Asymmetric extension and subsequent unroofing of the Pohorje pluton at the western margin of the Pannonian basin, NE Slovenia.
Tycho Z. de Bie*, Inge E. van Gelder & Ernst Willingshofer
Faculty of Geosciences, Department of Earth Sciences, Utrecht University, the Netherlands
References: (1) Fodor, L.I., Gerdes, A., Dunkl, I., Koroknai, B., Pecskay, Z., Trajanova, I., Horvath, P., Vrabec, M., Jelen, B., Balogh, K., and Frisch, W., Miocene emplacement and rapid cooling of the Pohorje pluton at the Alpine-Pannonian- Dinaridic junction, Slovenia, Swiss journal of Geosciences. (2008), (2) Modifed after Willingshofer, E. and Cloetingh, S., Present day lithospheric strength of the Eastern Alps and its relationship to neotectonics. Tectonics, vol 22, no. 6 (2003) , (3) Modified after Mioč P. & Žnidarčič M. Geological map of SFRJ 1: 100,000, Sheet Slovenj Gradec. Geol. Surv. Ljubljana, Federal Geol. Surv. Beograd, Slovenia (1977), (4) Kirst, F., Sandmann, S., Nagel, T., Froitzheim, N., Janák, M., Tectonic evolution of the southeastern part of the Pohorje Mountains (Eastern Alps, Slovenia). Geologica Carpathica 61, 451–461, 2010.
* Corresponding author: T.Z.deBie@students.uu.nl
Scan to read abstract
ISES
Netherland Research Center
for Integrated Solid Earth Science
Dinarides
Bohemian Massif
40003000 2000
1000
Munich
Treviso
Graz Innsbruck
Vienna
47oE 48oE
46oE
14oN 16oN
10oN 12oN
Po Basin Adriatic Sea
Zagreb MMF
PLF SEMP
Pannonian Basin
0 100 200 300 400(km)
Tertiary Basins Helvetic Units
Penninic & Flysch Units
Austroalpine Units
Thrust Faults
Strike-slip Faults Normal Faults Neogene Intramontane Basins
Southalpine Units
Abbreviations: SEMP= Salzachtal-Ennstal-Mariazell-Puchberg Fault, MMF = Mur-Mürz Fault, PLF = Pöls-Lavanttal Fault Maribor
Strike-slip Faults
Normal Faults
Dents towards hanging wall
Low angle detachment
Dents towards footwall
15 Dipsymbol Trace fold axis
10 km
N
Maribor A
B’
B
A’
25
10 73
22
30 15
Geological background
(2)
(3&4)
Introduction
The Pohorje mountains in northern Slovenia consist of two nappes of Austroalpine metamorphic rocks. This nappe stack has been intruded by a granodiritic-tonalitic pluton, which is situated in the center of the mountain range. U-Pb data
constrain a Miocene (18.6 ± 0.11 Ma) age of crystallization at mid-crustal levels. (Fodor et al., 2008)
Emplacement of the Pohorje pluton was followed by rapid cooling related to its exhumation to the surface within approx- imately 3 Myrs. Although the temperature-time evolution of the pluton and surrounding rocks is well known, the mecha- nism by which the igneous rocks got exhumed is still debated. This study addresses this fundamental question through detailed field- and micro structural analysis.
2000 m 1500 m 1000 m 500 m 0 m 2000 m
1500 m 1000 m 500 m 0 m
WNW
Pohorje mountains Mura BasinENE
Tectono-stratigraphic column and profiles
Gneisses and schistsEclogite Amphibolite
Marble
Orthogneiss Serpentinite Retrograde gneisses and schists
Chlorite rich amphibolite Phyllonite
Phyllites
Carbonitic micaschist Meta-limestone
Mudstone Phyllite & sandstone
Conglomerate Conglomerate Sandstone and marls
Fluvial deposits
Pre-Cambrium Paleozoic Mesozoic Neogene Q MioceneTriassicPermDevonianSilurianOrdovicium
2000 m 1500 m 1000 m 500 m 0 m
2000 m 1500 m 1000 m 500 m 0 m
SSE
>ĂǀĂŶƩĂůĨĂƵůƚ Pohorje Mountains Ribnica-Selnica trough Kozjak MountainsNNW
A
B’
A’
B
Nappe contact and strike slip Detachment Normal faults
S1 foliation
Sense of move- ment along detachment Sense of move- ment along nappe contact Sense of move- ment along strike-slip fault
LCA UCA-cover UCA-basement
UCA = Upper Central Austroalpine LCA = Lower Central Austroalpine
Field results
10 km
N
Maribor
D1: Recumbent isoclinal folds and S1 foliation
D2: Tight folds and S2 foliation
D3: Top to the east shearing, brittle faulting and large scale folding
W E
Isoclinal folds
W E
W E
Poles to S1 foliations (n=359) and eigenvectors
Tight fold and S2 foliation S2 foliation; great circles and poles (n=18)
Schmidt net projection showing dominant top-to-the-East sense of shear, independent of plunge direction of stretching lineations.
Top-to-the-East sense of shear Top-to-the West sense of shear Stretching lineation
•
Top-to-the-East sense of shear Top-to-the-West sense of shear
S1
S1
S2
C’ S1
Microstructures & metamorphism
• M1: Peak metamorphic conditions
A high grade metamorphic mineral assemblage consists of quartz, plagioclase, biotite, muscovite, garnet, staurolite, kyanite and opx, representing upper Amphibolite facies metamorphism.
• D3 deformation under retrograde metamorphism (M2)
1) Retrograde metamorphic overprint.
Chlorite replaces garnet
2) Replacement of muscovite by chlorite along shear band; top E sense of shear 3) Top to the east sense of shear; Indicated
by strain shadows around prorphyroclast 4) Rose diagram showing top E sense of shear
• Relative timing of deformation
1) D3 shearing displaces D1
isoclinal folds.
2) D2 tight folds are cross
cut by D3 shearbands.
3) The D3 phase affected the tonalite pluton
4) Dynamic recrystallization of feldspar in the tonalite
indicates temperatures of at least 400°C.
Musc Musc
Chl
E W W E
E W
E W
ZĞdž͛ĚŇĚƐ
ZĞdž͛ĚŇĚƐ Qtz
Bio
&ůĚƐ
&ůĚƐ
E W
SE
NW W E
PPL
PPL
PPL
PPL XPL
XPL XPL
XPL
C’
C
C’
S1 S1
S1
Interpretation
18.64 Ma (1)
t Post nappe stacking
t Intrusion pluton
~18 Ma
t Activation low angle detachment
t1st generation dykes
~ 18-16 Ma
Exhumation by normal faulting; Sedimentation on hanging wall
~ 16 Ma
t End of ductile phase of extension
t 2nd generation dykes
t Asymmetric exhumation profile; More exhumation in the East
t Erosion pluton
W E
LCA
UCA
-basementUCA
-coverinactive nappe contact
LCA = Lower Central Austroalpine UCA = Upper Central Austroalpine
