Assessing the impact of sedimentation on fault spacing at the Andaman Sea spreading center

Clément De Sagazan,Jean-Arthur Olive

doi:10.1130/g48263.1

Abstract

AbstractThe stabilizing effect of surface processes on strain localization, albeit predicted by several decades of geodynamic modeling, remains difficult to document in real tectonic settings. Here we assess whether intense sedimentation can explain the longevity of the normal faults bounding the Andaman Sea spreading center (ASSC). The structure of the ASSC is analogous to a slow-spreading mid-ocean ridge (MOR), with symmetric, evenly spaced axis-facing faults. The average spacing of faults with throws ≥100 m (8.8 km) is however large compared to unsedimented MORs of commensurate spreading rate, suggesting that sedimentation helps focus tectonic strain onto a smaller number of longer-lived faults. We test this idea by simulating a MOR with a specified fraction of magmatic plate separation (M), subjected to a sedimentation rate (s) ranging from 0 to 1 mm/yr. We find that for a given M ≥ 0.7, increasing s increases fault lifespan by ∼50%, and the effect plateaus for s &gt; 0.5 mm/yr. Sedimentation prolongs slip on active faults by leveling seafloor relief and raising the threshold for breaking new faults. The effect is more pronounced for faults with a slower throw rate, which is favored by a greater M. These results suggest that sedimentation-enhanced fault lifespan is a viable explanation for the large spacing of ASSC faults if magmatic input is sufficiently robust. By contrast, longer-lived faults that form under low M are not strongly influenced by sedimentation.

Highlights

Tectonic models commonly predict that erosion and sedimentation enhance strain localization onto a few major faults at subaerial plate boundaries such as orogens (e.g., Masek and Duncan, 1998; Willett, 1999) and continental rifts (Olive et al, 2014; Andrés-Martínez et al, 2019; Theunissen and Huismans, 2019)
We measured the distance between successive hanging-wall cutoffs visible in seafloor or basement topography at the end of each run, except in the s = 0 and M = 0.6 case, which produced large-offset detachments with spacings in some cases exceeding the half-width of the domain (Fig. 3A); in this case, we estimated fault spacing at different times during the run and discarded the very short-lived faults formed at the beginning of the simulation prior to steady detachment faulting
Erosion and sedimentation have been shown to enhance the lifespan of individual normal faults in numerical simulations (Olive et al, 2014; Andrés-Martínez et al, 2019; Theunissen and Huismans, 2019)

Summary

Introduction

Tectonic models commonly predict that erosion and sedimentation enhance strain localization onto a few major faults at subaerial plate boundaries such as orogens (e.g., Masek and Duncan, 1998; Willett, 1999) and continental rifts (Olive et al, 2014; Andrés-Martínez et al, 2019; Theunissen and Huismans, 2019). The ASSC is a divergent boundary marking a releasing step between the Sumatra and Sagaing strike-slip fault systems (Fig. 1A). It allows the opening of a 200-km-long, 120-kmwide pull-apart basin in the back-arc domain of the oblique Sumatra subduction (Curray et al, 1979; Sieh and Natawidjaja, 2000; Singh et al, 2013). Fault-induced topography within ∼30 km normal to the ASSC axis (≤2 Ma) is buried under a sedimentary layer as thick as 1.5–2 km This massive sedimentary input is largely provided by the Irawaddy River, 500 km to the north (Curray, 2005), and amounts to ∼0.75–1 mm/yr of syntectonic deposition over the past 2 m.y

Methods

Results

Conclusion