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

Jennifer Reeves,Craig Magee,Christopher Aiden-Lee Jackson

doi:10.30909/vol.01.01.0117

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 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, with magma transgression corresponds to localisation of uplift.

Highlights

Uplift of Earth’s surface in response to shallow-level magma movement provides crucial insights into volcano activity, potentially warning of impending eruptions [e.g. Sturkell et al 2006; Biggs et al 2009; Sparks et al 2012; van Wyk de Vries et al 2014]
Emplacement of shallow-level sills in sedimentary basins is commonly accommodated by overburden uplift to produce a forced fold that is expressed at the contemporaneous surface
The geometry and kinematics of these intrusion-induced forced folds reflects sill emplacement processes and sheds light on how ground deformation relates to magma movement at active volcanoes

Summary

Introduction

Uplift of Earth’s surface in response to shallow-level magma movement provides crucial insights into volcano activity, potentially warning of impending eruptions [e.g. Sturkell et al 2006; Biggs et al 2009; Sparks et al 2012; van Wyk de Vries et al 2014]. We typically assume that ground deformation results from elastic bending of the overburden (i.e. forced folding), such that the area of surface uplift is expected to directly correlate to the location and size of an underlying intrusion [Galland 2012]. Analyses of forced folds above sills and laccoliths exposed at Earth’s surface, generated in analogue models, modelled analytically, or imaged in seismic reflection data reveal that a combination of elastic bending and inelastic processes (e.g. faulting, fluidisation, and pore collapse) can accommodate magma emplacement [e.g. Because intrusion-induced forced folds and hydrothermal vents are expressed as topographic or bathymetric highs at the contemporaneous surface, numerous studies have used the age of overlying strata that onlap onto these structures as a method for determining the timing of magmatism [e.g. Our analysis of seismic-stratigraphic relationships between the hydrothermal vents, forced folds, and overlying strata reveals three main phases of forced fold growth and sill emplacement in Intrusion-induced forced fold kinematics

Results

Discussion

Conclusion