Page 101 - Handbook of Gold Exploration and Evaluation

82 Handbook of gold exploration and evaluation

Orogenic episodes throughout geological time from the earliest Precambrian
have combined numbers of old crustal elements into a generally rigid block.
These elements, including Precambrian shield units represent earlier plates that
have become progressively enlarged and welded together by successive
orogenic cycles along belts of tectonic activity; presumably representing former
collision or subduction zones between or marginal to the plates. In orogens, such
as the early Palaeozoic fold belts of North America, collision and deep levels of
erosion have erased the original topographic expression of the tectonic elements.
In other Palaeozoic orogens, magmatic and fold belts fringe the shield areas,
which include collision zones or sutures of late Palaeozoic age with later
additions or re-mobilisations in the Triassic and Cretaceous. Caenozoic tectonic
history is generally considered to closely relate to present-day plate interactions
and both active and passive tectonic elements can easily be recognised. The
major plate interactions were probably already established in the Mesozoic, but
structures of this age in current arc-trench zones and generally overprinted by
those of the main Tertiary orogenic phase, which is still active at the present day.
The Asian island arc system of the Pacific Ring of Fire is a type area for other
such systems and may be expected to yield results of significance to similar
systems in other parts of the world and throw additional light on the constitution
of older arc systems. The implication is clear that detailed examination of active
arc systems should provide models of processes at the higher crustal levels that
can be extrapolated downward. The essential elements of an active back-arc±
interarc±marginal basin and its associated arc-subduction complex are illustrated
in Fig. 1.19.

Epeirogenesis
Rock masses affected by epeirogenic movements are typically warped or tilted in
contrast to the more rapid folding and fracturing of orogenesis. Isostatic adjustment
occurs in response to denudation or sediment accumulation. As denudation
continues the material beneath the continent rises spontaneously to compensate for
the loss of surface material, thus maintaining a state of isostatic equilibrium
between the continental mass and the denser underlying sima. Retained stresses in
the crustal rocks are periodically released by denudation of the overlying surfaces,
so triggering a rapid isostatic compensation to restore equilibrium. Since the total
landmass is several times greater than the mass lying above base level, uplift occurs
at an amount less the thickness of the material removed.
Tilting of the crust produces a range of gradient changes that may include
aggradation and degradation in the same fluvial system. Local decreased
gradients in one sector may generate elevated pay streaks within an aggrading
stratigraphic section. Increased gradients in another sector may segment other-
wise payable pay streaks into potholes and other bedrock features. Downward
warping of the Lakekamu Embayment in Papua New Guinea led to the slumping