Crustal strain-dependent serpentinisation in the Porcupine Basin, offshore Ireland

Abstract

15Mantle hydration (serpentinisation) at magma-poor rifted margins is thought to play a key role in 16 controlling the kinematics of low-angle faults and thus, hyperextension and crustal breakup. 17However, because geophysical data principally provide observations of the final structure of a margin, 18 little is known about the evolution of serpentinisation and how this governs tectonics during 19 hyperextension. Here we present new observational evidence on how crustal strain-dependent 20 serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of 21 the Porcupine Basin, offshore Ireland. We present three new P-wave seismic velocity models that 22show the seismic structure of the uppermost lithosphere and the geometry of the Moho across and 23 along the basin axis. We use neighbouring seismic reflection lines to our tomographic models to 24 estimate crustal stretching (βc) of ~2.5 in the north at 52.5 o N and > 10 in the south at 51.7 o N. These 25 values suggest that no crustal embrittlement occurred in the northernmost region, and that rifting may 26 have progressed to crustal breakup in the southern part of the study area. We observed a decrease in 27 mantle velocities across the basin axis from east to west. These variations occur in a region where βc 28 is within the range at which crustal embrittlement and serpentinisation are possible (βc 3-4). Across 29 the basin axis, the lowest seismic velocity in the mantle spatially coincides with the maximum amount 30 of crustal faulting, indicating fault-controlled mantle hydration. Mantle velocities also suggest that 31 2 the degree of serpentinisation, together with the amount of crustal faulting, increases southwards 32 along the basin axis. Seismic reflection lines show a major detachment fault surface that grows 33 southwards along the basin axis and is only visible where the inferred degree of serpentinisation is > 34 15 %. This observation is consistent with laboratory measurements that show that at this degree of 35 serpentinisation, mantle rocks are sufficiently weak to allow low-angle normal faulting. Based on 36 these results, we propose two alternative formation models for the Porcupine Basin. The first involves 37 a northward propagation of the hyperextension processes, while the second model suggests higher 38 extension rates in the centre of the basin than in the north. Both scenarios postulate that the amount 39 of crustal strain determines the extent and degree of serpentinisation, which eventually controls the 40 development of detachments faults with advanced stretching. 41 42 3

Across‐axis variations in petrophysical properties of the North Porcupine Basin, offshore Ireland: New insights from long‐streamer traveltime tomography

“…An abrupt increase from 3.0–3.5 km/s to 5.0–5.5 km/s in less than 0.5 km indicates the transition between postrift sediments and basement rocks (Figure b). Below, the basement displays V p values of 5.0 to 6.0 km/s in agreement with previous tomographic studies on the area (Prada et al., ).…”

“…The Porcupine Basin formed in response to several rifting and subsidence phases during the Late Palaeozoic to Cenozoic, with the most pronounced rift phase occurring during the Mid to Late Jurassic (Naylor & Shannon, 2011). Extension in the Porcupine Basin led to a dramatic crustal thinning that increases from north-with stretching factors (β) 2.5-to south-β>10-along the basin axis (Prada et al, 2017;Watremez et al, 2016). According to recent tomographic studies, the increasing crustal thinning is associated with an increasing degree of serpentinization towards the centre of the basin (Prada et al, 2017), where crustal break-up occurred (O'Reilly, Hauser, Ravaut, Shannon, & Readman, 2006;Reston et al, 2004).…”

“…The latter episode occurred after the Early Eocene, following a Palaeocene-Eocene thermal uplift that some authors relate to the Icelandic hotspot activity (Jones, White, & Lovell, 2001;Tate et al, 1993), whereas others invoke a smallscale convection and plate reorganization model to explain it (Praeg et al, 2005). Regardless of the process that triggered these events, their combination contributed to the development of up to 8-km-thick postrift sedimentary cover in the Porcupine Basin (O'Reilly et al, 2006;Prada et al, 2017;Watremez et al, 2016).…”

“…We estimate the posterior uncertainty of the tomographic results related to the choice of the starting model by means of a Monte Carlo analysis (e.g., Korenaga et al, 2000 and references therein). The approach is similar to that followed in Prada et al (2017), but in this case, we only test the uncertainty related to the distribution of initial velocity models. An ensemble of 100 uniformly distributed velocity models is created by adding random perturbations of up to 10% to a given reference model, in which V p increases from 1.5 km/s at the top of the model to 5 km/s at the bottom (i.e., 6 km depth) ( Figure S1a).…”

Low‐Viscosity Crustal Layer Controls the Crustal Architecture and Thermal Distribution at Hyperextended Margins: Modeling Insight and Application to the Northern South China Sea Margin

“…For example, based on wide‐angle seismic data and Ocean Drilling Program drilling results from the West Iberia margin, Dean et al () concluded that the ocean‐continent transition zone is dominantly composed of either exposed upper mantle or serpentinized peridotite. Similar serpentinized mantle also has been found in the southeastern Canadian margin (Lau et al, ) and Porcupine Basin (Prada et al, ). Upper mantle exhumes when the crust ruptures before the lower lithosphere during rifting (Huismans & Beaumont, ), and the serpentinized peridotite is considered to be affected by extension velocity and lower crustal strength that control onset of melting and serpentinization (Pérez‐Gussinyé et al, ; Ros et al, ).…”