SHOCK METAMORPHISM

SHOCK METAMORPHISM

The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in the uplifted center of a complex crater, however.

High-temperature rock types, including laminated and welded blocks of sand, spherulites and tektites, or glassy spatters of molten rock. The impact origin of tektites has been questioned by some researchers; they have observed some volcanic features in tektites not found in impactites. Tektites are also drier (contain less water) than typical impactites. While rocks melted by the impact resemble volcanic rocks, they incorporate unmelted fragments of bedrock, form unusually large and unbroken fields, and have a much more mixed chemical composition than volcanic materials spewed up from within the Earth. They also may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt. Note: it is reported in the scientific literature that some “shock” features, such as small shatter cones, which are often reported as being associated only with impact events, have been found in terrestrial volcanic ejecta.

(AstroNotes October 2010, March 2011

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 and slickenslides, include brecciated rocks, suevites, impact melts and pseudotachylites. They attest to the destructive power of the impact event.

Schematic radial cross-section through one-half of a simple impact structure, showing locations of different impact-produced lithologies. Curved lines show isobars of shock pressures (in GPa) produced in the basement rocks by the impact.

The rocks at an impact 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 are quantified for reference:

Meteorite impact is a process in which a large object strikes an even larger one at hypervelocity a, which locally releases a huge amount of energy producing an impact crater b. This diagram documents that the magnitude of the shockc from an impactor is inversely proportional to the distance from the point of impact. The shock metamorphic effects in the country rock will then vary with shock magnitude (French 1998).

a Hypervelocity – 11.2 km/sec to 70 km/sec.

b Crater: impactor size ratio ranges from 20:1 to 50:1 (Shoemaker 1963, Baldwin 1963).

c The standard unit of pressure is the Pascal, abbreviated Pa, which is equivalent to 1 kilogram per square meter. A GPa is a gigapascal (giga means billion), a measurement of pressure, and is equal to 10,000 times the atmospheric pressure at the Earth’s surface.

SHOCK METAMORPHIC EFFECTS

PLANAR DEFORMATION FEATURES (PDF) – The passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions.

Shock-characteristic planar deformation features (PDFs) in a quartz grain (in distal ejecta from the Manson impact crater, found in South Dakota). Width of the grain ca. 100 mm. Multiple intersecting sets of PDFs are clearly visible (Christian Koeberl).
This photograph of a thin slice of plagioclase, 0.03 millimetre thick, is seen here in cross-polarised light, with a ‘sensitive tint’ plate. The original plagioclase is coloured yellow and the shock-changed mineral is purple. This sample is from the Manicouagan impact structure. (Courtesy Denis W. Roy & MIAC).
The recognition of unique shock-produced “deformation lamellae” or planar deformation features (PDFs) in quartz in the 1960s was a critical development in the identification of ancient meteorite impact structures: a) first published description of “deformation lamellae” identified in breccias from the Clearwater West impact structure, as an abstract in the Journal of Geophysical Research (McIntyre 1962) (reproduced by permission of the American Geophysical Union); b) photomicrograph of quartz grain in breccia from Clearwater West, showing multiple sets of PDFs (McIntyre 1968). Plane-polarized light. The quartz grain is about 1.4 mm long (French 2004).

PSEUDOTACHYLITE – is a fault rock that has the appearance of the basaltic glass, tachylyte. It is dark in color and has a glassy appearance. However, the glass has normally been completely devitrified into very fine-grained material with radial and concentric clusters of crystals. It may contain clasts of the country rock and occasionally crystals with quench textures that began to crystallize from the melt. It is formed when a high pressure from an impact is applied to country rocks and then abruptly released. This causes the rock along and within fracture lines or faults to partly melt. The fractures or faults containing the pseudotachylite are welded shut as soon as the motion created by the impact stops.

The entire period of activity of a fracture or fault filled with pseudotachylite may be measured in minutes. (e.g., Pseudotachylite is a rock type formed by friction-induced melting, during very rapid deformation) Philpotts 1964; Maddock 1983.

here
Sudbury pseudotachylite is illustrated as the black pulverized (by the impact) country rock injected into the pink gneiss country rock (the toe of my boot is for scale).
This is a possible pseudotachylite vein within the Manicouagan impact crater.

SUEVITE – is an impact fallback breccia, formed when a meteorite strikes the earth and blasts “target rock” high into the atmosphere. Some target rock falls back into the newly formed crater, and is compacted to form suevite. Suevite typically contains fragments of shock-metamorphosed rocks and glass set in a matrix of fine-grained minerals, rock, and glass fragments.

Lake Wanapetei suevite.
The suevite samples illustrated in the above images were found on the south shore of Lake Wanapitei crater. It was apparently scooped up from the lake bed by glacial activity and deposited in places along the southern shore of the lake.

COMMINUTION – the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. In geology, it occurs naturally during faulting in the upper part of the Earth’s crust.

<10 GPa – From the Barringer Impact Crater, the shocked Coconino Sandstone (top) is weakly shocked sandstone that lacks remnant porosity and contains abundant grain comminution and fracturing. Note the “rock flour” on the shocked sample. The unshocked Coconino Sandstone (bottom) consists of a fine to medium-grained, moderately well-sorted, rounded quartz arenite with ~ 20 vol% porosity. The shocked sample is from the Barringer impact structure. Coconino sandstone layers are typically buff to white in color. It consists primarily of sand deposited by eolian processes (wind-deposited) approximately 260 million years ago.

IMPACT MELTrock that has been made temporarily molten as a result of the energy released by the impact of a large colliding body. Impact melts include small particles, known as impact melt spherules, that are splashed out of the impact crater, and larger pools and sheets of melt that coalesce in low areas within the crater. They are composed predominantly of the target rocks, but can contain a small but measurable amount of the impactor.

Impact melt found in the Charlevoix impact structure. Note the country rock fragment in the inclusion.
An expample of impactite found by the author in the vicinity of the Pingualuit crater. The impact origin of the Pingualuit Impact Crater was finally confirmed in 1986 with the discovery of impactite similar to this in the vicinity of the structure.
Impactite from the bolide impact at Pingualuit.
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).
The Manicouagan impact structure – 10 m block of mafic gneiss embedded within the impact melt cliff, north shore of Memory Bay.

BRECCIA(from a Latin word meaning “broken”) is a rock that is composed of angular fragments of other rocks surrounded by a fine-grained “matrix” that may be of a similar or a different material. Breccias are extremely common in the central uplift, in crater-fill deposits, and in the ejecta blanket of meteorite impact craters.

These Brent impact structure breccia examples WERE NOT found in situ but were most probably transported here by the glaciers (glacial erratics). So, scientifically, without material analysis we cannot absolutely claim that this is breccia from the Brent structure, BUT, the circumstantial evidence is almost conclusive. The other explanation is that these deposits were from another impact site further to the north and just “happened” to be dropped off here within the Brent structure.
Impact melt is the “greyish” material between and cementing the country rock fragments. K-Ar dating of the recrystallized melt-bearing breccia gave ages of 310-365 Ma (Shafiquallah et al., 1968). Geochemical analyses show that the “melt” rocks are in fact melted target rock with ~1% contamination by chondritic material (Grieve, 2006).
Polymictic impact breccia; Glover Bluff impact structure (Wisconsin, USA). Structure diameter 8 km, Cambrian age, or younger (ERNSTSON CLAUDIN IMPACT STRUCTURES – METEORITE CRATERS).
Impact breccia from the Ile Rouleau structure.
The Manicouagan impact structure – breccia on the inner plateau of the central peak island – 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.
Macroscopic view of Newporte core sample – granitic frag-mental breccia D9462.0 (from Duerre 43-5 core) showing one quartz-nich angular granitic fragment (bright area top centre) with other darker granitic fragments in a ark, fine-grained, clast-rich matrix.(After Koeberl and Reimold, 1995)
In situ polymict breccia on Patterson Island east within the Slate Islands impact structure.
The Sudbury impact structure – grey Whitewater breccia.
The Sudbury impact structure – darker Whitewater breccia.
These are the breccia samples I had recovered from Opal Island within the Skelton Lake structure. The breccia deposit on Opal Island stands in stark contrast to the surrounding target rock in the Central Gneiss Belt.