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The Vredefort Structure. Misconceptions and Facts - Dr. Joe Mayer |
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   * How old is the Vredefort Dome and where did the meteorite collide with the earth? The Vredefort Structure can be considered to be a gigantic scar that was left behind when a huge meteor (estimated size 10km diameter) collided with the earth about 2023 million years ago. The impact site, in modern day terms, would be some odd tens of kilometres south of Parys. The collision, at an estimated speed of 150 000 k/h, took place on an ancient surface about 17 km higher than the mentioned modern-day target point. The reason for this was that at the time 17 km of layered sedimentary and volcanic formations were piled up upon what, in today’s terms, we refer to as the Parys granite. Although a crater with an estimated diameter in the order of 100 km and some odd tens of kilometres deep was formed on the ancient surface, it was destroyed by erosion over a period of millions of years.
What
happened to the meteorite? A misconception exists that the meteorite deeply penetrated the earth’s crust and is still intact at some deep level. Temperatures up to 2000°C were generated when the kinetic energy on impact was transformed to heat and shock pressure. This prevented the survival of the meteorite. In fact, the extreme temperatures caused the meteorite to be converted to melt and vapour. The explosion which followed and which exceeded any known nuclear explosion many times in intensity, caused vapour, melts, crushed rock and dust to be ejected from the crater.
What
are the present characteristics of the meteorite impact
site? The Vredefort Structure, that we see today, does not represent any of the original surface characteristics left by the meteorite impact 2023 million years ago. As stated formerly, the lapse of millions of years resulted in erosion destroying all of the superficial characteristics of the impact event such as the crater and the resulting ejecta deposits. In fact, what we observe at the present impact site is a structure that resulted at depth at the time (17km deep down in the earth's crust) due to the effect firstly of the primary shock event and secondly the rebound action that followed. This structure may be regarded as the "root structure" of the impact. It is now exposed after millions of years of erosion. An explanation of the events leading to the deformation of the deep-seated sedimentary and volcanic rock formations, in this root zone, is attempted in the following paragraphs.
Remember that at the time of the Vredefort event there were at least a17km thick succession of horizontal-lying sedimentary and volcanic formations resting on a floor of Archaean granite. On impact the primary shock event was responsible for the formation of a crater in the order of 100 kilometres in diameter and some odd tens of kilometres deep. The rebound action however, caused a plug-shaped part of the deep mantle*of the earth to bulge upwards and act like a huge punch, driven vertically upwards. This in turn was responsible for the upheaval, by tens of kilometres, of a similar circular plug of the Archaean granite crust. As the plug of Archaean granite (in plan 50 km wide) rose up in the middle of the crater it deformed the original horizontal lying sedimentary and volcanic formations by folding (bending) them upwards around the circumference of the granite plug. In addition an uplifted central highland in the middle of the crater floor was formed (similar to the " pimple" of high ground in the Venus craters, see Figure 5). It seems logical that over a period of two billions of years the root structure must too have been subjected to further crustal deformation explaining the asymmetry of the Vredefort Structure we see today (see Figure 3).
* The interior of the Earth is density zoned. The lighter crust is followed by the more dense mantle which in turn is followed by the earth's core consisting of nickel-iron.
The
following is a description of the present characteristics of the root structure i.e. the geological
structure referred to as the Vredefort Dome (Figure 1). In plan, it comprises concentric
arcs of successively tilted (dipping) formations arranged around a core
of ancient granite (or Archaean granite). The towns of Vredefort
and Parys are located on this granite core. We only see the
north-western
half of this Structure because towards the southeast it is covered by Karoo
sediments which are much younger than the Structure. The
variation in resistance to erosion of the arcuate dipping formations is
responsible for the hills and valleys which constitute the Vredefort Mountain Land,
as seen prominently developed around Venterskroon.
Fig
1. A simplified geological map of the most prominent part of the Dome
Structure. The dipping
formations are shown in order of age. The core formation, the ancient
granite (Archaean Granite), is the oldest and forms the floor on which the
sediments and volcanics were deposited. The West Rand
and
Central Rand Groups together form what is known as the Witwatersrand
Supergroup; these latter formations are also referred to as the collar
formations of the Structure and consist mainly of sediments. In the
Vredefort Structure they have been extensively prospected for gold. The
Ventersdorp Supergroup represents only lavas, whereas the Transvaal
Super Group once again is mainly a succession of sediments.
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