Application of Dip Analysis to the Lakes Tanganyika and

Figure 3.3 shows the distribution of multifold seismic (MFS) profiles and 28- kHz data used in this study. It also locates 170 MFS profile intersections which provide control points for dip analysis within Lakes Tanganyika and Malawi. The 108 control points in Lake Malawi are more evenly distributed throughout the rift zone and more numerous than those from Lake Tanganyika. At each intersection, the apparent dip of the base synrift reflection along the strike of both profiles has been measured. Although this is a time-consuming and tedious task, this is the only way to determine the actual dips of rotated or dipping intra-rift blocks. As mentioned in Chapter 2, a procedure to compute the dips at particular horizons automatically could and should be developed to aid in balancing structural interpretations in the third dimension. The direction of the maximum or "true" dip of the acoustic basement block underlying the synrift section has been

Figure 3.3 Trackline map of seismic data used in this study. Heavy lines locate multifold seismic coverage. Lighter lines are single channel and 28-kHz traverses. Intersections of heavy lines locate points where dip m easurements and calculations plotted in Figure 3.4 were made. Numbered and dashed lines show location of data shown in Figures 2.5, 3.5, 3.6, 3.9, 3.10, 3.11 3.14 and 3.15. Interpretations of areas presented in Figures 3.7, 3.8 and 3.12 are outlined by boxes.

Figure 3.4 (Next page) Rose diagrams and areal distribution of directions of true dips of subsided blocks underlying multifold seismic profile intersections. These data divide the rift zone into dip domains within which true dip directions are consistent within 5°. "Likoma Data" refers to data points that lie within the interpreted area in Figure 3.7. See text for further discussion.

LAKE TANGANYIKA

LAKE MALAWI

(NYASA)

FIG. 3.8

FIG. 3.12

2 9 ° E 30 ° E

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Chapter 3 East African Rift System

computed from the apparent dips. The directions of the true dips of blocks at each profile intersection are plotted in Figure 3.4. The rose diagram labeled ALL DATA demonstrates that the dip data are regionally consistent and indicate a tectonic transport direction of NW-SE to WNW-ESE for the underlying blocks (Figure 3.4). These data provide strong evidence that NW-SE regional extension has created these rift zones.

The N-S elongation and long, linear shorelines in some sub-basins of these lakes have been used to argue for E-W extension (e.g. Morley 1988), which should produce E-W synrift dip directions. However, dips of tilted blocks in the Lakes Tanganyika and Malawi rift zones are predominantly NW-SE. When data from each rift zone are plotted separately, the Tanganyika data record a more NW-SE dip direction at N50°-60° W compared to the Malawi data, which are clustered at N60°- 70° W. This is somewhat surprising given the concentration of data points in the most N-S trending portion of the Lake Tanganyika rift zone.

In both rift zones there are concentrations of dip directions in the N70°-90° W range (Figure 3.4). For example, WNW-ESE trends occur at the very southern end of Lake Tanganyika and adjacent to the NNE-SSW trending peninsula at about 4°S. There are only two E-W block dips in Lake Tanganyika; they lie in the NW-SE trending section of the rift at about 5°15' S and lie along the same E-W profile which may indicate a navigational problem (e.g. a kink in the the ship track). In Lake Malawi data, the WNW-ESE trend is concentrated in the extreme north and south of the rift zone. An isolated E-W dip direction occurs at about 11°S. A NW trend (N40°-50°W) trend is recognizable in both data sets.

There is an obvious compartmentalization of both lakes into dip domains dominated by SE-ESE or NW-WNW dips in Figure 3.4. Computed dips in each com partm ent, or dip dom ain, are internally consistent w ithin 5°-10°. Consideration of some specific examples of computed dip information shows how the use of the dip analysis helps to clarify the interpretation of structures and tectonic processes from these data.

Line 815 trends NW-SE and appears to image a series of "balanceable" tilt blocks, as described in Chapter 2 (Figures 3.5 and 2.5). Closer examination of the largest block reveals subvertical disruptions of synrift reflectors that separate changes in the dip of the acoustic basement reflector. Computation of the true dip direction of the blocks underlying profile intersections yields tectonic transport directions of N38°W, N42°W and N43°W. If this were an orthogonally extending system normal faults should trend approximately N50°E with transfer faults

Scott 1994 Oblique Lithospheric Extension

trending parallel to the direction of tectonic transport, approximately N40°W. However, fault correlations very close to profile ties constrain the orientations of the imaged faults to N20°E and N30°W, requiring oblique slip on both fault sets to accommodate the transport direction of the blocks.

An example of a limitation of dip analysis is the tie of Lines 908 and 8311 from Lake Tanganyika (Figure 3.6). In this case, Line 908 which trends 080° has an apparent acoustic basement dip that is flat lying or possibly to the west directly under the tie. This reflection geometry appears to be due to internal deformation of the synrift section by faulting at D. Other possibilities include "reverse drag" or folding next to the normal fault, differential compaction or velocity anomalies near the fault plane. Line 8311, trending at 184° has a base synrift reflection dip of 12.5° S. These relationships yield a computed dip to the S or SSW. However, the overall dip of acoustic basement on Line 908 is definitely easterly and measures about 9° from fault E to fault C. Dip direction data so derived indicate a tectonic transport direction for this block of S41°E consistent with the overall predominance of this trend.

Note that in both the Lake Malawi and Lake Tanganyika rift zones, where two apparent dips are measurable, computed true dip directions consistently trend between N40°-70°W or S40°-70°E, regardless of mapped fault trends. The two examples presented above are from good ties with deep (base) synrift reflection correlations which are convincingly top of basement. Block dips are aligned within 5° of each other, even though these measurements are from opposite polarity composite half graben nearly 1000 km away from each other along the rift. Data points that are inconsistent with the NW-SE trend usually can be correlated with dubious navigation evident from misties at water bottom levels, areas with strong pre-existing anisotropies (see discussion below), or mismatched reflectors in areas of shallow acoustic basement.

Figure 3.5 Multifold seismic Line 815 and coincident 28-kHz Profile 41 from Lake Malawi. Profiles are displayed at the same horizontal scale. Ticks on bottom of profile 41 indicate where other multifold seismic data tie with this line and dip computations were made. Vertical exaggeration (V.E.) is computed with respect to the lakefloor. Refer to Figure 3.3 for location of profile.

Figure 3.6 (Next page) Migrated multifold seismic Lines 8311 and 908 from Lake Tanganyika. Inset interpretations indicate faults and acoustic basement (heavy lines) and the water/sedim ent interface (dashed lines). Letters refer to faults discussed in text. Vertical exaggeration (V.E.) is computed with respect to the lakefloor. Refer to Figure 3.3 for location of profiles.

L

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.E

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Chapter3 East African Rift System

This study supports the contention that quantitative dip analysis, both on the domain and individual control point levels, is a robust method of constraining regional kinematics that can help to constrain fault geometries in extensional terrains and provides an important tool for determining structural patterns where data are sparse. For instance, changes in dip domains (Figure 3.4) from westerly to easterly trends document the half graben polarity switches noted by previous interpreters (e.g. Reynolds 1984, Rosendahl et al. 1986, Rosendahl 1987, Sander 1986, Sander & Rosendahl 1989, Specht 1987, Specht «Sc Rosendahl 1989, Ebinger et al 1987, Versfelt 1988, Morley 1988, Morley et al. 1990). Dip domain analysis identifies where faults are required to decouple basement dips even where no specific data are available.

In the study area a NW-SE tectonic transport direction is indicated which suggests that the major bounding faults are oblique slip. Given the apparent nonrectilinear fault geometries in the Lakes Tanganyika and Malawi rift zones, the consistency of the dip direction determinations requires that the internal bulk deformation mechanism be able to accommodate variably trending faults. Two possibilities: the inclined shear and "rule-of-the-normal" model recently developed by Braun et al. (in press) have been identified in Chapter 2. In the next section, the results of the dip analysis are used to reinterpret the deformation patterns within portions of the Lakes Tanganyika and Malawi rift zones.

3.2.3 A Reinterpretation of the Structure of Parts of the Lakes Tanganyika and