Indications for Late Miocene/Pliocene brittle fold and ...

The young structural evolution of Syros (Cyclades, Greece) - Indications for Late Miocene/Pliocene brittle fold and thrust tectonics? 1

2

1

Mark Keiter , Karsten Piepjohn , Christian Ballhaus 1

2

Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover

Palos

Alluvium

N

Marbles

Grizzas Kalogeros

Conglomerate of Palos

Introduction

Schists

Grammata

Kampos Lia

Metabasites, blueschist- to eclogite facies

Syringas

Metabasites, pervasive greenschist overprint Meta-Acidites

The Cycladic Blueschist Belt is currently located in the back arc area of the Hellenic Subduction Zone. After extensive prograde deformation under blueschist- to eclogite facies conditions and subsequent exhumation, it is generally assumed to be in an overall extensional stress field since at least the middle Miocene. Only minor occurrences of late Neogene crustal shortening structures are reported in that area.

Vari Unit: Gneisses with rim of sheared chloritic schists

Kini

Hermoupolis

Galissas

map Fig. 2 Azolimnos

This poster presents observations made on Syros Island, indicating that at least on a local scale, massive crustal shortening has taken place in the later Neogene.

Finikas

Vari

Fig. 3 a ki

a un

Komito

M

av

ra

38°

Vo

Andros Tinos Kea

Mykonos

Kithnos

Syros

Serifos

Paros

Ikaria

Naxos 37°

Siphnos

Amorgos

0

5 km

Milos 25°

Marble Quartzite Schist

26°

N

Fig. 1: Generalized geological map of Syros Island (modified after Hecht, 1984)

A'

Metabasite Serpentinitic Schist

A

50 km

Field Observations

normal fault thrust fault 0

100

200

300

400

500 m

Contour interval 20 metres

A'

A

ENE

WSW

N metamorphic foliation in marble metamorphic foliation in schist

Mapping work in the southwestern part of Syros (Fig. 1) provided evidence for significant horizontal thrust movements as a result of NE-SW directed crustal shortening. The identified thrust planes cross-cut late brittle vertical faults (Fig. 2), which are assumed to be of Late Miocene age or younger. This age is constrained from considerations about the ductile-brittle transition [1] and cross cutting relations of brittle faults with datable sediments throughout the Cyclades [2]. Therefore, thrusting must have taken place between 13 Ma and the present.

main thrust plane branch thrust plane striation lineation on main thrust plane

Num total: 18

Equal area projection, lower hemisphere

Associated with the deformation event responsible for the thrusts are abundant brittle upright to slightly vergent folds and reverse faults (Fig. 3). The folds and reverse faults show the same NE-SW directed shortening. These structures can be observed mainly in the southwestern part of the island in map scale as well as in outcrop scale.

Fig. 2: Map view, cross section and measured structural data of a relict thrust sheet between Finikas and Galissas (for location see Fig. 1).

generalized map view of the outcrop:

(a)

Implications

30/80

1m

fold axial plane

kink fold axis

(b)

N

fold axial plane reverse fault fold axial plane

Folding and thrusting, as seen in southwestern Syros, may indicate a previously not recognized or underestimated compressional event in the Neogene structural evolution of the Cyclades. Young deformation and its influence on the orientation of older fabrics has to be kept in mind when assessing the significance of structures related to the earlier metamorphic and deformation history.

References: [1] Lee, J., Lister, G.S., 1992, Geology 20, 121-124. [2] Boronkay, K., Doutsos, T., 1994, Journal of Structural Geology 16 (11), 1555-1573.

reverse fault

NE

SW

NE

fold axial plane

50 cm

SW

N metamorphic foliation

Fig. 3: Outcrop scale contraction structures N of Komito Beach (location: see Fig. 1). (a): pop up structure. Grey layer is a metabasite intercalated in meta-acidites. (b): top: late brittle kink folds and reverse faults in meta-acidites. bottom: stereographic plot of the data measured in this outcrop. Note the geometric coherence of the general shortening direction between structures shown in Fig. 2 and 3, indicating that they developed in the same general stress environment.

kink fold axes

reverse fault fold axial planes

Num total: 18

Equal area projection, lower hemisphere

Financial support by the DFG is acknowledged