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| | | | Geological Survey of Western Australia |
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| | | Mulka Tectonic Event (MK) | SP Johnson, S Sheppard, and FJ Korhonen | | | | | Event type | deformation: transpressional | Parent event | | Child events | | Tectonic units affected | | Tectonic setting | orogen: intracratonic orogen | Metamorphic facies | | Metamorphic/tectonic features | C-S fabric; mylonitic |
| | | Summary | The Mulka Tectonic Event is responsible for a series of anastomosing shear zones or faults that cut rocks of the Gascoyne Province and Edmund and Collier Groups across the southwestern part of the Capricorn Orogen. This tectonic event is characterized by fault reactivation, rather than reworking. The largest of the fault systems is the Chalba Shear Zone – Clere Fault. The shear zones and faults display consistent dextral strike-slip kinematics. Structures attributed to the Mulka Tectonic Event cut c. 755 Ma dykes of the Mundine Well Dolerite. White mica in the S-planes of an S–C fabric in the Chalba Shear Zone has been dated in thin sections using the ⁴⁰Ar/³⁹Ar method, yielding a single age of 570 ± 10 Ma. The Mulka Tectonic Event is coeval with the Petermann, Paterson, and King Leopold Orogenies, and reflects an episode of ‘pan-Gondwana’ intracontinental reactivation. | | | Distribution | The Mulka Tectonic Event comprises narrow shear zones that transect the Gascoyne Province, and Edmund and Collier Basins over a wide area. Shear zones attributed to this event have been identified as far north as the Mangaroon Syncline on MANGAROON, and south to the Chalba Shear Zone on YINNETHARRA. Where faults and shear zones reactivated during this event do not cut the 755 Ma Mundine Well Dolerite, they may be incorrectly attributed to one of the numerous Paleoproterozoic or Mesoproterozoic tectonic events that affect the western Capricorn Orogen. | | | Description | Rocks of the Gascoyne Province and Bangemall Supergroup are cut by a network of anastomosing shear zones with dextral strike-slip kinematics, which are attributed to the Mulka Tectonic Event. This event is marked by fault and shear zone reactivation, rather than pervasive reworking. Faults and shear zones active during this event range from a few centimetres to several tens of metres wide, and strike predominantly between east and southeast (a few trend northeast). In many (?most) instances this Neoproterozoic fault movement marks a reactivation of Paleoproterozoic or Mesoproterozoic structures.
Probably the most spectacular expression of the Mulka Tectonic Event is the Chalba Shear Zone – Clere Fault, a major structure in the western part of the Capricorn Orogen. On YINNETHARRA and LOCKIER, the Chalba Shear Zone comprises numerous, anastomosing narrow mylonites in a zone up to 5 km wide. The shear zone truncates a swarm of dykes trending north- to north-northeast, which belong to the Mundine Well Dolerite. Dextral offset of dolerite dykes is indicated across many individual mylonites, consistent with widespread lineations and sense-of-shear indicators. At one set of creek pavements adjacent to the Dairy Creek – Cobra Road on YINNETHARRA (SXSYIN6272, Zone 50, MGA 399994E 7257799N) strongly sheared porphyritic biotite monzogranite contains a subhorizontal mineral lineation in conjunction with climbing pegmatite veins, asymmetric folds, S–C fabrics, sigma porphyroclast tails, and extensional shear bands, all indicating dextral sense of shear.
Elsewhere in the Gascoyne Province, several southeast-trending faults within the Mangaroon Syncline on MANGAROON offset dykes of the 755 Ma Mundine Well Dolerite. A set of east-southeast- and southeast-trending mylonites also cut the Minnie Creek batholith on EUDAMULLAH and MOUNT PHILLIPS. These mylonites consistently have a mineral lineation plunging shallowly to the southeast, which is associated with S–C fabrics and sigma porphyroclasts indicating dextral shear. The mylonites are associated with the growth of fine-grained chlorite and sericite or muscovite. | | | | | | Geochronology | | | Mulka Tectonic Event | Maximum age | Minimum age | Age (Ma) | 570 ± 10 | 570 ± 10 | Age | Neoproterozoic | Neoproterozoic | Age data type | Isotopic | | References | | Bodorkos and Wingate (2007) |
| | Bodorkos and Wingate (2007) |
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| Only one date has been determined for the Mulka Tectonic Event. The following sample description and interpretation has been provided by Bodorkos and Wingate (2007). White micas were sampled from the Chalba Shear Zone on YINNETHARRA. The sample locality shows well-developed S–C fabrics and other shear sense criteria, indicating dextral strike-slip movement. White micas in different textural associations were dated in thin sections with an infrared laser using the ⁴⁰Ar/³⁹Ar method. Five analyses of coarse-grained crystals from C-planes yielded widely variable dates of 2790–630 Ma. They probably reflect a combination of heterogeneously distributed excess Ar and partial resetting of the Ar–Ar systematics. However, the fine-grained S-plane material yielded a single date of 570 ± 10 Ma. This reactivation may be associated with the development of dextral shear zones that offset dykes of the 755 Ma Mundine Well Dolerite. | | | Tectonic Setting | The Mulka Tectonic Event represents an episode of intracontinental reactivation coeval with other ‘pan-African’ or ‘pan-Gondwana’ events between 650 Ma and 500 Ma that reflect amalgamation of Gondwana (e.g. Veevers, 2003). Events of similar age in Australia include the Petermann Orogeny in central Australia, and the Paterson and King Leopold Orogenies in northern Western Australia. | | | | References | Bodorkos, S and Wingate, MTD 2007, The contribution of geochronology to GSWA's mapping programs: Current perspectives and future directions, in GSWA 2007 extended abstracts: promoting the prospectivity of Western Australia: Geological Survey of Western Australia, Record 2007/2, p. 10–11. View Reference | Veevers, JJ 2003, Pan-African is Pan-Gondwanaland: Oblique convergence drives rotation during 650-500 Ma assembly: Geology, v. 31, no. 6, p. 501–504. |
| | | | Recommended reference for this publication | Johnson, SP, Sheppard, S and Korhonen, FJ 2022, Mulka Tectonic Event (MK): Geological Survey of Western Australia, WA Geology Online, Explanatory Notes extract, viewed 19 October 2025. <www.dmp.wa.gov.au/ens> |
| | | This page was last modified on 24 February 2022. | | | | | Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are referenced using Map Grid Australia (MGA) coordinates, Zones 49 to 52. All locations are quoted to at least the nearest 100 m. Capitalized names in text refer to standard 1:100 000 map sheets, unless otherwise indicated. WAROX is GSWA’s field observation and sample database. WAROX site IDs have the format ‘ABCXXXnnnnnnSS’, where ABC = geologist username, XXX = project or map code, nnnnnn = 6 digit site number, and SS = optional alphabetic suffix (maximum 2 characters). All isotopic dates are based on U–Pb analysis of zircon and quoted with 95% uncertainties, unless stated otherwise. U–Pb measurements of GSWA samples were conducted using a sensitive high-resolution ion microprobe (SHRIMP) in the John de Laeter Centre at Curtin University, Perth, Western Australia. Digital data related to WA Geology Online, including geochronology and digital geology, are available online at the Department’s Data and Software Centre and may be viewed in map context at GeoVIEW.WA. | | | Further details of geological publications and maps produced by the Geological Survey of Western Australia are available from: Information Centre Department of Mines, Industry Regulation and Safety 100 Plain Street EAST PERTH, WA 6004 Telephone: +61 8 9222 3459 Facsimile: +61 8 9222 3444 |
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