Suggested conceptual modelfor Krafla

GGW2016 workshop, November 25., 2016

Knútur Árnason ISOR

Krafla

Geological map

Faults and fissures (blue)

Caldera,110000 yr. BP,(black hedged lines)

Eruptive fissures and craters(yellow)

Geothermal surface mani-festations (red)

S-wave shadows

Observed duringduring KraflaFires (1975-1984)between 3 and 7 km depth(green lines)

Faults mowedin Krafla fires

Boundary faults in Krafla Fires

Bimodal tectonicsetting

Hvannstóð branchActive 8-3 ky BP

Eastern branchActive before 8 ky BPand again since 3 ky BP

Buried innercaldera (80 ky BP)

Buried trans-secting “graben” ?

BouguerGravity map

N S

(Ármannsson et.al. 1987)

N-S lithological section

Resistivity 200 m a.sl. (1D inv. TEM)

ResistivecoreHigh T(> 240°C)alteration

Mostly conf.within innercaldera

Drilled wells in Krafla

(Location of sectionon next slide)

Resistivity, alteration and temperature

_

_

_

_

_

l

l l l l l

500m a.s.l

0

0 1000 2000 3000 4000 5000m

- 500

- 1000

- 1500

KG-8

KG-10

KJ-15

KJ-19

KJ-20

KJ-16

KJ-18

Calderarim

Alteration

Resistivity> 100 m10 - 100 m1 - 6 m low resistivity capHigh resistivity core

250TEM soundingTemperature °C

Unaltered rocksSmectite - zeolite zoneMixed layered clay zoneChlorite zoneChlorite-epidote zone

200

250

300

KraflaHveragil

Cooling

Different thermal character

The role of permeability

3D inversionof MT

S-wave shadows(purple lines)

Sesmicity(grey dots)

IDDP-01

3D inversion of MT at Krafla

3 km/s

P-wave velocity structure of Krafla(Brandsdóttir et al., 1997)

Recent seismic tomography (Schuler et al., J. Geophys. Res., in press)

Caldera

IDDP-1

S-waveshadows

Just earthquakes

Earthquakes andrefraction

Heat sources

Gravity and extend ofhigh temperature alterationat 200 m a.sl. (200 m depth)

Intrusions north of thegraben (WNW-ESE andNNE-SSW dykes?)

~E4°S

~E22°S

18°d

d c

c = 0.31*d

Closer look at thefissure swarm

Different spreadingdirections N and Sof Krafla

Leads to NNE-SSWspreading componentin Krafla

Conceptual model

• Repeated dike intrusions within the inner caldera and to the north of the buried “graben”• Not only NNE-SSW dikes. The extensional component along the fissure swarm favours WNW-ESE dikes• During the 5 ky (8 to 3 ky BP) a two phase geothermal system developed in the northern part of the inner caldera• When the spreading moved back to the eastern part, permeability (along the fissure swarm) increased dramatically at shallow depth west of Hveragil and the upper isothermal system developed• Repeated dike intrusions maintained suitable permeability for the deeper two phase system• Permeability did not increase much east of Hveragil, hence two phase system there

Conceptual model (cont.)• Intrusions (maybe at the end of glaciation10 ky BP), at the SW rim of the inner caldera also produced a geothermal system• Repeated intrusions have not maintained the heat sources at Hvíthólar. Just east of the active rift, the system is in its final stage with two phase conditions at shallow depth but temperature reversal below• Further to the west, within the presently active rift, extensive cooling has taken place• 3D inversion of MT suggests partially molten rocks at the edges of the S-wave shadows• WNW-ESE dike injected in the southern part of the dike complex in Mývatn Fires and the northern part in Krafla fires ?

The role of re-melt of altered basaltic rocksin heat transfer from basaltic intrusions

• Some evolved central volcanos, like Krafla and Askja, show bimodal eruptive behaviour. For long periods they produce basaltic magma, but occasionally they produce more silicic magma or pure rhyolite.• Isotopes show that the rhyolite has not differentiated form primitive melt but is a re-melt of (epidote, amphibole) altered basaltic rocks.• Hot (~1100 °C) basaltic intrusion partially melts the altered basalt producing hot silicic melt rich in volatiles.• The melt has low viscosity and is buoyant and migrates upwards

The role of re-melt (cont.)

• The rhyolite melt cools and degases as it migrates up• Viscosity increases• Can be trapped by minor structural obstacles at relatively shallow depth (~2 km)• Rhyolite melt has been encountered at least in two wells in Krafla at 2-2.5 km depth (K-39 and IDDP-01) and also in the Menengai caldera in Kenya.• The shallow emplaced rhyolite produces a zone of superheated steam• Re-melted rhyolite transports heat from deep basaltic intrusions to shallow depths