Growth of a thrust fault array in space and time: An example from the deep-water Niger delta

Abstract

The temporal and spatial evolution of thrust fault arrays is currently poorly understood, and marine fold and thrust belts at the toe of passive margin gravitational systems, imaged by commercial 3D seismic reflection datasets, afford a unique opportunity to investigate this problem in three dimensions. Using an extensive 3D seismic data set and age data, the total cumulative strain (shortening) and interval strain rates have been calculated for 11 thrust-related folds mapped in the toe-thrust region of the southern lobe of the Niger Delta. For the first time, the sequence of thrust nucleation, propagation and linkage through time at a scale of 10s km both along and across strike is documented. Short thrust segments had nucleated throughout the entire study area by 15 Ma. They then grew largely by lateral growth and linkage, increasing the fault trace length and generating asymmetric strain-distance plots, for the first 50% of their history. Thereafter, growth continued by shortening, with minimal along strike increase in fault length. Changes in shorteningdistance data between adjacent structures across strike suggest that the change in growth mode occurred once the thrusts had linked in 3D through the common underlying detachment. Over the entire thrust array the strain rate varies through time, starting slowly (<200m/Ma), then increasing between 9.5-3.7 Ma (200-400m/Ma) before slowing down in the last ~ 4 Ma (< 150 m/Ma). The variation in strain rate is attributed to a change in boundary conditions of the gravitational system.An increase in sediment supply to the delta occurred in the late Miocene-Pliocene, driving higher shortening rates in the toe area. A subsequent reduction in sediment supply in the last ~4 Ma led to a reduction in deformation rate and the cessation of activity on a number of the thrusts. Predictions of the critical taper wedge model are used to explain the near-synchronous growth of the entire thrust array over the last 15 Ma. Because sedimentation acts to lower the surface slope, the wedge can only continue to deform if shortening occurs over a wide area allowing the surface slope to build up. These new results suggest that models of piggyback fault propagation are not appropriate for deep-water fold and thrust belts.

Role of Shale Deformation in the Structural Development of a Deepwater Gravitational System in the Niger Delta

“…Thus, the main growth of gravity-driven structures started after ca. 9.5 Ma in the study area, nearly synchronous with adjacent regions (e.g., Pizzi et al, 2020). Within the late Miocene-to-present synkinematic Agbada Formation, several growth strata packages have been identified based on changing stratal geometries (Figure 4c), which were interpreted to represent different stages of structural growth evolution (Jolly et al, 2016;Krueger & Grant, 2011;Maloney et al, 2010).…”

“…The Niger Delta basin is one of the classic shale tectonic province in the world where deepwater gravitational systems are widely developed and studied (e.g., Cohen & McClay, 1996; Corredor et al., 2005; Damuth, 1994; Jolly et al., 2017, 2016; Morley & Guerin, 1996; Pizzi et al., 2020; Rouby et al., 2011; Wu & Bally, 2000; Wu et al., 2015). Previous studies found that the structural development of deepwater gravitational systems is closely related to deformation within the ductile substrate, composed of undercompacted and overpressured mobile shales (e.g., Bonini, 2003; Cohen & McClay, 1996; Deville et al., 2006; Duerto & McClay, 2011; Fillon et al., 2013; Hansberry et al., 2014; Morley, 2003; Morley & Guerin, 1996; Morley et al., 2011, 2017, 1998; Wu & Bally, 2000).…”

“…Incorporating the effect of local shale squeezing into classic fault kinematic models would enable a better understanding of the 3D deformation process of the basal shale unit. Within the sedimentary overburden, the sedimentary growth sequences deposited coevally with deformation have varying geometries and strain rates that record the structural evolution history (e.g., Burbank & Verges, 1994;Burbank et al, 1996;Jolly et al, 2016Jolly et al, , 2017Morley & Leong, 2008;Pizzi et al, 2020;Poblet et al, 1997;Suppe et al, 1992). Evidence of a correlation between 3D shale deformation and growth sequence evolution is still rare in existing literature.…”

Quantifying the relationship between structural deformation and the morphology of submarine channels on the Niger Delta continental slope

“…However, several of the most laterally persistent folds are marked by seabed scarps at their crests and actively deform the seabed. The structural and tectonic evolution of these structures have been documented by Jolly et al (2016) and Pizzi et al (2020): measurements of shortening and cumulative strain were calculated across four of the major thrust folds generating topographic relief. Line-length balancing was F I G U R E 3 (a) Three-dimensional (3-D) bathymetric map of the seabed showing the geomorphic expression of the two modern channels (black dashed lines), and structures that occur at, or near, the seabed (labelled a-g).…”

“…layer thickness and bed length are conserved during deformation). Recent work in the southern lobe of the Niger Delta shows that this approach is associated with measurement error of ±15% (Pizzi et al, 2020).…”