Unravelling intrusion-induced forced fold kinematics and ground deformation using 3D seismic reflection data

Abstract

Sills emplaced at shallow-levels are commonly accommodated by overburden uplift, producing forced folds. We examine ancient forced folds developed above saucer-shaped sills using 3D seismic reflection data from the Canterbury Basin, offshore SE New Zealand. Seismic-stratigraphic relationships indicate sill emplacement occurred incrementally over ~31 Myr between the Oligocene (~35-32 Ma) and Early Pliocene (~5-4 Ma). Two folds display flat-topped geometries and amplitudes that 10 decrease upwards, conforming to expected models of forced fold growth. Conversely, two folds display amplitudes that locally increase upwards, coincident with a transition from flat-topped to dome-shaped morphologies and an across-fold thickening of strata. We suggest these discrepancies between observed and expected forced fold geometry reflect uplift and subsidence cycles driven by sill inflation and deflation. Unravelling these forced fold kinematic histories shows complex intrusion geometries can produce relatively simple ground deformation patterns, where magma transgression corresponds to localisation 15 of uplift.

Saucers, Fingers, and Lobes: New Insights on Sill Emplacement From Scaled Laboratory Experiments

“…(2017) used a rigid‐plasticity approach to simulate sills in homogenous Mohr‐Coulomb material—gray curves in panel (c) plot numerically modeled damage zones, where magma is expected to intrude the host rocks, formed by inflation of horizontal cracks of variable starting length. Labels of saucer‐shaped sills in nature represent: (a) Sill 2, (b) Sill 1, (c) Sill 3 and (d) Sill 4 in Canterbury Basin, offshore SE New Zealand (Reeves et al., 2018); (e) Morskranes Sill, (f) Sundini Sill, (g) Kvívík Sill, (h) Fugloy Sill, (i) Eysturoy Sill, (k) Streymoy Sill and (m) Svínoy‐Fugloy Sill in Faroe Islands (Hansen, 2015); (l) Eocene Sill 1, and (n) Eocene Sill 2 in Faroe‐Shetland Basin (Moy & Imber, 2009).…”

Salt–magma interactions influence intrusion distribution and salt tectonics in the Santos Basin, offshore Brazil

“…between the late Albian and end of the Santonian) (e.g. Magee et al., 2014; Reeves et al., 2018; Schofield et al., 2017; Trude et al., 2003).…”

Post-collisional and volcanic features of the Late Miocene-Pleistocene foreland area, northern offshore Taiwan

“…In offshore Norway, layer parallel reflectors and saucer-shaped reflectors are recognized, showing the smoothness, size, continuity and depth of paleosurfaces of the sill (Planke et al 2005). It is commonly seen that a forced fold will be formed when sills are emplaced at shallow levels (Jackson et al 2013;Reeves et al 2018). Besides, the location and geometry of basaltic intrusion bodies are strongly controlled by preexisting structures and local stress (Valentine and Krogh, 2006).…”