Transcript Origin of Sudbury Impact Structure

Sudbury in the Context of Impact
French, 1998
One minute after the end of the Cretaceous!
How Common are Impact Craters on Earth? We all know how abundant
they are on the Moon where erosion and tectonic activity have not been
able to erode them.
On Earth there are about 200 reliably identified impacts but only a few are
really large.
After Dence, 1991
What Happens During an Impact?
First let’s look at a relatively small crater and the
various stages of its development
1. During the Contact/Compression Stage the projectile penetrates to a
depth approximating its diameter and initiates a supersonic shock wave
that propagates through the target rocks.
Projectile
Shock Wave
French, 1998
2. During the Compression Stage, the shock wave expands further
followed by a rarefaction or pressure release wave. material is physically
moved downward and outward away from expanding crater
Ejecta
Rarefaction or Release Wave
Shock Wave
material flows away from expanding crater
French, 1998
3. During the Excavation Stage, material is removed from the crater as an
ejecta curtain and also as vapour. A small melt sheet may form as a
lining on the cavity wall depending on the size of the crater. material
continues to be displaced downward and outward from the crater walls.
Vapour Plume
Melt Sheet
French, 1998
Ejecta Curtain
4. At the end of the Excavation Phase, the so-called transient cavity has
reached its maximum size
Transient Cavity
French, 1998
5. The Modification Stage represents the final phase of crater
development. Almost instantly, the transient cavity starts to be filled in by
centripetal slumping of material from the upper part of the crater. The
ejecta curtain collapses to form an ejecta blanket on neighbouring
surfaces and major fracture zones develop adjacent to the original crater
wall.
Ejecta
Centripetal Slumping
Crater Fill
Zone of
strongly
fractured
rock
French, 1998
Here is another image of a small crater
French, 1998
And here is an image of a small, simple bowlshaped crater on the moon.
NASA Website
What Happens during a larger impact?
Essentially it’s the same sequence of
stages that we have examined for small
craters with one major exception.
This summarizes what happens
Impact Site
Ejecta
Ejecta
Melt
Vapourized
Ejected
Ejected
Displaced
2. Excavation Stage (I’ve omitted Stage 1)
Ejecta
Rarefaction/release wave
Melt
material Flow
Shock Wave
French, 1998
3. End of Excavation Stage. Here’s the difference! In sufficiently large
impacts, the material in the center of the transient cavity starts to
rebound in what’s termed the Modification Stage.
Ejecta
material Flow. Initiation of
Central Uplift
French, 1998
4. And here’s the final structure after the completion of the Modification Stage.
There is a well-developed central uplift within a relatively wide flat crater which has
been modified by zones of marginal collapse/slumping. There is also a welldeveloped melt sheet. If the impact is really large, the ejecta layer can be traced all
around the globe (e.g. The K-T boundary layer from the Chixelub Impact).
Melt Layer
French, 1998
Central
Uplift
Marginal
Slump
Blocks
Ejecta
Layer
Another view of one half of a larger complex crater with a central
uplift and a melt sheet
Crater Rim
Central Uplift
Centripetal Faults
Fracture Zone beneath crater floor
French, 1998
Here’s a view of lunar crater Tycho, with a well-developed central
uplift. You can see the slumping that is taking place around the
circumference of the crater.
NASA Website
Another lunar crater being modified by exceptionally welldeveloped slump features. These may be slumping into the crater
along listric normal faults.
Possibly analogous to “superfaults” as proposed by Spray and
others
NASA Website
Even larger impacts form what are termed peak-ring craters where
the central uplift forms a well-defined ring. This is a view of the
Shrodinger crater on the moon.
Crater Rim
Outer Melt Sheet
Peak-Ring Structure
Inner Melt Sheet
NASA Website
Sudbury in the Context of a Peak Ring Crater
Outer Melt Ring
now Eroded
Except for
Offset Dykes
Central Melt
Sheet
(Preserved
SIC)
Peak Ring
Ames and Farrow
What Tells us We’re Looking at an Impact?
Indicators of Impact
– Microscopic Indicators (Shock Metamorphism)
• Kink Banding
• Planar Deformation Features (PDF’s)
• Destruction of Crystal Structure and Formation of Diaplectic
(Thetomorphic) Glasses
• Formation of High Pressure Mineral Phases (Quartz
Polymorphs, Diamond etc)
• Vesiculation and Formation of Melt
– Megascopic indicators (Field Geology)
• Shape
• Shatter Cones
• Breccia
• Ejecta Blankets
• Melt Sheets
Shock Metamorphism
What is Shock Metamorphism and why is it
different from normal metamorphic processes?
• Shock metamorphism refers to the various structural and
phase changes that occur in minerals during the passage of a
hyper-velocity shock wave.
• It is characterized by ultra-high pressures and temperatures
imposed during an extremely short period of time (fractions of
a second?).
• This differs from normal metamorphism where the effects are
active over a period of years (thermal effects adjacent to small
igneous intrusions) to millions of years (orogenies).
• The effects of shock metamorphism are thus due to nonequilibrium processes and this is reflected by the often erratic
distribution of these effects.
P-T Variables Comparing
Normal Metamorphism vs Shock Metamorphism
Field of “Normal”
Metamorphic P-T
Conditions
French, 1998
Stages of Shock Metamorphism
Possible Field of
“Normal”
Metamorphism
Low (Megascopic)
Shock Effects
PDF’s Start to Form
High Pressure Phases
Melting, Vesiculation etc
French, 1998
Comparison of P-T conditions, strain rates and reaction times for
various processes under regional (left) and shock (right)
metamorphism
Here are some photomicrographs illustrating
some of the more common petrographic
indicators of shock metamorphism
Kink-Banding in Biotite
French, 1998
Development of new mineral phases along pre-existing cleavages.
In this case the development of an iron oxide phase in
hornblende.
Development of multiple sets of planar deformation features
(PDF’s), often best developed in quartz but also common in other
minerals such as plagioclase.
PDF’S in slightly annealed rocks are commonly found as linear
arrays of fluid inclusions (these are known as decorated PDF’s).
With increasing degree of annealing the PDF’s become
progressively more diffuse (right hand picture) and will eventually
disappear. This has happened on the South Range of Sudbury.
Increasing intensity of shock metamorphism can lead to
destruction of the crystal structure of minerals. In this example
plagioclase (colourless phase in the left diagram has been
converted to isotropic maskelynite (black phase in the right
diagram) without disturbing the igneous texture of the rock.
Eventually, the target rocks are subject to wholesale melting and
vesiculation
Megascopic Indicators
Lithic breccias form annular rings adjacent to and under large impacts.
They are most commonly formed in the floor of the crater and thus their
distribution gives a general idea of the original size of the structure
This example is from the Vredefort Structure in South Africa.
Lithic breccias like this consist of randomly oriented clasts of local
country rock set in a very fine-grained matrix. The matrix can be either a
v.f.g. igneous-textured rock, glass or, very commonly, fine-grained rock
flour or cataclasite. This example is from Sudbury.
This map shows the general distribution of Sudbury Breccia
around the Sudbury Basin. Although this map suggests that
breccia mostly occurs within 10 km of the SIC, occurrences of
Sudbury Breccia have been reported more than 100 km from the
SIC – this suggests that the Sudbury Structure represents a very
large impact indeed with a diameter in excess of 200 km.
Sudbury Breccia
Ames et al, 2005
Shatter Cones
Anomalous Ir
Shatter cones are a more or less ubiquitous phenomenon at welldocumented impact sites and their presence has become an
expected criterion to their identification. The identification of
shatter cones at Sudbury by Robert Dietz in 1964 was the first real
indication of an impact origin for the Sudbury Structure.
French, 1998
This is a photograph of the original discovery site of shatter cones
at Sudbury.
This slide shows the distribution and orientation of shatter cones
around the Sudbury Basin. In undeformed impacts, cones point in
and upwards towards the point of impact and, if post-impact
deformation at Sudbury is removed, this is generally true here
also.
Naldrett (after Bray)
Distal Ejecta Sheets
Perhaps the most obscure megascopic indicator of an impact is
the discovery of distal ejecta horizons. Here is an example of the
probable ejecta from Sudbury which has recently been found in
Michigan, Minnesota, Western Ontario and possibly in Greenland.
The ejecta here is about 1 m thick.
Addison et al, 2005
This diagram shows the locations where the ejecta has been found as far
as 850 km west of the Sudbury Structure. Bear in mind that the ejecta
from the K-T impact at Chixelub has been found all around the world,
even if it is only a few mm thick in places.
Distal Ejecta Sites
After Addison et al, 2005
Impact Site
Here is the stratigraphic column showing the location of the ejecta layer
at the contact between the Gunflint and Rove Formations of the Penokean
Supergroup. Note the ages of the two dated tuff horizons at 1875 and
1836 Ma which bracket the age of the Sudbury impact at 1850 Ma.
After Addison et al, 2005
Dated Tuff Layers @ 1875 Ma & 1836 Ma
Although the ejecta sheet has not yet been radiometrically dated, its age
is determined as lying between that of tuff horizons found above and
below the ejecta. Here is a zircon concordia plot showing an age of ~1827
Ma for a tuff lying above the ejecta.
Addison et al, 2005
What’s the definitive proof that you’re actually looking at an
impact ejecta sheet? You look for evidence of shock
metamorphism such as this well-developed set of PDF’s found in
Minnesota.
Addison et al, 2005
So How Does All This Fit at Sudbury?
Paleo-setting of the Sudbury Structure
(The Red Units weren’t there at the time!)
At the time of
Impact (1850Ma),
the Superior
Province was
surrounded by
passive and active
marginal belts.
OOPS! What
Happened Here?
Tectonic-Structural-Metallogenic
History of the Sudbury District
Hydrothermal
Wanapitei
Sedimentation
Intrusions
Tectonics
Impacts
37 Ma
Magmatic
Mineralization
Grenville Orogeny is
Much Later and has
Nothing to do with
Sudbury Structure
Sudbury
1851 Ma
Penokean - Mazatzal
Orogenies
Blezardian Orogeny
Rifting and Start of
Huronian Deposition
After Ames and Farrow
The Situation Prior to the Blezardian Orogeny in the Sudbury area.
A thick sequence of basal volcanics is overlain by a southward
thickening wedge of clastic sediments. The basal portion of this
Huronian sequence is intruded by early mafic and felsic plutons.
Creighton and
Murray Granitoid
Intrusions at
~2400 Ma
East Bull and
River Valley Type
Intrusions at
~2450Ma
After Dutch
Blezardian (~2300 Ma) and early Penokean (start ~1900 Ma)
orogenic events deform the northern edge of the Huronian in the
Sudbury Area.
East Bull
Plutons
After Dutch
A large impact excavates a large transient cavity, perhaps 100 km
in diameter and 30 km deep. Material flow overturns rocks along
the crater wall
After Dutch
The overturn is complete. Rebound during the early modification
stage forms a peak ring crater. Impact melt floods the crater and is
capped by proximal fallback material (Onaping Formation)
After Dutch
Continued crater collapse and coeval late Penokean and even
later deformation events continue to modify the shape of the
impact site. Northward directing thrusting of uncertain age and 2
billion years of erosion complete the picture.
After Dutch
Leaving us with this!!
Riller
The Sudbury Impact Structure
Chemistry of the main mass of the SIC
Whole rock chemistry can also be used to track changes through the SIC. Here are
a number of plots showing selected major and trace element variations. Note the
abrupt, but gradational, changes that take place across the norite-gabbrogranophyre contacts (the so-called transition zone). Some trends (MgO, TiO2,
P2O5) may suggest crystallization from roof for the upper part of the granophyre?
SiO2
Mg-number
Lightfoot
MgO
Nickel
TiO2
P2O5
Sulphur
Copper
Enrichment in Nickel Sulphur and Copper towards base
Here are felsic norite normalized plots for mafic norite, quartz gabbro and
granophyre. The flat-lying character of the plots supports a co-magmatic
origin for all units of the main mass of the SIC.
Lightfoot et al, 1997
Here are some lithochemical data for the Offset Quartz Diorite phase of the SIC
normalized to average Upper Continental Crust (REE on left; extended element
spidergram on right). The generally flat profiles near a QD/UCC ratio of 1 suggest a
possible derivation of the SIC melt sheet from average crust.
AllQD_UCC_REE
Rock/UCC
10
Upper Cont. Crust REE Taylor-McLennan 1985
Upper Cont. Crust SPI. Taylor-McLennan 1985
1
1
.1
Rock/UCC
10
All QD with Grand Average
.1
All QD with Grand Average
.01
.01
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Cs Rb Ba Th U K Ta Nb La Ce Sr Nd Hf Zr Sm Ti Y Yb Lu
Here are the same data normalized to a North American Shale Composite.
Again the flat-lying profile near a QD/NASC value of 1 support a crustal
origin for the SIC melt sheet.
Rock/NASC
10
North American Shale Comp REE Gromet-1984
1
.1
.01
All QD's with Average
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Shape
•
•
•
•
•
Over the years many have commented on the lack of circularity of the
Sudbury Basin. There are good reasons for this lack.
As we have seen, the Sudbury Structure has been affected by a
number of post-impact orogenic events.
Not all impact craters are circular! The Barringer Crater in Arizona is
almost square and many terrestrial and extra-terrestrial craters have
polygonal outlines.
This may be due to immediately post-impact modification processes
such as slumping of blocks of crater rim material into the transient
cavity, possibly along structures akin to what Spray (1997) has termed
“super-faults”. Or possibly due to structural features inherent in the
target rocks which might control some of the shape.
Most likely, most of the lack of circularity is due to later episodes of
compressive tectonism.
Here, for example, is are a sketch and aerial view of the
conspicuously square Barringer Crater.
French, 1998
Here is a simple sketch of how centripetal crater collapse could
have affected the shape of the Sudbury structure. Blocks of crust
slide into the partially melt-filled crater along listric normal faults.
After Thompson, 1991
This is the latest model for the shape of the Sudbury Basin based
on results from seismic profiling across the basin by Lithoprobe.
The model has been constrained to fit near surface drill results
and the trace of prominent seismic reflectors.
Wu and Milkereit, 1994
Alternatives have been proposed! Here we see two variants: the upper
section is as the previous slide, proposed by Wu & Milkereit – the lower
section was proposed by Card & Jackson. As you can see it is very
similar to the old folded sill model. Which is more nearly correct?
This figure shows a comparison between observed and calculated
gravity profiles across the Sudbury Basin. The calculated profile
is based on the seismic interpretation of Wu & Milkereit. The
agreement is excellent.
Similarly, this figure shows a comparison between observed and
calculated magnetic profiles across the Sudbury Basin. The
calculated profile is based on the seismic interpretation of Wu &
Milkereit. The agreement here is also excellent.
This plan view shows the nature of the presently visible
megascopic structural elements which have affected the shape of
the basin.
This is a 3-D block model of the same structural elements at the
east end of the basin. The agreement with the seismic
interpretation is excellent.
A Few Key References
•
French, B.M., 1998; Traces of Catastrophe; a handbook of shock-metamorphic
effects in terrestrial meteorite impact craters. LPI Contribution #954. (Impact and
shock metamorphism)
•
•
•
•
•
•
Addison, D.A. et al; Discovery of distal ejecta from the 1850 Ma Sudbury impact
event. Geology, 2005, V33, p 193-195. (Distal ejecta)
Spray, J.G., 1997;Superfaults. Geology, V25, p 579-582. (Crater Collapse mechanism)
Scott, R.G. and Spray, J.G., 2000; The South Range bre ccia belt of the Sudbury
impact structure: A possible terrace collapse feature.
Ames, D.E. et al, 2005; Sudbury bedrock compilation; map and digital tables:
GSC Open File 4570. (Sudbury geology)
Cowan, E.J., Riller, U., and Schwerdtner, W.M., 1999; Emplacement geometry of
the Sudbury Igneous Complex: Structural examination of a proposed impact
melt-sheet, in Proceedings Volume from the LPI 1997 Sudbury Conference:
Large Meteorite Impacts and Planetary Evolution. (Structure and deformation)
Milkereit, B., White, D.J., and Green, A.G., 1994; Towards an improved seismic
imaging technique for crustal structures: The LlTHOPROBE Sudbury
experiment: Geophysical Research Letters, v. 21, no.10, p. 927-930. (Seismic profiling)