Abstract
The entire Earth was bombarded c.4100-3800 Ma, establishing initial
conditions for all later regional geology. Deep impact-fractures have
been regenerated upward from the brittle-ductile boundary by the action
of convection, outgassing, circulating fluids, the twice-daily
earth-tide, and earthquakes throughout all subsequent time. In
consequence, many such fractures have never entirely healed or been
eliminated. The two-dimensional map-outlines and circular curvature of
numerous three-dimensional “craterform” scars can be readily seen,
once the observer has been alerted to the possibility of their
existence. Many other Hadean/EoArchean impact-scars are covered over, as
is the case at present, at any given time. These impact features have
been regenerated “cold” from below and are fundamentally different
from astroblemes, as presently defined, whose rocks were directly
subjected to the high temperatures and pressures that accompany
hypervelocity extra-terrestrial impacts. Melt rock filled the largest
impact sites and produced cratons, with overflow producing platforms. In
later times, craton rims buckled during collisions, producing orogens.
Crater rims originally entered the Earth at near-vertical angles but
after sufficient net erosion following Snowball Earth episodes, deeply
exposed rim-zones entered the Earth at lower angles, thereby
facilitating deep subduction. Renewed activation of earliest Precambrian
fractures from below is a recurrent geological phenomenon. The largest
scar, approximately 5350 kilometers in diameter, encompasses Asia and
has Novaya Zemlya as part of an outer rim. Our vision has greatly
improved since 1788 when James Hutton could find “no vestige of a
beginning”.