IDENTIFYING IMPACT STRUCTURES, Part VII - SHOCK METAMORPHISM
An obvious craterform is an excellent indicator of a possible impact origin; particularly, if it has the appropriate morphometry as illustrated in Part II of this series. But as noted, such features are rare and short-lived in the terrestrial environment. The burden of proof for an impact origin generally lies with the documentation of the occurrence of shock-metamorphic effects. Impacts produce distinctive "shock-metamorphic" effects that are found in situ within the crater and allow impact sites to be distinctively identified. Such shock-metamorphic effects, in addition to the shatter cones (documented in Parts V & VI), include brecciated rocks, suevites, impact melts and pseudotachylites. They attest to the destructive power of the impact event.
The rocks at the target site are melted, shattered, and mixed during the impact explosion. When the site finally settles and cools, a new composite rock, impact breccia in bodies tens to hundreds of meters in size, is the result. Lithologies showing these unique diagnostic shock effects, formed at pressures ≥10 GPa, tend to be restricted to two locations: (1) crater-fill materials (suevites, melt breccias, and fragmental impact breccias) deposited in the crater; and (2) brecciated basement rocks, often containing shatter cones, near the center of the structure. The magnitudes of the impact shock relative to the point of impact that form the shock metamorphic effects were quantified for reference in Part V.
Starting with this article, I will document the impact shock metamorphic effects found within impact craters, starting with the Manicouagan Crater.
Approximately 214 million years ago an estimated 10 kilometre wide hypervelocity meteorite impacted at between 12 and 30 kilometres per second and formed the Manicouagan Impact Structure.
The resultant 100 kilometre diameter crater (image left - Courtesy NASA/LPI) is one of the largest impact craters still preserved on the surface of our planet. The Copernicus crater on the moon (image right - Courtesy NASA - A12) has a diameter of 93 kilometres. For comparison, the Manicouagan's annular moat would fit comfortably within the rim of the Copernicus Crater.
Morphological elements of the Manicouagan structure are based on topographical expression and are:
- outer circumferential depression - ~150-km outer diameter;
- outer disturbed zone - ~150 km diameter;
- inner fractured zone - ~100 km outer diameter;
- annular moat - ~65 km outer diameter;
- inner plateau - ~55 km outer diameter;
- central region - ~25 km outer diameter. (Grieve, Head 1983)
In the summer of 2006, Eric Kujala and I explored the interior of the Manicouagan impact structure by canoe and on foot. We entered the structure from the east, crossing the all these morphological elements and concluding in the Memory Bay inlet. This inlet is on the east portion of the island forming the central peak of the structure. To read about our harrowing experience while in the crater please see my web site about the trip (O'Dale 2006).
At the point of impact, the country rocks were instantaneously evaporated/melted/shattered by the energy released leaving a 200 to 600 cubic kilometre sheet of impact melt directly on basement rocks. We observed changes in these impactite textures as a progressively increasing proportion of superheated melt and a decreasing fraction of cold fragmented country rock material toward the interior of the crater (Simonds 1976). The following images will illustrate these observations.
Outer Circumferential Depression, Outer Disturbed Zone and Inner Fractured Zone
In Part IV of this series I documented the shattered rock we encountered at approximately 40km from the central peak as we entered the outskirts of the crater. We did could not identify the outer circumferential depression.
The water filled circular annular moat that is prominent in space images is only one third of the size of the original crater. The water in the annular moat fills a ring where impact-brecciated rock was eroded away by glaciation. Before flooding of the reservoir, isolated outcrops of tilted and deformed limestone, siltstone and shale were found on the inner edges of the moat (Murtaugh, 1975).
This rock formation is found at the extreme eastern portion of the annular moat on one of the small islands. Note the rock structure is breccia free gneiss. The central peak of the structure is visible over 10 km in the distance.
The Inner Plateau of the Manicouagan structure is bounded by the annular moat, overlain by melt sheet, underlain by shocked basement rock (Orphal, Schultz 1978). We found a "lunar landscape" here containing various breccia types. The astronauts exploring the moon found that impact-melt breccias, similar to what we found here, were the most common rock types at the Apollo highland sites (Apollos 14, 15, 16 and 17) (Haskin 1998). We documented impact breccias formed by similar and very different country rocks like those found on the moon!
Impact breccias were melted, mixed, crushed and compressed by shock waves at various stages in the cratering process: (1) during the initial shock-wave expansion and transient crater formation; (2) during the subsequent modification of the transient crater. Even within the brief formation time of an impact crater, it is possible for the multiple generations of breccia to develop and to produce distinctive differences, even though the time between one breccia generation and the next may be measured in seconds or minutes (French 1998). The extremely small size of the grains within the matrix between the country rock fragments were formed by the very high pressure of the gas generated when the bolide impacted.
This photo illustrates a breccia outcrop found within the inner plateau area of the Manicouagan Impact Structure. Note the different types of rock fragments forming the breccia within the fine grained matrix impact melt.
This images documents "uniformly white" shattered country rocks imbedded in a fine grained matrix impact melt. This breccia outcrop is found in an inlet, cut into the central peak of the impact structure, known as Memory Bay.
Further west into Memory Bay I noticed a possible shock pseudotachylite vein within a breccia outcrop. The pseudotachylite veins associated with impacts are much larger than those associated with faults and are thought to have formed by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift. In Part IV of this series I illustrated the pseudotachylite found in the Sudbury Impact Structure. Impact related pseudotachylite was first recognized at the Vredefort crater in Africa.
The Central Region of the Manicouagan Structure is a complex zone of uplifted, shocked and metamorphosed basement rocks with small tabular bodies of impact melt and pseudotachylite veins (Orphal, Schultz 1978). Recent U-Pb zircon dating of the impact melt gave an age of 214 ± 1 million years.
The illustrated impact melt cliff and talus (debris at the base of the cliff) is found in the central region area of the Manicouagan Impact Structure. It is composed of target rock that was made temporarily molten from the energy released during impact. There are not any detectable meteorite components in the Manicouagan structure melt rock (Palme et al., 1978).
I tried to climb the talus slope up to the cliff face but it became very unstable the higher I climbed. I got to the point that I was creating dangerous rock slides without making any progress. I stopped to take this picture; looked down and found the "Manicouagan shatter cone" I documented in Part VI of this series. Serendipity at its best!
While we explored the impact melt cliffs on the north shore of Memory Bay we noticed an odd feature in one of the cliffs. Eric took this image of the feature. It shows a 10 m block of mafic gneiss (indicated in the image) suspended about 20 m above the base of the melt sheet. Such a block is 0.3g/cm³ denser than the melt and should settle at a minimum of 5 cm/sec (Stokes Law) through a Manicouagan composition melt with 2% H2O (water) if it were still liquid at 1000°C. In order for that block to remain suspended, the melt must have begun to crystallize rapidly enough to trap the block before it settled to the bottom of the sheet (Simonds 1976).
Documented in this article were impact metamorphic rocks found in the Manicouagan impact structure to illustrate shock metamorphism in an impact crater. Separately, each of these shock metamorphic features could be explained by naturalistic means (other than impact), but taken together they strongly suggest evidence for an impact. Similar rock formations found in other craters and will be documented in future articles.
Grieve and Head, 1983. R.A.F. Grieve and J.W. Head, The Manicouagan impact structure: An analysis of its original dimensions and form.
Dence, M. R. 1976 The Manicouagan impact structure. NASA Spec. Pub.
French, Bevan M. 1998. Traces of Catastrophe, A handbook of Shock-Metamorphic effects, Lunar and Planetary Institute.
Haskin, L et al 1998, The case for an Imbrium origin of the Apollo thorium-rich impact-melt breccias. Meteoritics & Planetary Science, vol. 33, no. 5, pp. 959-975.
Murtaugh, J.G. 1972, Shock metamorphism in the Manicouagan cryptoexplosion structure, Quebec. Proc. 24th Int. Geol. Congr.
O'Dale, C.P. 2006; Manicouagan Impact Structure
Orphal, D & Schultz, P, An alternative model for the Manicouagan impact structure. Proc Lunar Planet Sci Conf 1978.
Simonds, C.H. et al 1976, Thermal model for impact breccia lithification: Manicouagan and the moon. Proc. Lunar Sci. Conf. 7th (1976) p. 2509-2528.