the Pn velocities are more than 6.5 km/s, 5-10 % higher than those outside the rift, normal crustal velocities, P=6.0-6.8 km/s occur from 30-35 km deep. It has been suggested that mafic intrusions, associated with centres of magmatism, are probably reflected by these differences. At depths of 20-25 km, and a Moho depth of 30 km, the presence of an intracrustal reflector indicates thinning of the crust beneath the axis of the rift. Crust that is about 45 km thick, displaying a high velocity lower crustal layer that is about 15 km thick, and with a velocity of 7.4 km/s, lies beneath the western flank of the rift. On the eastern side of the rift there is no equivalent layer beneath the flank, the crust being about 35 km thick. The high velocity lower crustal layer that has been found beneath the western flank has been interpreted to be underplated material that is associated with pre-rift flood basalt from the Oligocene, and with magmatic activity that is possibly more recent. The presence of igneous intrusions directly beneath the rift valley is suggested by variations of intracruastal seismic reflectivity.

Additional evidence for the permanent modification of the crustal structure of rift zones by magmatism occurring before as well as after rifting, has been provided by gravity data. Long wavelength (>1000 km) negative Bouguer gravity anomalies coincide with major topographic uplifts that are about 2 km high, the Ethiopian Plateau and the Kenya Dome, in Ethiopia and Kenya, part of the East African Plateau. On the Ethiopian Plateau the highest parts rise to a height of more than 3 km. A high volume continental flood basalt eruption occurred between 45 and 22 Ma, resulting in the great heights reached by parts of the Plateau, the majority of this volcanism coinciding with the opening of the Red Sea and the Gulf of Aden about 34 ma (Wolfenden et al., 2005). The presence of upper mantle that is of anomalously low density and elevated geotherms are reflected by the negative gravity anomalies (Tessema & Antoine, 2004).

Upper mantle that is of low velocity and high density with high heat flow. In many rift basins temperature gradients of 50-100o C/km, higher than those in the adjacent rift flanks or the nearby cratons, are suggested by heat flow measurements that average 70-90 mWm-2 and low seismic velocities. In places such as East Africa, where the asthenosphere has been found to be anomalously hot, domed uplift and pervasive volcanism are the result. Among rifts the temperature and volcanic activity there is a high degree of variability. An example is the Baikal Rift that is much cooler, displaying low regional heat flow, 40-60 mWm-2 (Lysack, 1992) and no volcanic activity. Elevated temperatures are suggested by relatively low Pn wave velocities, 7.7 km/s, beneath the Adama Rift Basin in Ethiopia in the upper mantle (Mackenzie et al., 2005). Upper mantle Pn wave velocities elsewhere are 8.0-8.1 km/s, a velocity expected for stable areas with normal heat flow. Beneath the rift, the low velocity zone, is indicated to be of tabular form, about 75 km wide and extending to 200-250 km deep, by tomographic inversion of P- and S-wave data (Bastow et al., 2005). In the upper 100 km the zone is segmented and offset away from the axis of the rift, becoming more central about the axis of the rift at deeper levels. The low velocity anomaly broadens laterally below 100 km in the northern section of the Ethiopian Rift that is more highly extended, possibly being connected to deeper structures of low velocity beneath the Afar Depression. During the Pleistocene to Recent the Main Ethiopian Rift propagated toward older spreading centres, the Red Sea and the Gulf of Aden, with which the broadening of the low velocity zone is consistent.

The zones of low velocity beneath rifts, in addition to high temperatures, have been suggested to reflect partial melting. The alignment of partial melt in steep dikes in the upper 70-90 km of the lithosphere or the lattice preferred orientation of olivine in the asthenosphere, as the lateral flow of hot material into the rift zone, is suggested by shear wave splitting and delay times of teleseismic waves that travel beneath the Kenya Rift (Ayele et al., 2004) and the northern Ethiopian Rift  (Kendall et al., 2005). The upper mantle is characterised by material of low velocity, low denisty and temperatures that are anomalously high, as indicated by observations.

See Source 1 for more detailed information on all aspects of plate tectonics

Sources & Further reading

  1. Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.
Last updated: 19/05/2011


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