Strain Localization and Migration During the Pulsed Lateral Propagation of the Shire Rift Zone, East Africa

We investigate the spatiotemporal patterns of strain accommodation during multiphase rift evolution in the Shire Rift Zone (SRZ), East Africa. The NW-trending SRZ records a transition from magma-rich rifting phases (Permian-Early Jurassic: RiftPhase 1 (RP1), and Late Jurassic-Cretaceous: Rift-Phase 2 (RP2)) to a magma-poor phase in the Cenozoic (ongoing: RiftPhase 3 (RP3)). Our observations show that although the rift border faults largely mimic the pre-rift basement metamorphic fabrics, the rift termination zones occur near crustal-scale rift-orthogonal basement shear zones (Sanangoe (SSZ) and the Lurio shear zones) during RP1-RP2. In RP3, the RP1-RP2 sub-basins were largely abandoned, and the rift axes migrated northeastward (rift-orthogonally) into the RP1-RP2 basin margin, and northwestward (strike-parallel) ahead of the RP2 rift-tip. The northwestern RP3 rift-axis side-steps across the SSZ, with a rotation of border faults across the shear zone and terminates further northwest at another regional-scale shear zone. We suggest that over the multiple pulses of tectonic extension and strain migration in the SRZ, pre-rift basement fabrics acted as: 1) zones of mechanical strength contrast that localized the large rift faults, and 2) mechanical ‘barriers’ that refracted and possibly, temporarily halted the propagation of the rift zone. Further, the cooled RP1-RP2 mafic dikes facilitated later-phase deformation in the form of border fault hard-linking transverse faults that exploited mechanical anisotropies within the dike clusters and served as mechanically-strong zones that arrested some of the RP3 fault-tips. Overall, we argue that during pulsed rift propagation, inherited strength anisotropies can serve as both strain-localizing, refracting, and transient strain-inhibiting tectonic structures.

1 This manuscript is a preprint and has been submitted for consideration in Tectonophysics. It is 1 our expectation that it will undergo peer review after which it will hopefully be accepted for 2 publication. Subsequent versions of this manuscript may differ due to peer review or the editorial 3 process. If accepted for publication, the final version will be available through the "Peer-reviewed 4 publication DOI" link on this preprint server. We hope you find this paper interesting and would 5 welcome your feedback on it. 6 Kindly contact Folarin Kolawole (folarin.kol@gmail.com or f.kolawole@columbia.edu) with any 7 feedback that you may have. 8  Fig. 1a), 299 Oesterlen and Blenkinsop (1994) inferred an NNE-regional extension direction. 300 The third phase of extension (RP3) is associated with the currently active East African 301 Rift System. In the region of the SRZ, RP3 is thought to have begun in the Late Tertiary 302 (Delvaux, 1989) or Quaternary (Castaing, 1991), and is associated with localized deposition  Choubert et al., 1988;Castaing, 1991). Although the RP1 and RP2 tectonic extension in the 306 SRZ were associated with widespread volcanism (magma-rich rifting), RP3 is non-magmatic. 307 The Quaternary Lower Shire depocenter (also known as "Shire Graben" or "Lower Shire  DATA AND METHODS 328 We integrated field observations from this study and previous studies, digital 329 elevation model (DEM) hillshade maps, intra-basinal borehole penetration logs, and 330 aeromagnetic data to generate an updated tectonic and structural framework for the SRZ, 331 allowing us to evaluate its multiphase rifting history.

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To create an updated geologic map of the SRZ, we compile published geological maps fault mapping mostly consists of buried faults and buried extensions of exposed faults 350 interpreted from filtered aeromagnetic maps, we also interpreted additional surface-351 breaking fault segments from topographic hillshade maps (see Figure S2). Below, we provide 352 the details of the field data collection, borehole data, and aeromagnetic datasets, and data 353 analysis techniques used in the study.  For the mapping of buried fault segments, dikes, and buried volcanic centers, and the 385 modeling of the depth-to-magnetic basement, we utilize two aeromagnetic datasets: a lower 386 resolution (2 km spatial resolution) regional grid covering the entire basin (both the Malawi part of the basement exposed in Figure 3f). (h) Compact linear bands of chaotic magnetic fabrics (i.e. exposed  Figure 2). In the Chiuta area, which is in Mozambique, we utilized the available 2 km-510 resolution regional aeromagnetic grid ( Figures S1a and 4b). Whereas, in the Lower Shire 511 Graben, located in Malawi, we utilized the original (unmerged) Malawi part of the legacy 512 regional grid ( Figure S1b). Our preference for the unmerged legacy Malawi aeromagnetic grid for source-depth estimation is due to its moderate resolution and suppression of high-514 frequency noise (e.g., related to intra-sedimentary mafic dikes).

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To perform an automatic calculation of depth-to-the top of the magnetic basement, where widespread RP3 Quaternary sedimentary cover is localized (Figures 2 and 5b). The Lupata Sub-basin hosts a major Mesozoic volcanic zone, the Lupata Volcanic Province (LVP).

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The LVP and the Salambidwe Igneous structure define the main intra-rift igneous zones, and 545 the Chilwa Alkaline Province (CAP) defines an off-rift syn-rift igneous province. 2), we also note that the northern and southern Lengwe Sub-basin show evidence of partial 579 reactivation (see partial Quaternary sediment cover in Figure 2). An understanding of the 580 first-order subsurface structure of the two prominent RP3 sub-basins is critical for 581 elucidating the multiphase evolution of the SRZ.  b and Table S1), and 693 aeromagnetic fabric patterns (Figures 4c-h). First, these datasets show that the RP1-RP2  Quaternary sediments as the unconsolidated sediments directly overlie the gneissic 701 basement ( Figure S3a; Table S1). The magnetic structure of the Lengwe Sub-basin where

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In the absence of high-resolution aeromagnetic data over the Txizita Horst, we map 748 basement fabrics in both the low-resolution SADC aeromagnetic grid (Figures 4b, 5a) and 749 the topographic relief map (red lines in Figure 9a). The frequency-azimuth distributions of  Figure S5b). 19 km diameters) distributed over a distance of 140 km (Figures 4b and 5a). These ring-944 shaped anomalies do not correspond to any distinct surface topographic feature. However, 945 the anomalies delineate a NW-trending belt along the rift axis.  (Figures 5c-e, 6a-b). However, we note that  hard-linked by the Chisumbi segment but soft-linked with the Camacho Fault (Figures 6a-b).

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The northwesternmost segment, the Thyolo Fault, side-steps basinward to the right and Quaternary sediments (Figures 10a-b). This buried Muona Fault continuation is truncated 1238 and separated from the exposed southeastern section of the fault by the NE-trending 1239 Chisumbi Fault which physically connects the exposed Muona Fault to the Thyolo Fault 1240 (Figures 10a-b). Similarly, the Chisumbi Fault defines the boundary between the Fault (Figures 5a and 6b) where the fault and its sub-basin developed within a zone of NW-1272 trending basement fabrics (Figures 4b and 5a). The alignment of the Chiuta Fault with the 1273 bounding basement fabrics (Figures 4b, 9a, and 9f) suggests that the nucleation of the fault 1274 also exploited the basement fabrics. Thus, we infer that the initial termination and stagnation of the RP1-RP2 SRZ rift tip 1294 was controlled by the SSZ which possibly represented a mechanical barrier to continued early-phase lateral propagation of the rift zone. Also, we note that although in RP3 tectonic 1296 strain migrated further northwest of the SSZ, represented by the Chiuta Sub-basin, the RP3 1297 sub-basin also terminates near another zone of ENE-trending basement fabrics with a 1298 plunging ductile shear zone (Techigoma Shear Zone, TSZ; Figures 4b, 5a, and 5a inset).

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Furthermore, we suggest that the establishment of the Chiuta Sub-basin was facilitated by 1300 strain localization within an isolated crustal block of NW-trending basement fabrics that is 1301 located ahead, but proximal to and colinear with the earlier established RP1-RP2 rift zone.

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To the southwest, the RP1-RP2 border fault system either terminates at the NE-trending  In addition to the larger scale influence of intra-basement shear zones on rift 1343 termination, we also note that the cooled early-phase (RP1) magmatic plumbing structures This may also imply that at this initial stage of development, the maximum lengths of the fault segments are delimited by the inherited cooled dikes.

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In summary, during the pulsed or episodic propagation of a rift segment, inherited