Throw Rate Variability on Gravity-Driven Normal Faults; Constraints from the Gudrun Fault, South Viking Graben, Offshore Norway

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Authors

Christopher Aiden-Lee Jackson 

Abstract

The growth and throw rate variability on normal faults can reflect fault interaction, plate tectonic forces and, in gravity-driven systems, variations in sediment loading. Because earthquakes may occur as faults slip, it is important to understand what processes influence throw rate variability on normal faults to be able to predict seismic hazards in extensional terranes. Furthermore, the rate of normal fault growth directly controls rift physiography, sediment erosion, dispersal and deposition, and the distribution and stratigraphic architecture of syn-rift reservoirs. Instrumental (e.g. geodetic) data may constrain the co-seismic movement on, or relatively short-term (i.e. <103) throw rate history of, normal faults, whereas palaeoearthquake data may provide important information on medium-term (i.e. 103-105 years) rates. Constraining longer-term (i.e. >106 Myr) variations typically requires the use of seismic reflection data, although their application may be problematic because of poor seismic resolution and the absence of, or poor age constraints on, coeval growth strata. In this study I use 3D seismic reflection and borehole data to constrain the growth and (minimum) long-term throw rate variability on a gravity-driven, salt-detached normal fault (Middle-to-Late Jurassic) in the South Viking Graben, offshore Norway, and to assess the impact of throw rate variability on the thickness and character of syn-rift reservoirs. I recognise five kinematic phases: (i) Phase 1 (early Callovian) - fault initiation and a phase of moderate fault throw rates (0.06 mm yr-1); (ii) Phase 2 (early Callovian-to-end Callovian) - fault inactivity, during which time the fault was buried by sediment; (iii) Phase 3 (early Oxfordian-to-late Oxfordian) - fault reactivation and a phase of moderate throw rates (up to 0.03 mm yr-1); (iv) Phase 4 (late Oxfordian-to-end Oxfordian) – a marked increase in throw rate (up to 0.27 mm yr-1); and (v) Phase 5 (early Kimmeridgian-to-middle Volgian) – a decline in throw rate (0.03 mm yr-1) and eventual death of the fault. These rates are comparable to those observed on other gravity-driven normal faults, with the variability in this example apparently kinematically coupled with the growth history of the thick-skinned normal fault system bounding the western margin of the basin. Fluctuations in sediment accumulation rate and loading may have also influenced throw rate variability. Shallow marine reservoirs deposited when throw rate was relatively low (Phase 1) increase in thickness but do not change in facies across the fault, principally because sediment accumulation rate outpaced fault throw rate. In contrast, deep-marine turbidite reservoirs, despite being characterised by relatively high sediment accumulation rates, were deposited when the throw rate was relatively high (Phase 4), thus are only preserved in the fault hangingwall. Variations in throw and sediment accumulation rate may therefore act as dual controls on the thickness and distribution of syn-rift reservoirs in salt-influenced rift basins.

DOI

https://doi.org/10.31223/osf.io/tpyb9

Subjects

Earth Sciences, Geology, Physical Sciences and Mathematics, Tectonics and Structure

Keywords

earthquake, Tectonics, normal fault, normal faulting, North Sea, fault slip rate, Gudrun fault, South Viking Graben, structura geology

Dates

Published: 2017-11-24 12:58

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Academic Free License (AFL) 3.0

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