How do tectonics influence the initiation and evolution of submarine 1 canyons? A case study from the Otway Basin, SE Australia

The architecture of canyon-fills can provide a valuable record of the link between tectonics, 9 sedimentation, and depositional processes in submarine settings. In this study, we investigate the 10 role of plate tectonics in the initiation and evolution of submarine canyons. We demonstrate that 11 plate tectonic-scale events (i.e. continental breakup and shortening) have a first-order influence on 12 submarine canyon initiation and development. Initially, the Late Cretaceous (c.65 Ma) separation 13 of Australia and Antarctica resulted in extensional fault systems, which then formed a steep stair- 14 shaped paleo-seabed. Subsequently, the Late Miocene (c.5 Ma) collision of Australia and Eurasia 15 has resulted in substantial uplift and exhumation in the SE Australian continental margin. These 16 tectonic events have resulted in elevated seismicity that ultimately gave rise to the gravity-driven 17 processes (i.e. turbidity currents and mass wasting processes) and formed the canyon base. The 18 inherited stair-shaped topography then facilitated gravity-driven processes which established a 19 mature sediment conduit extending from the shallow marine shelf to the abyssal plain . We indicate 20 that the canyon stratigraphic architecture can be used as an archive to record tectonic movements. 21 Moreover, the factors which preconditioned and triggered gravity-driven processes can also induce 22 canyon initiation and facilitate canyon development.


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Submarine canyons are ubiquitous in deep-water settings and have long been considered as one 26 of the major conduits for transporting sediment from the shelf edge, across the continental slope, 27 and into the deeper abyssal plain (Shepard, 1981;Normark et al., 2003;Antobreh and Krastel, 28 2006). They are normally characterised by U-shaped cross-sectional geometries, that represents 29 incision of hundreds of metres into the underlying stratigraphy, extend for several kilometres in 30 width, and up to hundreds of kilometres long (i.e. Lewis and Barnes, 1999;Baztan et al., 2005;Su 31 et al., 2020). When buried, the coarse-grained canyon-fill (i.e. sand-rich turbidites) may act as 32 reservoirs for hydrocarbons and/or long-term carbon storage, in many submarine settings, (e.g. calculates the variability of a trace to its neighbour over a particular sample interval and produces 151 interpretable lateral changes in acoustic impedance (Van Bemmel and Pepper, 2000), low variance 152 response represents similar traces, and high variance response represents discontinuities (Brown, 153 2011). Therefore, coupled with seismic facies analyses, variance attributes can contribute to better 154 imaging and mapping intra-canyon deposits.  Seismic facies-2 (SF-2) is characterised by sheet-like, medium-to low-amplitude, laterally 169 continuous reflections that cap SF-1 and SF-3 ( Figure 6). SF-2 consists of a flat base and top surface 170 with fair cross-sectional continuity, and no erosive features have been observed. In general, the SF-171 canyon stratigraphy. The thickness of SF-2 is constant, ranging from 150-190 m. Based on the seismic characteristics and previous seismic facies-based studies, SF-2 is interpreted as a mix of 174 fine-grained turbidite complexes and mud-rich hemipelagic deposits (Symons et al., 2017;Maier 175 et al., 2018), representing a low-energy depositional environment (Prather et al., 1998).  3D seismic data area, expanding from the lower slope to the abyssal plain. In the following section, 228 we take BC-1 (the biggest buried canyon in the study area) as an example to further investigate its 229 facies association and infill patterns.

Upper segment 232
The upper segment of the BC-1 truncates into the paleo-lower continental slope (Figure 7a, 7b). 233 The maximum width and relief of the buried canyon is c. 3 km and 300 m, respectively. In this 234 segment, the lowermost and the uppermost section of the buried canyon is commonly filled with 235 SF-1, suggesting that turbidite complexes are the most dominant depositional elements during the 236 initial and final phase of the buried canyon-fill (Figure 8a    gravity-driven processes (most likely mass wasting processes) during Late Miocene. 331

How do Late Miocene tectonics dictate the canyon initiation? 332
During Late Miocene, the SE Australia margin has experienced an extremely intense episode of  The Late Miocene erosion period corresponds to the time when the entire shelf was exposed and 352 thus heavily incised by frequent deposition of MTCs, that were transported down to the deeper 353 part of the basin. We infer that mass failures during episodes of intense tectonic, rather than other 354 factors, caused of incision on the continental slope to initiate the development of buried canyons. 355 The Late Miocene tectonics have helped establish a mature sediment conduit system that 356 extended from shallower marine down to the abyssal plain. 357

How do Late Cretaceous tectonics influenced the canyon evolution? 358
The late Cretaceous fault systems are generally NW-SE striking (Figure 11a The dip-and cross-seismic sections have revealed these faults cutting vertically beneath the 360 thalwegs of the BC-1 and BC-2 (Figure 4b, 5b, 9b). The seismic dip line along the canyon axis shows 361 the presence of the faults has created a stair-shaped structure within the Lower segment, which is 362 truncated by the canyon (Figure 5b, 9b). The seismic dip line along the area outside the buried 363 canyons show that after deposition of the pre-canyon succession (sedimentation between horizon 364 H1-H2), the paleo-seafloor (at the time of H2) may have inherited the geometry created by the 365 Cretaceous fault systems, showing a stair-shaped structure with a high-gradient (Figure 11c). This 366 can be clearly seen from the onlapping patterns of sediments onto the local topographically high 367 created by buried faults (Figure 11c). 368 The stair shaped geometry is interpreted as the hanging walls of the deeply sourced fault systems 369 may have created a local structure high on the Late Cretaceous seafloor when horizon H1 is 370 deposited. The footwalls of the deeply sourced fault systems have created a local structure low on 371 the Late Cretaceous seafloor (Figure 12a). After the burial, the buried footwalls acted as a local 372 high (buried hanging walls are locally low), causing an elevation difference between two adjacent 373 footwalls and hanging walls (Figure 12a). When the canyon initiates, the stair-shape paleo 374 geometry can cause an immediate increase in currents (e.g., turbidity currents or debris flow) 375 energy and erosivity, thus facilitating the canyon development (Figure 12b). The subsequent 376 canyon-fills was also influenced by the inherited topography created by the previous canyon infill 377 and the stair-shape canyon base (Figure 12c). In modern analogues, the local gradient variation of 378 the seabed has played a key role in canyon evolution (e.g., expansion in canyon width and depth), as demonstrated by modern canyon systems (Qin et al., 2017;Wu et al., 2022). Therefore, we 380 suggest that the late Cretaceous fault-controlled zones may have pre-determined the location of 381 the canyons by facilitating the erosional downcutting during the formation of the canyon base, this 382 influence has not been instantaneous, instead the impact on the canyon evolution can be felt as 383 late as tens of million years (or more). 384

Implication 385
Previous studies show that the tectonically active settings tend to develop small-scale, short-lived 386 canyons (Eyles and Lagoe, 1998), while canyons in tectonically stable passive margin settings tend 387 to develop relatively large scale canyons which are active for longer periods (Coleman et al., 1983). 388 However, we reveal that in the tectonically active regions, uplift and tilting due to tectonic 389 deformation induce an increased in sediment supply and seismicity, which can promote mass 390 failure events thus contribute significantly to the formation of large-scale submarine canyons. 391 Therefore, we indicate that the factors which preconditioned and triggered mass-transport 392 complexes can also induce canyon initiation and facilitate canyon development. We suggest that 393 the plate tectonic scale events (i.e. continental breakup and shortening) have a first-order influence 394 on the submarine canyon initiation and evolution. The impact from the regional tectonics to the 395 buried canyons can be instantaneous (i.e. directly trigger canyoning processes), or their influence 396 can also be postponed (i.e. indirectly influence the seabed topography thus the canyon geometry). 397  We suggest that repeated mass failure is the most likely driving mechanism of the buried canyon inception, in conjunction with increased sediment flux due to exhumation of the margin. 408 3. We interpret the extensional faults associated with the late Cretaceous plate separation 409 between Australia and Antarctica as responsible for the inception and evolution of the buried 410 canyons by increasing the steepness of the paleo-seabed, thus controlling the canyon geometry 411 and location.