Deformation and Melting in Delamerian Migmatites, Kangaroo Island, South Australia

 

Coastal outcrops on the south coast of Kangaroo Island expose the interaction between deformation and migmatization of rocks of the Kanmantoo Group. We document four deformation phases, all of which associated with anatexis that took place during the Delamerian Orogen. These phases and their progression were defined by combining the nature of the deformation, orientation and overprinting relationships: D1 and D2 are both related to crustal shortening, at different orientations, and regions dominated by D2 are characterized by a well-defined, folded early lineation trending at high angles to the D2 fold axis. D3 and D4 are both extensional at high angles to one another, both overpring D1 and D2 folds, and D4 is better developed and overpring D3.

D1 involved thrusting and isoclinal folding with main fabric deformed by subsequent deformations. D1 structures overprint earlier structures, but we interpret this to be related to the evolution of the same shortening event, where new gently dipping thrusts overprint steepened locked thrusts and folds. In places (Six Mile Lagoon, 6ML6), banded migmatites are transposed in a zone over 50 m wide and trend parallel to the axial plane of D1. D1 gave rise to an intense fold axis parallel lineation that tends to plunge either gently NNE or SSW as it is folded around D2 open folds.

D2 is dominated by gentle, upright folds trending N60-80W, locally leading to transposition of pre-existing fabric (VB2 area) in melt-rich area, very similar to the pattern in D1 but in a different orientation. L1 is folded around the hinges of metric D2 folds, associated with intrafolial isoclinal F1 folds with the same fold axis.

D3 gently to steeply dipping N-S trending normal shear zones intruded by narrow leucosomes parallel to C-S fabric (VB6).

D4 moderatly to steeply dipping normal and dextral oblique shear zones, either intruded by pegmatites or associated with sheared diatexites. There is a large zone of diatexite (Six Mile Lagoon, 6ML1) characterized by N60E-trending dextral-normal shearing during melting that is ascribed to D4 based on orientation and movement sense, and lack of later overprinting.

This section is divided into descriptions of different areas where key relationships are best exposed.

 

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Part I. Outcrop Vivonne Bay VB6: Four deformation phases

Figure Ia) Open D2 fold intruded by a dyke, with an axial plane trending parallel to the central dyke (060/sv).	Figure Ib) D2 fold limb overprinted by a normal D3 shear zone trending N10E/75E.

Figure I CENTRAL) Click on it for a full geological map. General Outline of the Outcrop at VB6: this outcrop has a well-defined set of open, upright, gently plunging D2 folds, cross cut by a D3 shear zone, that in turn is cross cut by D4 shear zones. D1 is locally preserved on F2 limbs. Boxes indicate position of the photographs Fig. 1 a to d)

Figure Ic) Normal D3 shear zone trending N10E/75E overprinted by D4 normal shear zone.	Figure Id) Open D2 fold overprinted by a D4 normal shear zone on its northern boundary (looking E).

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D1 thrusting and isoclinal folding: the features above overprint and refold small scale D1 folds

 

Figure If) Tight folds of the interlayered psammite-pelite of the Kanmantoo Group. Note what seems to be a refolded fold. Thrust plane with melt= N08E/38W. Vertical surface.	Figure Ig) Tight D1 folds. Vertical surface. Looking down plunge of fold axis 037/30.

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D4 Normal Shear Zones Trending N60-80E

 

 

Figure Ih) Isoclinal folds in stromatic migmatite, overprinted by a conjugate set of normal folds related to D4. Vertical surface.	Figure Ii) Hinge of a D2 overprinted by a conjugate set of normal D4 faults filled with granite (vertical exposure looking E).
Figure Ij) Conjugate set of normal faults filled with granite (vertical exposure looking E).	Figure Ik) Conjugate set of normal faults filled with granite (vertical exposure looking E).

 

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Part II- Vivonne Bay-Cape Elen VB4: Magma breaking through apex of doubly plunging anticline

Figure IIa) Magma pool at the hinge of the doubly plunging anticline, looking south.	Figure IIb) Detail of a.

Figure II CENTRAL). Doubly plunging anticline to East and West indicated by thick black arrows, with magma breaking through the apex.

Figure IIc) Ripped margin of metatexite: margin of the magma pool (looking west).	Figure IId) Ripped margin of metatexite: margin of the magma pool (looking northwest).	Figure IIe) Branching channel linking with magma pool to the right (looking west).

 

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Part III- Six MIle Lagoon: Diatexite flow, extraction and deformation during D4

Figure IIIa) Schollen with asymmetric shape indicative of dextral shearing and broken up in situ.	Figure IIIb) Channels converging upwards.	Figure IIIc) Channels funneling upwards and stretching schollen.

Figure III CENTRAL) Field sketch of sheared diatexite in Six Mile Lagoon. a) Dextral shearing on N60-70E of schollen preserved in magma-rich diatexite, b)Funnel-shaped N-directed channels draining magmas from same diatexite a few meters to the right (East) of (a). This outcrop has therefore two parallel processes occurring simultaneously during anatexis: dextral shearing on N70E and melt extraction through channels trending N-S. Boxes mark details shown on photographs.

Figure IIId) Sheared and broken up trailing edge of schollen.	Figure IIIe) Flow asymmetry at the side of a larger schollen to the right.	Figure IIIf) Flow asymmetry around a schollen indicative of dextral shearing.

 

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