Strike-slip overprinting of initial co-axial shortening within

14 Submarine landslides (slides) are some of the most voluminous sediment gravity-flows on Earth 15 and they dominate the stratigraphic record of many sedimentary basins. Their general kinematics 16 and internal structure are relatively well-understood. However, how slides increase in volume and 17 internally deform as they evolve, and how these processes relate, in time and space, to the growth 18 of their basal (shear) zone, are poorly understood. We here use three high-resolution 3D seismic 19 surveys from the Angoche Basin, offshore Mozambique to map strain within a shallowly buried, 20 large, and thus seismically well-imaged slide (c. 530 km). We document several key kinematic 21 indicators, including broadly NW-trending lateral margins and longitudinal shears bounding and 22 within the slide body, respectively, and broadly NE-trending symmetric pop-up blocks in the slide 23 toe. Approximately 7 km downdip of the slide toe wall, thrusts and related folds also occur within 24 otherwise undeformed slope material, with thrusts detaching downwards onto the downslope 25 continuation of the basal shear zone underlying the slide body. Based on the style, trend, and 26 distribution of these features, and their cross-cutting relationships, we propose an emplacement 27 model involving two distinct phases of deformation: (i) bulk shortening, parallel to the overall SE28 directed emplacement direction, with contractional shear strains reaching c. 8%; and (ii) the 29 development of broadly emplacement direction-parallel shear zones that offset the earlier-formed 30 shortening structures. We infer that the contractional strains basinward of the slide body formed 31 due to cryptic basinward propagation of the basal shear zone ahead of and to accommodate updip 32 sliding and shortening associated with, the entire slide mass. Our study demonstrates the value of 33 using 3D seismic reflection data to reveal slide emplacement kinematics, especially the 34 multiphase, non-coaxial nature of deformation, and the dynamics of basal shear zone growth. 35

We begin by providing a general description of the studied slide using the larger, time-migrated 172 seismic dataset; in contrast to the depth-migrated seismic dataset, which provides good imaging 173 of the slides toe region, the time-migrated images the slides headwall and lateral margins, and 174 the full range of its contained seismic facies. scarp zone that defines the up-dip limit of the slide, and downslope into a frontal ramp that defines 184 its downdip limit. The basal shear surface steps up through stratigraphy to define the slides lateral 185 margins; beneath the slide, the basal shear surface comprises several ramps (Fig. 4). The basal 186 shear surface is generally defined by a relatively continuous, negative polarity (i.e., trough) 187 reflection, although it becomes locally discontinuous near ramps, where it is discordant to 188 underlying stratigraphy. 189 The slide terminates across-strike against a lateral margin (Figs 4b-c, 5) that trends parallel to the 190 gross, SE-directed emplacement direction. The lateral margin is easy to trace downslope, defined 191 by a clear, straight, steep, continuous scarp that is up to 300 ms high and ultimately links to the 192 frontal ramp in the toe region (Fig. 5b). En echelon tension cracks locally flank the lateral margin, 193 such as in the NW part of the toe region (Fig. 5b). 194 The top of the slide is rugose and has a vertical relief of up to 989 ms (957 m), measured from the   The basal shear surface deepens basinward, before steepening upwards in the toe region to define 226 the frontal ramp and the downdip limit of the contractional region (Fig. 8). The frontal ramp trends 227 broadly perpendicular to the gross, SE-directed transport direction of the slide and has a complex 228 morphology (Fig. 8). In the SE, the basal shear surface is defined by a c. 230 ms-high frontal 229 ramp, deepest immediately adjacent to the frontal margin (Fig. 8). To the NE, however, the frontal 230 ramp has a more complex, staircase-like geometry, consisting of two steep-dipping ramps 231 separated by an intermediate, strata-parallel detachment (Figs. 8a-b). There is considerable 232 variation in relief (up to 450 ms) along the basal shear surface due to the presence of these ramps 233 (Fig. 5a). Slide material covers the ramps in the NE, the SE and extends basinward onto the proto-234 seafloor, beyond the most distal ramp (Fig. 7). Thus the slide falls into the frontally emergent

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The toe region is divided in two parts: an inner thrust-belt and an outer thrust-belt (Fig. 8c). The 254 inner thrust-belt is dominated by symmetrical, thrust-bound pop-up blocks, within which internal 255 reflections are relatively well-preserved (Fig. 8c). These internal reflections are similar in terms of 256 overall seismic character to adjacent, undeformed strata located outside the slide body (Fig. 10c).  Seismic sections across PB10 illustrate how its geometry changes along strike from the SE to the 281 NE ( Fig. 12e-f). In the SE, it is defined by a single pop-up block bound by forethrust FT1 and 282 backthrust BT1 (Fig. 12f), passing along-strike to the NE into two pop-up blocks bounded by two 283 forethrusts (FT2 and FT3) and two backthrusts (BT2 and BT3) (Fig. 12g).

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Ideal T-x profiles display maximum throws near the center of a fault with a progressive taper of 285 the separation to zero at the fault tips. For this study, throw is measured at the best imaged parts  i.e., offset across shearing-related structures was not sufficient to passively juxtapose throw 301 profiles with strongly differing throws.   Despite these observations being broadly supportive of scenario 1, we note that the slide has a total 351 volume of c. 530 km 3 , with the thickness ratio between the relatively thick slide (c. 300 ms) and

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We argue this model can describe the styles and patterns of deformation observed here, given 406 contractional strains (e.g., thrusts and folds) are present downslope of the present toe wall. We 407 infer that the entire sediment mass between the slide toe wall, and the downdip limit of 408 contractional strains beyond the toe wall, has undergone cryptic lateral translation. The relatively 409 weakly deformed strata in this region (Fig. 10b, c) is essentially a giant megaclast. Only a few within the contractional domain.

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In the cases cited above, the shear zones occurred during downslope formation of the slide, 499 whereas we suggest the shear zones in the studied slide formed after shortening. There appears to 500 be a spatial relationship between the two main NW-trending longitudinal shears bounding areas A 501 and B, and areas B and C, and two major along-strike bends (from NE-to SE-trending) in the plan-502 view trace of the slide toe wall (e.g. , Figs 8 and 9). However, it is clear there are numerous similar 503 bends that are not associated with longitudinal shears, i.e., there are many more bends than there 504 are shears, suggesting shear zone development is not genetically linked to along-strike/across-flow 505 changes in slide toe wall geometry and degree of confinement. We thank Instituto Nacional de Petróleo Mozambique (INP) and WesternGeco for supplying the 527 data used for this study. We thank the editor and reviewers for the time and effort dedicated to 528 providing feedback on our manuscript and are grateful for the detailed comments and valuable 529 improvements made to our paper.