© G. Berberich Autonomous Underwater Vehicle (Atlas Sea Cat, Atlas Elektronik GmbH) By the Sea Cat detected gas exhalations at the lake bottom Why does the Size of the Laacher See Magma Chamber and its Caldera Size not go together? – New Findings Ulrich Schreiber & Gabriele Berberich University of Duisburg-Essen, Essen, Germany EGU General Assembly 2013 | Poster EGU2013-5908 | B444 | GMPV36/TS3.4 Corresponding author: [email protected] References Schmincke U (2009): Vulkane der Eifel. Spektrum Verlag Heidelberg. Holohan E, van Wyk de Vries B & Troll V. (2008): Analogue models of caldera collapse in strike-slip tectonic regimes. Bull. Volcanol., 70:773-796 Acocella V, Funiciello R, Marotta E, Orsi G, and de Vita S. (2004): The role of extensional structures on experimental calderas and resurgence, J. Volcanol. Geotherm. Res., 129(1-3), 199-217. Roche O, Druitt TH, Merle O (2000): Experimental study of caldera formation. Journal of Geophysical Research, 105: 395 Schmincke U (2000): Vulkanismus. Wissenschaftliche Buchgesellschaft, Darmstadt, pp. 264. Loos J, Juch D & Ehrhardt W (1999): Äquidistanzen von Blattverschiebungen - neue Erkenntnisse zur Lagerstättenbearbeitung im Ruhrkarbon.– Zeit. f. angew. Geol., 45: 26 – 36. Ochmann N (1988): Tomografische Analyse der Krustenstruktur unter dem Laacher See Vulkan mit Hilfe von teleseismischen Laufzeitstudien. Mitt. Ing.- u. Hydrogeol. 30, pp. 108 Viereck LG & v.d. Bogaard P (1986): Magma- und Wärmeinhalt der Magmakammer des Laacher Sees und des Riedener Vulkans. Forschungsbericht T86-174, Bundesministerium für Forschung und Technologie (BMFT), Karlsruhe, pp 1-98. Bahrig B (1985): Sedimentation und Diagense im Laacher Seebecken (Osteifel). Bochumer geol. geotechn. Arb. 19, pp 231 v.d. Bogaard P & Schmincke HU (1984): The Eruptive Center of the Late Quarternary Laacher See Tephra. – Geol. Rundschau, 73: 933-980. v.d. Bogaard P (1983): Die Eruption des Laacher See Vulkans. Dissertation, Ruhr-Universität Bochum, 348 S. Block bounding fault Thrust Normal Zone bounding fault Conjugate Strike slip Rotation Marker Voids Source: Dibblee, 1977 a) b) c) d) R’-Shear plane Development of scales & duplex structures en-échelon R-Shear plane Connection with P-Shear planes Faults 2 nd order Position and conncention of Riedel shear planes along of a dextral strike-slip fault (after Swanson 2006, J. Struct. Geol. 28, 456-473) Our findings: Erupted magma volume incl. bedrock: 0.5 km³ Bed rock Bed rock Bed rock Drawn by: G. Berberich Pre-eruptive Szenario 1a (Schmincke 2009): Magma chamber Volume: 18 km³ Width: 1km Depth of top: 3 km 3 km 6.0 km³ erupted Magma Post-eruptive Szenario 1b (Schmincke 2009): Erupted bed rock volume: 0.5 km³ Erupted magma volume: 6.0 km³ Total erupted volume: 6.5 km³ Magma chamber Volume: 18 km³ Width: 1km Depth of caldera bottom: ~7 km 1 km 10.6 km 2.5 km Today‘s caldera width Bed rock Bed rock Pre-eruptive Szenario 2a (Schmincke 2009): Magma chamber Volume: 18 km³ Width: 1.6 km Depth of top: 3 km Surface 1 km 18 km³ Magma chamber volume 12 km³ Magma chamber volume Bed rock Bed rock 3 km Bed rock 1.6 km 6.0 km³ erupted Magma Surface 18 km³ Magma chamber volume Bed rock Bed rock Bed rock 0.5 km³ erup. Magma 3 km Bed rock Surface Bed rock Bed rock 2.5 km Today‘s caldera width Debris of bed rock Debris of bed rock Bed rock Surface 6 km 1.6 km 12 km³ Magma chamber volume Post-eruptive Szenario 2b (Schmincke 2009): Erupted bed rock volume: 0.5 km³ Erupted magma volume: 6.0 km³ Total erupted volume: 6.5 km³ Magma chamber Volume: 18 km³ Width: 1.6 km Depth of caldera bottom: ~3 km 2.5 km Today‘s caldera width Debris of bed rock >1 km³ ? Magma chamber volume 3.5 km 6.8 km 2.7 km Munich Hamburg Amsterdam Brussels Vienna Budapest Marseille Rome Bonn Stockholm Kopenhagen Warsaw Berlin Prague Genf Ice border 11 000 BP oc o o Sto o c Turin Stockholm Turin 0 200 km O Genf nf nf nf f f T urin Turin urg rlin erlin rlin urg Be e e 10 05 15 15 05 70 65 0 50 25 10 10 02 30 11 16 03 02 06 05 09 02 02 03 05 05 05 10 06 05 06 05 2 mm 5 mm 1 cm 5 cm 1cm 5cm 10cm 10cm 10cm 5 cm 1 cm 5 mm Gotland Turin 03 Total thickness of Laacher See ashes and tephra fans in Central Europe (after: van den Bogaard, 1983) New EQ-Center of the last 40 years Laacher See strike-slip fault (LSSSF) Processed mining field Examples A and B Mining exposure Tectonic projection Projection due to equidistances of the large-scale strike-slip faults } Analogue Models of Caldera Collapse in Strike-Slip Tectonic Regimes (from: Holohan et al. 2008) Ruhr carboniferous: Equidistances of strike-slip faults (from: Loos et al. 1999) 10 m 8 m 4 m 2 m 1 m 1,1 %/ 1,1 % 0,3 %/1,0% 0,13 Mio. ppb 0,10 Mio. ppb/0,21 Mio. ppb 0,16 Mio. ppb/0,22 Mio. ppb 0,7 Mio. ppb 0,8 Mio. ppb 0,23 Mio. ppb 0,16 Mio. ppb/ 0,21 Mio. ppb 0,16 Mio. ppb 0,30 Mio. ppb 0,17 Mio. ppb/0,11 Mio. ppb 0,38 Mio. ppb 0,16 Mio. ppb 0,18 Mio. ppb/0,46 Mio. ppb Strike-slip faults and crustal block rotation with voids (after: Dibblee, 1977; Swanson 2006) a) Pre-caldera strike-slip deformation Outline of magma chamber at depth Y-shear Chamber-localised graben and fault Regional faults (Riedel shears) b) Early downsag phase Diffuse zone of peripheral extension Elliptical downsag zone inside chamber outline c) Initiation of reverse faulting Reverse fault scarp forms a short axis end of chamber Focus of asymmetry is initially to the NW Component of horizontal movement during early central zone subsidence d) Final caldera structure Reverse fault propagates toward chamber long axis Regional fault reactivated Regional fault reactivated to accomodate peripheral extension and/or central subsidence Second reverse fault forms. Central zone subsidence becomes less asymmetric and purely vertical e) Interpretation Subsidence controlling reverse fault Zone of diffusive peripheral extension Peripheral extension localised on reactivated pre-existing fault Subsidence controlling reverse fault Reactivated Riedel shear Pre-collapse regional faults (Riedel shears) Pre-collapse faults (unreactivated) f) Excavation of remnant chamber Subsided, almost flat, upper surface of remnant chamber Ridge bends sharply into Y-shear trend. Displacement transfer from reverse fault to Y-Shear? Cut into chamber made by regional R- and Y-shears. Associated with arrest of reverse fault propagation Ridge with parallel outward dipping furrow Vertical ridge. Inward dipping at SW end Scale 10 cm Scale 10 cm Scale 10 cm Scale 10 cm Scale 10 cm Scale 10 cm N N N N N N Under pressure experiments of Roche et al. (2000) Plate with hole Sand pack Silicone Tube a) Apparatus, section view Small deep chamber Breccias Roof Chamber Chamber Roof Large shallow chamber Breccias Breccias c) Final model summarizing the deformation pattern for funnel (small deep chamber) and piston (large shallow chambers) calderas (modified after Roche et al. 2000) Sand-pack Silicone Sand-pack Silicone Sand-pack b) Section view of 3 experiments characterized by very different aspect ratios of the chamber roof Silicone Schematic models of Laacher See magma chamber sizes and erupted volumes Sea Cat tracks of 2010 and 2011 campaigns Profile of Laacher See (reflection seismic; Bahrig 1985) Break-up zone below Laacher See (Ochmann 1988) 2010 2011 Sonar image of rising gas bubbles from Laacher See bottom (300 kHz side scan sonar) New model concepts on the tectonic evolution of the young East Eifel lead to a contradiction in terms of size of the postulated magma chamber and the Laacher See Caldera Due to the slow movement rates of active tectonic faults (mm per year), an estimated 18 km³ magma chamber beneath the Laacher See (v. d. Bogaard & Schmincke 1984) cannot be confirmed. Discrepancies are given by • the volume of the Laacher See caldera of approx. 0.5 km³ with regard to the pre-eruptive surface (Viereck & v.d. Bogaard 1986) and the erupted volume of 6.3 km³ dry rock equiva- lent of lava and bed rock (v. d. Bogaard & Schmincke 1984) resp. 6.5 km³ magma (Schmincke 2009), • a comparison of modeling of caldera evolution with the Laacher See Caldera formation (Holohan, de Wries & Troll 2008; Acocella, Funiciello, Marotta, Orsi & de Vita 2004), • no geophysical prove of such a large magma chamber, • a volume compensation of approx. 6 km³ by ascending magma from the mantle which could have prevented a further subsidence of the magma chamber (over a period of several days of the estimated duration of eruption) appears unrealistic, • performed sonar recordings of the post-eruptive Laacher See sediment layers (Bahrig 1985) that do not show any displacements that might indicate a doming caused by magma. Our findings • No statistical significant data set with regard to spatial distribution of the erupted tephra volume, e.g. only one sample point for North Italy (v.d. Bogaard 1983). • Overestimation of the tephra thickness in published isopach maps of the Westerwald and other regions. • More critical evaluation of interpretations of tephra samplings from old maps and literature is required. • Inclusion of atmospheric effects (e.g. atmospheric turbulences, dune formation, dust storms long after the eruption, congestion of air masses at the alpine orogene) is required. • An order of magnitude smaller magma chamber stretched over a longer vertical crustal section can help to better match the given tectonic movement rates and the size of the caldera. • All sampling locations would also be explained by an erupted volume of only 10% of the estimated one by Schmincke (2009). © J. Kalwa, ATLAS Elektronik GmbH © J. Kalwa, ATLAS Elektronik GmbH © J. Kalwa, ATLAS Elektronik GmbH © J. Kalwa, ATLAS Elektronik GmbH Pre-existing fault reactivated to accomodate peripheral extension Reverse fault truncates against regional fault