A new model for the growth of normal faults developed above pre-existing structures

Emma K Bramham,Tim J Wright,Douglas A Paton,David M Hodgson

doi:10.1130/g48290.1

Abstract

AbstractConstraining the mechanisms of normal fault growth is essential for understanding extensional tectonics. Fault growth kinematics remain debated, mainly because the very earliest phase of deformation through recent syn-kinematic deposits is rarely documented. To understand how underlying structures influence surface faulting, we examined fault growth in a 10 ka magmatically resurfaced region of the Krafla fissure swarm, Iceland. We used a high-resolution (0.5 m) digital elevation model derived from airborne lidar to measure 775 fault profiles with lengths ranging from 0.015 to 2 km. For each fault, we measured the ratio of maximum vertical displacement to length (Dmax/L) and any nondisplaced portions of the fault. We observe that many shorter faults (&lt;200 m) retain fissure-like features, with no vertical displacement for substantial parts of their displacement profiles. Typically, longer faults (&gt;200 m) are vertically displaced along most of their surface length and have Dmax/L at the upper end of the global population for comparable lengths. We hypothesize that faults initiate at the surface as fissure-like fractures in resurfaced material as a result of flexural stresses caused by displacements on underlying faults. Faults then accrue vertical displacement following a constant-length model, and grow by dip and strike linkage or lengthening when they reach a bell-shaped displacement-length profile. This hybrid growth mechanism is repeated with deposition of each subsequent syn-kinematic layer, resulting in a remarkably wide distribution of Dmax/L. Our results capture a specific early period in the fault slip-deposition cycle in a volcanic setting that may be applicable to fault growth in sedimentary basins.

Highlights

Two end-member models that explain strain distribution during the evolution of fault systems are (1) the isolated model, whereby faults increase in length with displacement accumulation and fault tips propagate and interact incrementally; and (2) the constant-length model, whereby faults reach their final lengths near instantaneously, and fault tips interact by linkage with increasing displacement accumulation (e.g., Walsh et al, 2002; Rotevatn et al, 2019)
We investigate fault growth in syn-kinematic deposits using data from the northern Krafla fissure swarm (KFS), Iceland, a region where an established fault system was near-instantaneously magmatically resurfaced by the Storaviti lava flow at ca. 10 ka (Sæmundsson, 1991; Jóhannesson and Sæmundsson, 1998) and fractured by ∼20 subsequent rifting episodes (Björnsson et al, 1979; Buck et al, 2006)
Facility (ARSF) Dornier aircraft; these surveys have an optimal resolution of ∼0.5 m on the xand y-axes and ∼0.2 m on the z-axis, resulting in a high-resolution (0.5 m) digital elevation model (DEM) of the KFS

Summary

INTRODUCTION

Two end-member models that explain strain distribution during the evolution of fault systems are (1) the isolated model, whereby faults increase in length with displacement accumulation (the ratio of maximum vertical displacement to length, Dmax/L, remains approximately constant) and fault tips propagate and interact incrementally; and (2) the constant-length model, whereby faults reach their final lengths near instantaneously, and fault tips interact by linkage with increasing displacement accumulation (e.g., Walsh et al, 2002; Rotevatn et al, 2019) These end-member models and the associated global compilation of Dmax-L data The data quantity and quality and the presence of syn-kinematic deposits provide an unparalleled opportunity to consider a single fault system with Dmax-L spanning two orders of magnitude

KRAFLA FISSURES The KFS lies within the Northern Volcanic

DATA AND METHODS

DATA DISTRIBUTION AND INTERPRETATION

Findings

Vertical displacement evident across the whole fault profile