LiDAR and InSAR analysis of deformation in the Krafla rift zone, NE Iceland

Current models of fault growth examine the relationship of fault length (L) to vertical displacement (D) where the faults exhibit the classic fault shape of gradually increasing
vertical displacement from zero at the fault tips to a maximum displacement (Dmax) at the middle of the fault. These models cannot adequately explain displacement length observations at the Krafla fissure swarm, in Iceland's northern volcanic zone,
where I observe that many of the faults with significant vertical displacements still retain fissure-like features, with no vertical displacement, along portions of their lengths.
I have created a high resolution digital elevation model (DEM) of the Krafla region using airborne LiDAR and measured the displacement/length profiles of 775 faults, with lengths ranging from 10s to 1000s of metres. I have categorised the faults based on the proportion of the profile that was still fissure-like. Fully-developed faults (no fissure-like regions) were further grouped into those with profiles that had a flattened appearance (large regions of constant vertical displacment), those with a classical fault shape and those that show regions of fault linkage. I measured the Dmax/L ratio of each identifiable original fault within the linked fault profiles, evidencing that the majority of the original faults had reached the maximum D/L prior to linkage. I suggest that a
fault can most easily accommodate stress by displacing regions that are still fissure-like, and that a fault would be more likely to accommodate stress by linkage once it has reached the maximum displacement for its fault length. My results demonstrate that there is a pattern of growth from fissure to fault in the Dmax/L ratio of the categorised faults and propose a model for this growth. I suggest it is possible to better understand how faults grow in their earliest stages of development and that the proposed model can be incorporated as an early stage of fault growth for current models which only model behaviour of a fault once it has acquired the classical D/L profile. The range in the distribution of the published Dmax/L data is mainly attributed to tectonic setting, rock type and resolution limiting the choice of sample rate and fault
length range. Using the LiDAR data I have examined the effect that data resolution has on the interpretation of the D/L relationship. I have resampled the LiDAR point data to
produce two additional DEMs of 10 m and 30 m resolution, from which I have measured 90 and 40 fault profiles respectively. I have compared (Dmax)/L for all of these fault
profiles with those of the published data. I have shown that by varying resolution the
interpretation of the (Dmax)/L relationship gives trends for each resolution that together
account for the spread in results of the combined published data for the length of faults measured. I have proposed that it may be possible to identify whether a measured fault
is a single structure or if it is actually a segmented structure, when measured at a higher resolution, based on its location in the Dmax/L published distribution. The currently available surface displacement data in post-rifting Krafla and interrifting Askja are limited either to single point time series of displacement or regional displacement maps that are averaged over time and do not provide details of changes in
rate through time. I have created a 24-epoch InSAR time series from ERS-1 and ERS-2 satellite SAR images over the 16-year period between 1992-2008. Using this I have extracted time series at 39 locations, both along- and across-axis at Krafla and Askja, and have identified trends in displacement rates over time. I have produced cumulative displacement profiles, based on the trends in displacement rate, both along- and acrossaxis and identifed key periods of displacement behaviour in the NVZ. I suggest that Krafla has three possible major sources of surface displacement: the shallow magma chamber under the Krafla caldera provides a decaying surface deflation between 1992 and 1999 and two possible deeper sources further north, the first between the Krafla and Fremrinamar fissure swarms creating uplift between 1992 and 1999 and the second
beneath the Theistareykir volcanic centre between 2004 and 2008. In Askja I observe that the displacement rate in the caldera, previously thought to be a slowly decaying inflation, incurred a significant increase in rate to ~30 mm/yr in 1996-2004 followed by a decrease in rate to ~10 mm/yr.