Since ‘regional’ and ‘local’ scales can be different for different elec- tromagnetic sounding periods, it is clear that whilst the decomposi- tion hypothesis may fail for some period ranges of a dataset, it may yet be satisfied for more limited period ranges. Dynamic processes related to plate tectonics continually modify the Earth’s structure.
Whilst some plates break apart, others collide and merge; plates alter their directions of motions; stress fields rotate. In this balance of destruction and renewal, remnants of past tectonic events are often preserved, giving rise to tectonic environments in which structural lineaments trend in more than one direction. One such environment exists in southern Kenya, where a major NW–SE trending suture
102 Dimensionality and distortion
zone between the Archaean Nyanza craton and the Mozambique Mobile Belt coexists with the N–S trending Rift Valley.
In 1995, MT data were acquired along an E–W profile cross- ing the N–S trending Kenya rift (Figure 5.13 (a)). Assuming a 2-D electromagnetic structure controlled by the rifting process, a profile of sites perpendicular to the rift seemed to be an appropriate experi- mental configuration. Contrary to expectation, the electromagnetic strike was found to be both period- and site-dependent. Close to the western boundary of the rift marked by a 1500-m escarpment, a N–S strike was recovered, but away from the escarpment, on the western flank of the rift, an approximately NW–SE strike was detected, followed by a second N–S trending boundary (delineated by the Oloololo escarpment). Close to the eastern margin of the rift, both the N–S strike of the rift and an approximately NW–SE strike influence the data (Figure 5.13 (b)). Both N–S and NW–SE striking structures are also reflected strongly in the induction arrows (Figure 5.13 (c)).
The electromagnetic strike hinted at structural trends that were not detectable from seismic refraction or gravity surveys previously
(a) Figure 5.13(a) Geological map
of East Africa showing locations of MT sites (indicated by stars) collected in 1995 as part of the Kenya Rift International Seismic Project (KRISP).
(b) Pseudosection showing variations in decomposition angle (Groom and Bailey, 1989) as a function of period and distance along a profile crossing the southernmost part of the Kenya rift. (c) Real (solid lines) and imaginary (dotted lines) induction arrows at MT sites shown in (a) for discrete periods between 64 s and 4096 s (Simpson, 2000).
5.6 How many strike directions are there? 103
carried out along the same profiles, but posed a problem (analogous to side-swipe in seismic techniques) with regard to 2-D modelling:
because MT provides a volume sounding, a 2-D model can contain artefacts at depth that are actually due to structure that is laterally displaced from the profile, unless the data are rotated into an appropriate co-ordinate frame that decouplesE- andB-polarisations.
In this case study, the profile runs sub-parallel to one of two dominant regional strikes, making adequate decoupling and subse- quent 2-D modelling of the data untenable. Instead, 3-D forward modelling was used to demonstrate that the NW–SE electromag- netic strike probably relates to shear zones delineating the suture (b)
0 50 100 150 200 250 300 350 400
Distance along profile (km) 4
3 2 1 0 –1 –2
0 15 30 45 60 75 MAG
Rift
Decomposition angle ( )ο
logperiod (s)10[]
(c)
Figure 5.13(cont.).
104 Dimensionality and distortion
zone between the Mozambique Mobile Belt and Archaean Nyanza craton that collided during the Proterozoic (Simpson, 2000).
A model consisting of the near-surface overburden of the N–S striking rift, and more deeply penetrating NW–SE conductive linea- ments, could explain not only the induction arrows, apparent resis- tivities and impedance phases, but also the period and site dependence of phase-sensitive strike and skew (Figure 5.14). Site MAG (Magadi), within the rift, exhibits an anomalous strike that is neither N–S nor NW–SE. Comparison of the measured and model- led decomposition angles (Figure 5.14) reveals the anomalous strike to be a virtual one, arising from coupling between the N–S and NW–SE strikes. At a neighbouring site (SIN) closer to the eastern flank of the rift, N–S and NW–SE strikes are retrieved in different period ranges.8
The Kenya case study demonstrates that appropriate applica- tion of the impedance tensor decomposition hypothesis can facili- tate decoupling of structures lying laterally closer, or shallower, on the one hand, and laterally further away, or deeper, on the other.
Therefore, we shouldn’t be perturbed when we see a period- or location-dependent strike, or try to average out the differences that may be apparent. MT data can potentially constrain direction- ality and its source better than seismological data – something that should be viewed as an asset, rather than as an inconvenience of the MT method. However, we have to exercise caution, because, in certain circumstances, the electromagnetic strike retrieved by imped- ance tensor decomposition may be a virtual strike only, as in the case of site MAG.
101 102 103 104 101011
Period (s) Period (s)
102 103 104
101 102 103 104
Period (s)
Modelled Measured (b)
Decomposition angle()°
90 45 0
Skew
0.3 0.2 0.1 Skew 0.0
0.3 0.2 0.1 0.0 (a)
90 45
Decomposition angle()° 0 Modelled
Measured
101 102 103 104
Figure 5.14Measured and modelled decomposition angles (phase-sensitive
‘strikes’8) and phase-sensitive skews at (a) an MT site (MAG) close to the axis of the Kenya rift (Figure 5.13); and (b) an MT site (SIN) close to the eastern boundary of the rift.
(Redrawn from Simpson, 2000.)
8Inverted commas around ‘strike’ are in recognition of the fact that, strictly speaking, a strike belongs to a 2-D structure, whereas the period dependence of the decomposition angle belies the idea of two-dimensionality.
5.6 How many strike directions are there? 105