TY - JOUR
T1 - The Timing of the Cape Orogeny
T2 - new Ar⁴°/Ar³⁹ age constraints on deformation and cooling of the Cape Fold Belt, South Africa
AU - Hansma, Jeroen
AU - Tohver, Eric
AU - Schrank, Christoph
AU - Jourdan, Fred
AU - Adams, David
PY - 2016/4
Y1 - 2016/4
N2 - We conducted a ⁴⁰Ar/³⁹Ar thermochronological study of samples from three transects across the Cape Fold Belt of South Africa. Metamorphic conditions along these transects decrease from the greenschist facies in the southern hinterland to the anchizonal foreland to the north. Nine new ⁴⁰Ar/³⁹Ar plateau ages and two mini-plateau ages from handpicked muscovite are combined with one earlier published biotite mineral age and reveal two age modes. The older population ranges from 261 ± 3 to 276 ± 5 Ma (n = 4) and the younger from 248 ± 2 to 254.6 ± 2.1 Ma (n = 4). The remaining three ages (> 400 Ma) are detrital and reflect unreset ages. Electron microprobe analyses and X-ray diffraction studies indicate little mineralogical variation of micaceous material across the belt; samples dominantly host muscovite, clay minerals such as illite or smectite are absent. Microstructural characterization of rock fabrics across the belt reveals different mechanisms of strain partitioning and heterogenous deformation of different lithologies. Specifically, pressure solution, fracture, and limited crystal plasticity are the dominant deformation mechanisms in the shale-dominated Bokkeveld Group and Witteberg Groups in the foreland region, resulting in recrystallization of phyllosilicate minerals. Strain partitioning into narrow, locally deformed horizons of the Table Mountain Group is accompanied by dynamic grain boundary recrystallization in quartz. A weak trend of southward younging ages is interpreted as the preservation of deformation ages (ca. 275–260 Ma) and younger cooling ages (ca. 255–245 Ma), although it remains plausible that these populations represent two deformation events. These new age constraints significantly narrow the temporal window of Cape Fold Belt tectonic activity, and provide a firm basis for evaluating the depositional patterns in the foreland Karoo Basin.
AB - We conducted a ⁴⁰Ar/³⁹Ar thermochronological study of samples from three transects across the Cape Fold Belt of South Africa. Metamorphic conditions along these transects decrease from the greenschist facies in the southern hinterland to the anchizonal foreland to the north. Nine new ⁴⁰Ar/³⁹Ar plateau ages and two mini-plateau ages from handpicked muscovite are combined with one earlier published biotite mineral age and reveal two age modes. The older population ranges from 261 ± 3 to 276 ± 5 Ma (n = 4) and the younger from 248 ± 2 to 254.6 ± 2.1 Ma (n = 4). The remaining three ages (> 400 Ma) are detrital and reflect unreset ages. Electron microprobe analyses and X-ray diffraction studies indicate little mineralogical variation of micaceous material across the belt; samples dominantly host muscovite, clay minerals such as illite or smectite are absent. Microstructural characterization of rock fabrics across the belt reveals different mechanisms of strain partitioning and heterogenous deformation of different lithologies. Specifically, pressure solution, fracture, and limited crystal plasticity are the dominant deformation mechanisms in the shale-dominated Bokkeveld Group and Witteberg Groups in the foreland region, resulting in recrystallization of phyllosilicate minerals. Strain partitioning into narrow, locally deformed horizons of the Table Mountain Group is accompanied by dynamic grain boundary recrystallization in quartz. A weak trend of southward younging ages is interpreted as the preservation of deformation ages (ca. 275–260 Ma) and younger cooling ages (ca. 255–245 Ma), although it remains plausible that these populations represent two deformation events. These new age constraints significantly narrow the temporal window of Cape Fold Belt tectonic activity, and provide a firm basis for evaluating the depositional patterns in the foreland Karoo Basin.
KW - Cape Fold Belt
KW - Argon–Argon
KW - Geochronology
KW - Gondwanide Orogeny
UR - http://purl.org/au-research/grants/arc/LP0991834
UR - http://www.scopus.com/inward/record.url?scp=84925780468&partnerID=8YFLogxK
U2 - 10.1016/j.gr.2015.02.005
DO - 10.1016/j.gr.2015.02.005
M3 - Article
VL - 32
SP - 122
EP - 137
JO - Gondwana Research
JF - Gondwana Research
SN - 1342-937X
ER -