Tectonic interpretation of orogenic belts - Introducing Tectonics, Rock Structures and Mountain Belts - page 81

plate A

plate B

foreland
fold/thrust
belt

central crystalline core complex

foreland

10

1 2

3 4 5

high-grade
gneisses
post-tectonic
pluton

shear zones

platform
sediments

ophiolite
crystalline
nappe

foreland
fold/thrust
belt

slate
belt

slate
belt

schist
belt

foredeep

foredeep

80 81

continental basement

continental basement - plate A

continental basement - plate B

re-activated
extensional
faults

A

volcanic arc

continent
plate A

continent
plate B

continental
platform

subduction
zone

deep
ocean

thrust front

foreland
basin

foreland
deposits

continental
platform

evolving
orogen

continental crust - plate A

C

slope

shelf deep

ocean

slope

trench

carbonates

banded cherts

clastics

B

characteristic deposits greywackes, volcanics

Figure 10.1 An idealised collisional orogenic belt. A. Cross-section, reading from left to right. (1) Plate A foreland: continental basement of plate A is
overlain by a thin cover of platform sediments, which thicken towards the continental slope; they are overlain in turn by the foredeep or foreland basin,
filled by sediments derived from the rising orogen. (2) The foreland fold-thrust belt: the outer part of plate A basement and its sedimentary cover are
sliced up and telescoped by a set of thrusts and related folds; these are largely un-metamorphosed, but the outermost sediments may be of slate grade,
forming the slate belt. (3) The central crystalline core complex: at the base of this tectonic unit is an ophiolite sheet that represents part of the oceanic
crust and uppermost mantle scraped off the down-going slab and accreted to the base of the overlying continental plate. (4) Overlying the ophiolite
sheet are a series of crystalline nappes (thrust sheets) consisting of high-grade, metamorphosed, sedimentary and plutonic igneous rocks affected by
ductile folds and shear zones; these are initially directed towards the foreland but, towards the interior of the complex, they are affected by more upright
structures; the complex is intruded by post-tectonic granite plutons. (5) Plate B foreland: the inner zones consist of a foreland fold-thrust belt directed
towards the foreland, in a mirror image of the plate A margin; the highest unit consists of schist-grade metamorphics, succeeded in turn by a slate belt,
un-metamorphosed foreland cover and a foredeep, overlying continental basement of plate B. Based on a diagram by Hatcher & Williams (1986). B.
Schematic representation of the pre-collisional plate margin of plate A, showing the various depositional environments: shelf, slope, deep-ocean, trench
and far slope, together with their characteristic deposits. C. Development of a foreland basin or foredeep. The weight of the advancing orogen depresses
the thinned continental margin of plate A, causing a basin to form, which is filled by clastic sediments eroded from the rising mountains of the orogen.

The foreland fold-thrust belt
In order to understand the geometry
of this belt, it is necessary to recon-
struct the original make-up of the
passive margin, as shown in Figure
10.1B. The depositional environments

change ocean-ward as follows:
1. the continental shelf or platform,
usually dominated by carbonates;

where the continental crust thins and
is replaced by oceanic crust;
3. the deep ocean environment, char-
acterised by thin-bedded cherts and
mudstones;
4. the ocean trench , which is typically

2. the continental slope, typified by

coarse to fine clastic sediments depos-
ited in deeper water, above the zone