Publication Date:
2021-03-29
Description:
The 1.85 Ga Sudbury Igneous Complex (SIC) in central Ontario is now widely considered to be the erosional remnant of a deformed paleo-horizontal impact melt sheet, about 2.5 km in thickness. Deformed impact melt breccias of the Onaping Formation and postimpact metasedimentary rocks overlie
the layered SIC, which in turn rests on shocked Archean basement and Paleoproterozoic cover rocks. The main
mass of the Igneous Complex is subdivided from top to bottom into granophyre, quartz-gabbro and norite layers. Previous workers considered noncylindrical folding and NW-directed reverse faulting as the main structural processes that formed the asymmetric, syn-formal geometry of the SIC apparent in map view and seismic section. Structural studies support this
model in the southern part of the impact structure, where greenschist-facies metamorphic tectonites of the South
Range Shear Zone (SRSZ) accomplished structural uplift of the southern SIC by NW-directed reverse shearing. However,
little evidence for pervasive ductile strain has been reported from the weakly metamorphosed eastern part of the SIC, the East Range, which is characterised by steep basal dips and maximal curvature in plan view. The objective of this study is to assess the
structural inventory of the East Range in terms of post-emplacement deformation
mechanisms. Our interpretation is
based on published and newly acquired
structural data.
Planar mineral shape fabrics of cumulate
plagioclase and pyroxene are developed
in the intermediate quartz-gabbro
and lower norite layers of the southern
East Range SIC. Microstructures
show little intracrystalline deformation
in quartz. Euhedral cumulate plagioclase
retains an angular outline indicating
magmatic mineral fabric development.
This magmatic foliation is concordant
to SIC contacts or large-scale
discontinuities in their vicinity (Fig. 1).
Magmatic fabrics are observed rarely in
the northern portion of the East Range.
Here, tectonic foliations and S–C fabrics
are developed sporadically at, and
concordant to, brittle structures striking
N–S. A weak tectonic foliation defined
by chlorite that replaces magmatic
minerals is developed in the upper granophyric
SIC of the NE-lobe that connects
the SIC’s North and East Ranges
via a 105° arc. This foliation grades
into a shape-preferred orientation of primary,
i.e., magmatic, mafic minerals observed
in the lower granophyre and underlying
layers of the SIC. Mineral fabrics
observed in the NE-lobe SIC are
concordant to metamorphic foliations
developed in the overlying Onaping Formation
breccias. Both foliations strike
parallel to the NE-Lobe’s acute bisectrix
and, thus, display an axial-planar
geometry typical for fabrics formed in
the core of a buckle fold (Fig. 1). Brittle
structures including centimetre-scale
shear-fractures to kilometre-scale faultzones
are observed in the eastern SIC and its host rocks. Largescale
faults striking N–S cut the NElobe’s
eastern limb causing variable
magnitudes of strike separation of
SIC contacts. Centimetre- to metrescale,
brittle faults and chlorite-filled
brittle-ductile shear-zones occur pervasively
in the eastern SIC, often causing
centimetre-scale offset of markers.
Microstructures from first-order
fault-zones indicate deformation at, and
below, greenschist-facies metamorphic
conditions.
The concordance of magmatic and tectonic
mineral shape fabrics in the NElobe
indicates progressive deformation
of the SIC during cooling from the
magmatic state to lower greenschistfacies
metamorphic conditions. Synmagmatic
deformation of the SIC suggests
that it was emplaced during ongoing
orogenic deformation. Furthermore,
maximum principal stress directions
inferred from inversion of faultslip
data collected in the Onaping Formation
are orthogonal to metamorphic
foliation surfaces at the same localities.
This points to a similar deformation
regime in the Onaping Formation
during ductile and brittle deformation.
The concordance of magmatic,
metamorphic and brittle fabrics is explained
best by a single progressive deformation
event that was active while
the SIC cooled and solidified. The lack
of pervasive ductile deformation fabrics
in the East Range SIC can be explained
by rapid cooling of the impact
melt sheet (within 100–500 ka) with respect
to natural tectonic strain rates.
While the geometry of mineral fabrics
in the study area is compatible with
large-scale, non-cylindrical folding, the
low levels of ductile deformation suggest
that shape-change of the eastern SIC
has been accomplished mainly by discontinuous
deformation. This deformation
mechanism may have accomplished
bulk NW-SE shortening that was accommodated
by reverse shearing within
the SRSZ, resulting in large strike separations
of SIC contacts observed in the
western part of the impact structure.
By contrast, the eastern SIC may have
accomplished such shortening by brittleductile,
non-cylindrical folding at the
eastern terminus of the SRSZ. The complex
post-impact deformation pattern of
the central Sudbury Structure results
from impact into an active orogen.
Description:
conference
Keywords:
551
;
VEV 127
;
VAE 150
;
VKB 280
;
VAE 400
;
Ontario {Geologie}
;
Strukturelle Erscheinungen {Strukturgeologie}
;
Impaktgesteine {Petrologie}
;
Tektogenese {Geologie}
;
Sudbury 〈Ontario, Region〉
;
Mehrfachringbassin
;
Tiefengestein
;
Impaktmetamorphes Gestein
;
Deformation 〈Geologie〉
Language:
German
Type:
anthologyArticle
,
publishedVersion
Format:
application/pdf
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