Download Orogenic Belts and Orogenic Sediment Provenance

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Orogenic Belts and Orogenic Sediment Provenance
_____________________
Eduardo Garzanti1, Carlo Doglioni2, Giovanni Vezzoli, and Sergio Andò
Laboratorio di Petrografia del Sedimentario, Dipartimento di Scienze Geologiche e Geotecnologie,
Università di Milano-Bicocca, Piazza della Scienza 4, 20126 Milano, Italy
(e-mail: [email protected])
1
Author for correspondence
2
Dipartimento di Scienze della Terra, Università “La Sapienza”, Piazzale Aldo Moro 5, 00185
Roma, Italy (e-mail: [email protected])
Key Words: Subduction geometry, Orogenic domains, Modern sands, Heavy minerals,
Sedimentary petrology, Provenance models.
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ABSTRACT
By selecting a limited number of variables (westward vs. eastward subduction polarity; oceanic vs.
continental origin of downgoing and overriding plates), we identify eight end-member scenarios of
plate convergence and orogeny. These are characterized by five different types of composite
orogenic prisms uplifted above subduction zones to become sources of terrigenous sediments (IndoBurman-type subduction complexes, Apennine-type thin-skinned orogens, Oman-type obduction
orogens, Andean-type cordilleras, and Alpine-type collision orogens).
Each type of composite orogen is envisaged here as the tectonic assembly of sub-parallel geological
domains consisting of genetically associated rock complexes. Five types of such elongated orogenic
domains are identified as the primary building blocks of composite orogens: 1) magmatic arcs; 2)
obducted or accreted ophiolites; 3) neometamorphic axial belts; 4) accreted paleomargin remnants;
and 5) accreted orogenic clastic wedges.
Detailed provenance studies on modern convergent-margin settings from the Mediterranean Sea to
the Indian Ocean show that erosion of each single orogenic domain produces peculiar detrital
modes, heavy-mineral assemblages and unroofing trends that can be tentatively predicted and
modelled. Five corresponding primary types of sediment provenances (“Magmatic Arc”,
“Ophiolite”, “Axial-Belt”, “Continental-Block”, and “Clastic-Wedge” provenances) are thus
identified, which reproduce, redefine, or integrate provenance types and variants originally
recognized by Dickinson and Suczek (1979). These five primary provenances may be variously
recombined in order to describe the full complexities of mixed detrital signatures produced by
erosion of different types of composite orogenic prisms. Our provenance model represents a flexible
and valuable conceptual tool to predict the evolutionary trends of detrital modes and heavy-mineral
assemblages produced by uplift and progressive erosional unroofing of various types of orogenic
belts, and to interpret petrofacies from arc-related, foreland-basin, foredeep, and remnant-ocean
clastic wedges.
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Introduction
“The overall geometry of orogenic belts produced by subduction ..... is fundamental for provenance
analysis on a global scale.” W.R.Dickinson (1988, p.18)
Orogens are stacks of rock units uplifted above subduction zones by tectonic and magmatic
processes, which depend on subduction geometry as well as on the nature and geological history of
converging plates. Although distinct types of orogenic prisms may be identified, natural processes
are so varied and complex that any attempt to classify orogenic belts sounds like a hopeless
challenge; each mountain chain has its own peculiarities and stands as a case apart.
As a consequence, modelling provenance of orogenic sediments and sedimentary rocks is an
arduous task, and a systematic quantitative treatment of orogenic sediment provenance is wanting.
Classic provenance models have dealt with such an intricate problem only in a general way, loosely
discriminating among three distinct types of orogenic settings (subduction complex, collision
orogen/suture belt, and foreland fold-thrust belt) and three corresponding types of “Recycled
Orogen Provenances” (Dickinson and Suczek 1979; Dickinson 1985).
This article focuses on topographically elevated sources of detritus found within thrust belts and
arc-trench systems (rather than on the associated basins representing sediment sinks; Dickinson
1988), and proposes a simplified classification of orogenic domains, which is intended to represent
a useful reference model for a better description and understanding of orogenic sediment
provenance.
Subduction Polarities and Orogen Types
“Current circum-Pacific arcs include east-facing island arcs and west-facing continental arcs in a
consistent pattern that implies net westward drift of continental lithosphere with respect to underlying
asthenosphere.” W.R.Dickinson (1978, p.1)
Two opposite subduction polarities have long been recognized (Nelson and Temple 1972; Uyeda
and Kanamori 1979). Whereas backarc spreading is characteristic of east-facing arc-trench systems
(westward subduction), backarc thrusting is most widespread in west-facing arc-trench systems
(eastward subduction; Dickinson 1978).
Such contrast has often been ascribed to the age of subducting oceanic lithosphere, older
lithosphere being denser and consequently prone to generate a more efficient slab pull (Royden
1993). Systematic analysis of subduction dip and convergence rate at the trench, however, fails to
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show any significant correlation between age of downgoing lithosphere and slab inclination
(Cruciani et al. 2005).
Global tectonics is considered here as fundamentally controlled by Earth’s rotation, with
lithospheric plates drifting westward with respect to the mantle (Bostrom 1971; Dickinson 1978;
Scoppola et al. 2006). Plate motions follow a sinusoidal flow, oriented roughly W-ward in the
Atlantic Ocean, but turning WNW-ward in the Pacific Ocean, and finally bending SW-ward in Asia
(Crespi et al. 2006). Referring to such mainstream of plate motion, we identify two end members of
subduction zones, associated with two fundamental types of orogens.
Westward subduction zones oppose mantle flow, and tend to be steeper. Décollement planes are
shallow and only affect the upper layers of the downgoing plate. Low-relief, thin-skinned, singly
vergent orogens are produced, chiefly consisting of volcanic or sedimentary rocks (e.g., Lesser
Antilles, Sandwich, Apennines, Carpathians, Banda, Tonga, Marianas, Nankai, Kurili, Aleutians;
Doglioni et al. 2006).
Instead, eastward subduction zones follow mantle flow and are less inclined. Decollement
planes cut across the whole crust, allowing the exhumation of deep-seated rocks. High-relief, thickskinned, doubly vergent orogens are produced (Koons 1990; Willett et al. 1993), largely consisting
of neometamorphic and plutonic rocks (Andes, Alps, Caucasus, Zagros, Himalaya, Indonesia,
Taiwan, New Zealand Alps; Doglioni et al. 2006).
As a general rule, the subduction hinge moves away from the upper plate in westward
subduction zones, and towards the upper plate in eastward subduction zones. This dichotomy
corresponds with the distinction between "pull arc" and "push arc" orogens (Laubscher 1988). Note
that the term “eastward” is used loosely here to designate subduction zones that follow mantle flow,
even if these are actually oriented northeastward in Eurasia (e.g., Himalaya) because of the
undulated mainstream of plate motion (Crespi et al. 2006).
If the net westerly drift of lithospheric plates is controlled by Earth’s rotation, an astronomical
mechanism that cannot switch, this model should be valid also for the geologic past.
Geometries of Plate Convergence
“If the present is the key to the past, perhaps global paleotectonic and paleogeographic reconstructions
should be based on the actualistic hypotheses that ... backarc spreading occurs where arc orogens face
east, and that backarc thrusting occurs where arc orogens face west.” W.R.Dickinson (1988, p.20)
Subduction involves a lower downgoing plate and an upper overriding plate, both of which can be
either oceanic or continental. By considering all possible basic combinations between subduction
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polariy (westward vs. eastward) and nature of converging plates (oceanic vs. continental), the types
of plate convergence are reduced to eight end-members (fig. 1). The pro-side of the orogen is the
one facing the subduction zone.
A) Westward Intraoceanic Subduction. Westward intraoceanic subduction is widespread along
the western Pacific (e.g., Philippines and Marianas), and occurs in the western Atlantic (Lesser
Antilles and South Sandwich Islands) and western Mediterranean Sea (Aeolian Islands). The age of
the oceanic crust is syn-subduction in the backarc basin, and generally much older in the lower
plate. The arc-trench system formed above the subduction zone includes calc-alkaline igneous rocks
(arc massif) and oceanic rocks scraped off the subducting plate (oceanic prism).
B) Westward Subduction of Oceanic beneath Continental Lithosphere. This setting is typical
of newly formed westward subduction zones, which may develop along the retro-side of thickskinned orogens generated by pre-existing eastward subduction (e.g., initiation of Barbados
subduction; Doglioni et al. 1999). Large crustal slices of the inactive orogen are boudinaged during
progressing backarc extension, and dragged eastward while the subduction hinge migrates away
from the upper plate (e.g., Calabria and pre-Pleistocene of northern Japan; Doglioni et al. 1998).
The pro-side of the orogen is an accretionary prism, developed at the expense of the uppermost
layers of the subducting plate, largely represented by oceanic sediments (e.g., Nankai Trough;
Moore et al. 1990).
Perhaps the best modern example is northern New Zealand, where the oceanic Pacific Plate
subducts beneath stretched continental lithosphere (Henrys et al. 2006). Upper-plate crustal
extension is propagating southward across the low-topography North Island, unzipping the nascent
Taupo backarc basin (Parson and Wright 1996; Beanland and Haines 1998). Subduction polarity
changes farther south, where South Island is instead characterized by a doubly vergent, hightopography compressional orogen produced by continental collision (Beaumont et al. 1996a).
C) Westward Subduction of Continental beneath Oceanic Lithosphere. At the final stage of
oceanic subduction, or laterally to an active oceanic segment (e.g., Banda Arc), thinned continental
lithosphere may be pulled down to 100-250 km (Müller and Panza 1986; Ranalli et al. 2000), until
subduction is eventually throttled by buoyant lithosphere of closer-to-normal thickness (e.g.,
Southern Apennines).
East-facing singly vergent orogens formed above westward subduction zones are characterized
by low structural relief (Doglioni et al. 1999). The pro-side of the orogen is a thin-skinned thrust
belt (including sedimentary sequences originally deposited on a continental paleomargin and
frontally accreted turbidites), whereas its retro-side is a magmatic arc (better developed if
subduction is faster; Tatsumi and Eggins 1995).
W
E-NE
WESTWARD SUBDUCTION
LOW-RELIEF, THIN-SKINNED, SINGLY VERGENT, “PULL-ARC” OROGENS
volcanic arc
backarc basin
incipient
backarc basin
trench
boudinaged
oceanic
belt
prism
oceanic crust
LID
0 km
A
B
N. New Zealand-type
boudinaged belt & prism
Marianas-type
intraoceanic arc
100
100
km 0
OCEAN beneath CONTINENT
OCEAN beneath OCEAN
backarc basin
forebelt
forebelt
ensialic
backarc basin
foredeep
foredeep
continental crust
C
asthenosphere
D
Carpathian-type
boudinaged belt & prism
Apennine-type
boudinaged belt & prism
CONTINENT beneath CONTINENT
CONTINENT beneath OCEAN
W
E-NE
EASTWARD SUBDUCTION
HIGH-RELIEF, THICK-SKINNED, DOUBLY VERGENT, “PUSH-ARC” OROGENS
oceanic
prism
remnant
ocean
accreted
ophiolite
retrobelt
forebelt
pull-apart
basin
trench
retro
foreland basin
continental crust
E
F
Andaman-type
intraoceanic prism
Andean-type
cordillera
OCEAN beneath OCEAN
pro
foreland basin
G
forebelt
Oman-type
obduction orogen
CONTINENT beneath OCEAN
OCEAN beneath CONTINENT
obducted
ophiolite
remnant
ocean
pro
foreland basin
H
forebelt
Alpine-type
collision orogen
CONTINENT beneath CONTINENT
Figure 1 - Garzanti et al.
axial
belt
retrobelt
retro
foreland basin
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Because little sediment is produced by the low-relief orogen and tectonic subsidence is one
order-of-magnitude greater than for eastward subductions (Doglioni 1994), the sedimentary basin
formed both in front of and above (Ori and Friend 1984) the growing accretionary prism typically
remains persistently in deep-water conditions (“foredeep”). Foredeeps formed above westwardsubducting continental margins are contrasted here with less-subsident foreland basins associated
with high-relief Alpine-type collision orogens, which are rapidly overfilled with shallow-marine to
fluvial sediments (Doglioni 1994).
D) Westward Subduction of Continental beneath Continental Lithosphere. This case differs
from the one described above only because extension on the retro-side of the orogen is insufficient
to tear the crust, and an ensialic backarc basin develops (e.g., Pannonian Basin in the rear of the
Carpathians; Horvath 1993). This has no systematic influence on the structure of the orogen, and
thus on terrigenous supply.
E) Eastward Intraoceanic Subduction. The not numerous modern examples include the
Vanuatu (New Hebrides), where a doubly vergent intraoceanic prism has been produced by
collision of the volcanic arc with oceanic plateaux, submarine ridges and seamounts (Taylor et al.
1995; Meffre and Crawford 2001). Eastward subduction also takes place along highly oblique
intraoceanic convergence zones beneath the Macquarie Ridge (Massell et al. 2000) or the Andaman
Sea, a young pull-apart oceanic basin (Curray 2005). The Andaman-Nicobar Ridge is a tectonic
stack of arc-derived and remnant-ocean turbidites capped by forearc ophiolites (Allen et al. in
press). Ophiolites represent the only subaerial exposure of the Macquarie Ridge (Rivizzigno and
Karson 2004).
F) Eastward Subduction of Oceanic beneath Continental Lithosphere. The classic example is
the eastern Pacific, bordered by a continuous high-relief cordillera from Alaska to the Andes
(Jaillard et al. 2002). Because viscosity is much higher in the lower oceanic plate, shortening is
mostly concentrated in the upper continental plate, and the orogen chiefly consists of upper-plate
material (Table 1). The lower plate may be subducted entirely, and even tectonic erosion commonly
takes place because of this marked rigidity contrast (von Huene and Lallemand 1990; Ranero and
von Huene 2000). Instead, accretion typically occurs where the lower plate is overlain by thick
deep-sea turbidites (e.g., Alaska and Sumatra; Dickinson 1995; Ingersoll et al. 2003).
The pro-side of the orogen is an arc-trench system, with a forebelt of variable width cored by
basement rocks (von Huene 1986). The retro-side is a thick-skinned thrust belt also involving
basement, but frequently propagating foreland-ward in thin-skinned mode (e.g., Rocky Mountains;
Bally et al. 1966).
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The orogen generally undergoes compression and uplift, but extension may develop where
motion of the lower plate has been inverted after subduction of a mid-ocean ridge (e.g., Basin and
Range), or where distinct sub-plates ovverride the lower plate at different velocities (e.g., Aegean
Sea; Doglioni 1995). When considering the undulated mainstream of plate motions, the SumatraJava arc belongs to this category.
G) Eastward Subduction of Continental beneath Oceanic Lithosphere. This setting represents
the final stage of eastward intraoceanic subduction, when a continental margin arrives at an
intraoceanic trench. Virtually intact slabs of dense oceanic upper-plate lithosphere can thus be
emplaced onto buoyant continental crust (“obduction”; Coleman 1971; Karig 1982). Such a process
gives rise to “ophiolite-capped thrust belts” (Cawood 1991), the archetypal example being the
Northern Oman orogen assembled at Late Cretaceous times (Searle and Stevens 1984). The pro-side
of the obduction orogen is a thick-skinned thrust belt, beneath which continental blueschists and
eclogites of the axial belt may be exposed. Its tectonic lid consists of a complete section of forearc
mantle and crust, generated synchronously with subduction and detached along a mechanically
weak boundary as deep as the asthenosphere (Spray 1984; Searle and Cox 1999).
H) Eastward Subduction of Continental beneath Continental Lithosphere. This setting
represents the final stage of eastward oceanic subduction, when two continental margins collide.
Doubly vergent Alpine-type orogenic prisms with high structural and topographic relief are thus
produced (Doglioni et al. 1999). The axial part of the orogen includes slivers of strongly-thinned
outer continental margins and adjacent oceanic lithosphere, which underwent high-pressure
metamorphism in the early subduction stage of collision (Beaumont et al. 1996b). Because thrust
planes cut across the lithosphere, deeply subducted eclogitic rocks can be exhumed in a few million
years (Rubatto and Hermann 2001; Baldwin et al. 2004). Thick-skinned thrust belts formed along
both external sides of the orogen include inner parts of collided paleomargins underlain by
continental crust of closer-to-normal thickness, as well as frontally accreted foreland-basin clastics.
In external belts, close to the contact with the axial belt, orogenic metamorphism may reach
amphibolite facies (Frey and Ferreiro Mählmann 1999).
Collision orogens are by no means all alike, and major differences are displayed by two of the
best-known examples belonging to the same orogenic system, the Alps and the Himalaya. The
Himalaya has a much better-developed arc-trench system, which can be traced for ~3000 km from
Pakistan to Myanmar (Gansser 1980). The axial belt of the Alps includes a stack of both continental
and oceanic nappes showing widespread high-pressure metamorphism. In the Himalaya, instead, the
axial belt is confined to a narrow wedge largely consisting of amphibolite-facies metasediments
extruded between a main thrust zone at the base and a major detachment system at the top (Hodges
EXAMPLES
SOURCE OF ACCRETED
ROCK UNITS
PROCESS OF
PRISM BUILDING
OROGEN TYPE
WESTWARD SUBDUCTION
LOW RELIEF, THIN-SKINNED, SINGLY VERGENT, "PULL-ARC" OROGENS
RAPID SUBSIDENCE OF TRENCH OR FOREDEEP
OCEANIC (OR ENSIALIC) BACKARC BASIN
O/O
MARIANAS
SANDWICH
Lower plate
oceanic
offscraping
intraoceanic
arc
C/O
N. NEW ZEALAND
CALABRIA
Inherited upper plate
+ Lower plate
oceanic
offscraping
boudinaged
belt & prism
O/C
BANDA
S. APENNINES
Inherited upper plate
+ Lower plate
continental
offscraping
boudinaged
belt & prism
C/C
CARPATHIANS
N. APENNINES
Inherited upper plate
+ Lower plate
continental
offscraping
boudinaged
belt & prism
EASTWARD SUBDUCTION
HIGH RELIEF, THICK-SKINNED, DOUBLY VERGENT "PUSH-ARC" OROGENS
SLOWER SUBSIDENCE OF TRENCH OR FORELAND BASIN
NO TRUE BACKARC BASIN
O\O
ANDAMANS
VANUATU
Upper plate
+ Lower plate
oceanic
collision
intraoceanic
prism
O\C
ANDES
CASCADES
Mostly upper plate
continental
buckling
cordillera
C\O
OMAN
PAPUA
Upper plate
+ Lower plate
obduction
obduction
orogen
C\C
HIMALAYA
ALPS
Upper plate
+ Lower plate
continental
collision
collision
orogen
Table 1 - Garzanti et al.
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2000); oceanic units are lacking, and eclogites have been recognised only locally so far (Lombardo
and Rolfo 2002). The retro-side of the Alps consists of a well defined, 50-100 km-wide thickskinned thrust belt, whereas the deformed retro-side of the Himalaya is much broader and includes
the vast Tibetan plateau as well as a series of highly elevated mountain chains (e.g., HindukushKarakorum, Pamir, and Tian Shan). Both external belts of the Alps include deep-sea turbidites
ranging in age from Cretaceous to Miocene, whereas foreland-basin clastics accreted along the front
of the Himalaya chiefly include Neogene fluvial sediments (Burbank et al. 1996; Najman 2006).
Classification of Orogenic Sediment Provenances
“Given the potential diversity of recycled orogenic sediment, it is a severe challenge to devise a scheme
for its identification and classification that has empyrical validity for interpretation of the sedimentary
record” W.R.Dickinson (1985, p.347)
Because orogenic belts are composite structures including diverse rock complexes assembled in
various ways by geodynamic processes, orogenic detritus embraces a varied range of signatures.
Unravelling provenance of clastic wedges accumulated in foreland basins, foredeeps, or remnantocean basins is, therefore, an arduous task, which requires a sophisticated conceptual reference
scheme.
The Dickinson Model. Orogenic detritus derives either from volcano-plutonic rock suites
generated along active intraoceanic and continental arcs (“Magmatic-Arc Provenance”), or from
mainly sedimentary or metamorphic rocks tectonically uplifted within subduction complexes
(“Subduction Complex Provenance”), foreland fold-thrust belts (“Foreland Uplift Provenance”),
and collision orogens (“Collision Orogen Provenance”; Dickinson and Suczek 1979). “Subduction
Complex Provenance” is typified by chert-rich detritus from offscraped oceanic slivers and abyssalplain sediments, whereas “Foreland Uplift Provenance” is characterized by varied sedimentary
lithics, moderately high quartz, and minor feldspars and volcanic lithic grains. Sediments from
collision orogens also have typically intermediate quartz contents, high quartz/feldspar ratio, and
abundant sedimentary and metasedimentary lithic fragments (Dickinson and Suczek 1979;
Dickinson 1985).
Dickinson and Suczek (1979) and Dickinson (1985, 1988) recognized the intrinsically
composite nature of detritus shed from orogenic belts into associated sedimentary basins, but did
not attempt to establish clear conceptual and operational distinctions among the three identified
types of “Recycled Orogenic Provenances”. For instance, “Subduction Complex” and “Collision
Orogen” Provenances may both include detritus from ophiolitic mélanges, and “Collision Orogen”
and “Foreland Uplift” Provenances may both include detritus from magmatic-arc remnants in
90
90
80
70
Dissected
Arc
RO
50
70
60
60
50
Dissected Arc
40
g
nd
OPHIOLITE
PROVENANCE
20
L
Lv+Lu
Ls
Ls+Lm
Q 50
50
Unroof
in
A
90
80
80
70
30
70
60
60
50
Undissected
Ophiolite 30
50
40
Dissected
Ophiolite
Undissected
Ophiolite
Dissected
Ophiolite
F 50
AXIAL-BELT
PROVENANCE
20
Unro
10
L
Q
Lv
90
Amphibolite
facies
Amphibolite
facies
10
Metaophiolites
F
CONTINENTALBLOCK
90
PROVENANCE
L
Q
Lv+Lu
r
Un
10
Lv+Lu
90
Fluvial
foreland -basin
sandstones
F
50
40
Remnant-ocean
turbidites
30
(recycled
river sands)
destruction of labile shale/slate grains
in high-energy environments
&tHM
Anchimetamorphic
P+O+S
90
turbidites
80
70
Clastic Wedge
HMC 0.8±0.6
tHMC 0.4±0.4
60
50
40
30
20
10
L
A
70
60
20
10
80
10
Ls
Lm
HMC 8±5 tHMC 6±4
f
eo s
rad lith
g g oto
sin pr
rea ous
Inc igne
r
ter
Foredeep
turbidites
nd
tre
30
Dissected
Block
20
Undissected
Block
(recycled
beach sands)
50
40
HMC 0.7±0.6
tHMC 0.3±0.4
30
20
L
Undissected
Block
50
40
90
sandstone protholiths
60
for foredeep turbidites
70
g
fin
oo
Un
r
tre oofin
nd g
P+O+S
&tHM
60
30
Q
70
A
40
Undissected Block
80
Ls
90
Dissected
Block
50
F
CLASTIC-WEDGE
PROVENANCE
HMC 10±4
tHMC 9±3
80
70
10
Amphibolite-facies
basements
20
90
60
20
HMC 7±4
tHMC 7±4
10
Lm
70
Dissected
Block
Amphibolite-facies
metacarbonates
50
80
50
HMC 10±2
tHMC 9±2
60
30
Metaophiolites
60
30
90
80
40
field of
recycling
80
Amphibolite-facies
metapelites
70
Low-grade
covers
20
10
HMC 2.2±0.9
tHMC 0.9±0.6
40
30
20
P+O+S
&tHM
HMC 22±8
tHMC 19±8
50
Low-grade
covers
A
Eclogite-facies
metaophiolites
60
Un
ro
tre ofin
nd g
HMC 23±8
tHMC 19±9
Greenschist-facies
covers
70
50
40
Lu
HMC 25±11
tHMC 22±13
Undissected
Ophiolite
20
10
90
70
40
trend
80
60
30
(mantle)
Lm
80
30
ofing
(mantle)
(crust)
Dissected
Ophiolite
40
(crust)
10
P+O+S
&tHM
90
40
20
HMC 29±23
tHMC 26±20
g trend
10
U
tre
Undissected Arc
F
fin
20
Undissected
Arc 10
nr
oo
fin
MA
Undissected
Arc
30
g
oo
nr
20
tre
n
U
30
50
40
HMC 19±7
tHMC 16±5
Dissected
Arc
d
40
80
70
CB
10
90
80
60
30
&tHM
Lm
Q
MAGMATIC-ARC
PROVENANCE
20
Unmetamorphosed
turbidites
Lv+Lu
10
Ls
Figure 2 - Garzanti et al.
A
depleted heavy-mineral assemblages
inherited from diagenized parent sandstones
P+O+S
9
relationship with relief distribution and drainage patterns (Dickinson and Suczek 1979, p.21762178; Dickinson 1985, p.350).
Observations from modern settings, where all factors affecting sediment composition can be
verified, reveal discrepancies with the Dickinson model. Ophiolitic detritus is only marginally dealt
with. Large, subaerially exposed subduction complexes chiefly consist of offscraped turbidites and
thus typically shed abundant shale/slate grains rather than chert (Garzanti et al. 2002a; own data
from the Indo-Burman Ranges). Detritus from remnants of continental paleomargins incorporated
within thrust belts shows close affinities with anorogenic detritus of “Continental-Block
Provenance” (Garzanti et al. 2003, 2006). Collision orogens produce a wide range of
lithic/quartzolithic to quartzofeldspathic signatures, including all possible mixtures of both firstcycle and multicycle detritus from neometamorphic, paleometamorphic, plutonic, volcanic, and
sedimentary rocks. Further problems are caused by recycling of clastic wedges accreted at the
orogenic front (Garzanti et al. 2002a, 2004b, 2005).
As pointed out by Ingersoll (1990, p.733), most of these complexities cannot be succesfully
handled because “third-order fields (magmatic arc, recycled orogen, and continental block) are
useful for continental-scale analyses”, whereas “second-order fields (e.g., undissected, transitional,
and dissected arcs of Dickinson 1985) are useful at the scale of mountain ranges and basins”. In
order to improve on the resolution of provenance models, this article focuses on the structure of
single orogenic belts rather than on whole continents and ocean basins.
A Refined Two-Step Model for Orogenic Sediment Provenance. The scheme presented here is
based on the simple observation that the complicated tectonic structure of composite orogens results
from the juxtaposition and superposition of a limited number of geological domains, each including
genetically associated rock complexes elongated subparallel to the orogen’s strike. Because major
rivers reach well into the core of the composite orogen, sediments supplied to the associated basins
are invariably a mixture of detritus from different types of such linear domains (Dickinson and
Suczek 1979). If mixed detrital signatures are too varied to be modelled directly, then their
complexity can be handled by operating in two steps. We focus first on detrital modes produced by
each distinct type of orogenic domain (“primary orogenic provenances”), and only subsequently
recombine the appropriate types of primary provenances to model detrital suites recorded in
sedimentary basins (“composite orogenic provenances”).
Five types of orogenic domains are identified here as the primary building blocks of composite
orogenic prisms : 1) magmatic arcs (autochthonous or allochthonous sections of arc crust); 2)
accreted or obducted ophiolites (largely intact allochthonous sections of oceanic lithosphere); 3)
neometamorphic axial belts (polydeformed slivers of distal continental-margin crust and adjacent
COMPOSITE OROGENS
Indo-Burman-type
Subduction Complexes
MAGMATIC-ARC
PROVENANCE
OPHIOLITE
PROVENANCE
PRIMARY
OROGENIC
PROVENANCES
Minor
CLASTIC-WEDGE
PROVENANCE
Dominant locally
recycled)
Minor locally
(incorporated
Dominant
(remnantocean turbidites)
Major
(early
stage -> undissected arc; late stage ->
dissected arc)
Minor
Table 2 - Garzanti et al.
(pro-
Significant locally
side)
(late
Significant locally
(late
Dominant
(pro
Dominant
(retro-
Major
(late
stage)
(retroside)
(late
stage)
side)
Major
(early
stage)
side)
stage)
(foredeep
turbidites)
Significant locally
stage)
Dominant locally
Major
terranes)
(early
Significant locally
Dominant locally
(boudinated remnants)
Significant locally
(pro-side)
stage)
Alpine-type
Collision Orogens
Cordilleras
Dominant
(retro-
Dominant
(subduction-complex remnants)
Insignificant
Andean-type
side)
Dominant locally
(retro-
side)
Significant locally
(retro-
Oman-type
Obduction Orogens
side)
Significant locally
AXIAL-BELT
PROVENANCE
CONTINENTAL-BLOCK
PROVENANCE
(largely
Apennine-type
Thin-skinned Orogens
Major
(forelandbasin clastics)
10
oceanic lithosphere which have undergone high-pressure to high-temperature metamorphism during
subduction and subsequent exhumation); 4) paleomargin remnants (only weakly metamorphosed
allochtonous sections of continental basement and/or overlying platform to pelagic strata); and 5)
orogenic clastic wedges (accreted foreland-basin, foredeep, or remnant-ocean-basin terrigenous
sequences).
As shown by provenance studies in modern orogenic settings from the Mediterranean Sea to the
Indian Ocean, detritus produced by the erosion of each single orogenic domain is characterized by
unique detrital modes, heavy-mineral assemblages and unroofing trends, which can be tentatively
predicted and modelled. The five types of orogenic domains thus correspond to five types of
primary sediment provenances (fig. 2).
Our scheme expands on the original Dickinson model, with the following main modifications :
1) “Magmatic-Arc Provenance” is unchanged; 2) “Ophiolite Provenance” is recognized as a new
provenance type; 3) “Axial-Belt Provenance” is defined as the most distinctive type of “CollisionOrogen Provenance”; 4) marked affinities between “Foreland-Uplift Provenance” (a type of
orogenic-sediment provenance in Dickinson and Suczek 1979) and “Continental-Block
Provenance” (the anorogenic sediment provenance in Dickinson and Suczek 1979) are emphasized;
5) “Clastic-Wedge Provenance” is newly defined, in order to single out, and put emphasis on, the
thorny problem of sediment recycling. This latter provenance type consists entirely of recycled
orogen-derived detritus, whereas the former four types chiefly consist of first-cycle detritus (with
the limited exceptions of grains recycled from forearc-basin strata exposed in arc domains or from
sandstone-bearing passive-margin strata intercalated within paleomargin successions).
Such a moderate increase in complexity is held as necessary and sufficient to appropriately
handle the full variety of cases observed along modern subduction zones. The proposed scheme is
flexible enough to reproduce the complete range of mixed detrital signatures issued during erosional
denudation of composite orogens, and to predict the evolution of detrital modes and heavy-mineral
assemblages as recorded in space and time by clastic wedges deposited in arc-related, foreland,
foredeep, and remnant-ocean basins (Table 2).
Primary Provenances and Unroofing Trends
“Data for modern marine and terrestrial sands from known tectonic settings provide standards to evaluate
the effect of tectonic setting on sandstone composition” W.R.Dickinson and C.A.Suczek (1979, p.2164).
The information contained in this paragraph is based on provenance studies of modern
sediments produced and deposited in areas mostly characterized by arid climate and/or high relief.
A
ALPINE SUBDUCTION COMPLEX
ALPINE-DERIVED
FOREDEEP TURBIDITES
BOUDINAGED ALPINE BELT
Greenschist-facies covers
Remnant-ocean turbidites
D
C
10°
Amphibolite-facies basements
15°
Accreted ophiolites
45°
IC
AT
RI
AD
A
SE
E D
A C H
F
G
40°
TYRRHENIAN
SEA
B
AN
NI
IO SEA
B
E
Q
Lm
Axial-Belt
Provenance
80
90
Amphibolite-facies basements
Plutonic rocks
Foredeep turbidites
Volcanic rocks
Remnant-ocean turbidites
Paleomargin covers
(boudinaged 70
basements)
DissectedMagmatic-Arc
Provenance
Clastic-Wedge Provenance
60
80
Adriatic
rivers
(pro-side)
Axial-Belt
Provenance
(boudinaged covers)
60
(foredeep turbidites)
40
50
Axial-Belt
Provenance
Clastic-Wedge
Provenance
40
(boudinaged covers)
20
10
F
Clastic-Wedge
Provenance
Thyrrhenian
rivers
(remnant-ocean
20
turbidites)
(retro-side)
Ophiolite
Undissected- Provenance
Magmatic-Arc
Provenance
30
(foredeep
turbidites)
Ophiolite
Provenance
F
Thyrrhenian
rivers
Undissected-Continental-Block
L
Provenance
Adriatic
rivers
(pro-side)
(retro-side)
10
Provenance
Lv+Lu Undissected-Magmatic-Arc Clastic-Wedge
Ls
(remnant-ocean turbidites)
MAGMATIC ARC (Tuscan and Roman Provinces)
Monzogranites
(boudinaged basements)
70
50
30
Axial-Belt
Provenance
Accreted ophiolites
Greenschist-facies covers
90
Ultrapotassic lavas
G
Provenance
PALEOMARGIN COVERS (Apulia)
Pelagic limestones and cherts
H
Figure 3 - Garzanti et al.
11
The described compositional signatures can thus be considered as unaffected by significant
chemical weathering and diagenetic dissolution, and to faithfully reflect plate-tectonic setting and
lithology of source terranes. Such primary detrital modes can be used as a reference for assessing
provenance of ancient clastic suites deposited in comparable geodynamic settings, or for inferring
compositional modifications caused by intense weathering in hot humid climates or by diagenesis.
High-resolution bulk-petrography data were collected by the Gazzi-Dickinson method (Ingersoll
et al. 1984; Garzanti and Vezzoli 2003) on river and beach sands derived from single homogeneous
orogenic domains (“first- to second-order sampling scales” of Ingersoll 1990). Further quantitative
information on heavy-mineral assemblages is provided in Garzanti and Andò (2006a).
Magmatic-Arc Provenances. Volcanic detritus from basaltic, andesitic, and rhyodacitic lavas
and ignimbrites representing the arc cover consists of volcanic lithic grains, plagioclase, and
pyroxenes (fig. 3G,“Undissected-Arc Provenance”; Marsaglia and Ingersoll 1992). Plutonic detritus
from diorite-granodiorite batholiths representing the plutonic roots of the arc massif chiefly includes
quartz, plagioclase, K-feldspar, and mainly blue-green hornblende (fig. 3F, 4D; “Dissected-Arc
Provenance”). The ideal compositional trend recorded by terrigenous sequences accumulated in
forearc and other arc-related basins during unroofing of the arc massif is, therefore, characterized by
the progressive increase of quartz, K-feldspar and blue-green hornblende at the expense of volcanic
lithic grains and pyroxenes (Dickinson 1985; Garzanti and Andò 2006a).
Ophiolite Provenance. Tectonically accreted or obducted sections of oceanic lithosphere, which
escaped subduction and orogenic metamorphism, shed mafic to ultramafic detritus with peculiar
petrographic and mineralogical features (Nichols et al. 1991). Distinct signatures characterize
sediments supplied during unroofing of progressively deeper stratigraphic levels of the multilayered
oceanic lithosphere. Pillow lavas and sheeted dikes of the upper crust shed lathwork volcanic to
altered diabase lithic grains and clinopyroxene or low-grade minerals grown during oceanic
metamorphism (e.g., actinolite and epidote). Orthopyroxene-phyric boninite grains may be common
in detritus from supra-subduction-zone ophiolites (fig. 5D; Bloomer et al. 1995; Garzanti et al.
2000). Plutonic rocks of the lower crust supply cumulate, gabbro and plagiogranite rock fragments,
calcic plagioclase, diopsidic clinopyroxene, and green/brown hornblende; hypersthene grains may
reflect the hydrous, arc-related character of late-stage magmatic source rocks. Serpentinized mantle
harzburgites supply lizardite-serpentinite grains with pseudomorphic cellular texture, enstatitic
orthopyroxene, olivine, and rare chrome spinel (fig. 3E, 5E; Garzanti et al. 2002b).
Axial-Belt Provenance. Detritus supplied by the axial pile of neometamorphic nappes
representing the central backbone of collision orogens is influenced by several factors, including
metamorphic grade of source rocks and relative abundance of continental versus oceanic protoliths.
NEOMETAMORPHIC AXIAL BELT
(Penninic Domain)
Blueschist-facies calcschists
Amphibolite-facies gneisses
A
Retrogressed eclogite-facies serpentineschists
C
B
Lm
Q
Neometamorphic gneisses
Neometamorphic covers
90
Neometamorphic ophiolites
Calcalkaline batholiths
80
Axial-Belt
Provenance
(gneiss domes)
Paleometamorphic
basements
Unmetamorphosed
covers
Remnant-ocean
turbidites
Axial-Belt
Provenance
70
major
rivers
60
DissectedContinental-Block
Provenance
90
80
70
Axial-Belt
Provenance
(gneiss domes)
60
DissectedMagmatic-Arc
50
Provenance
Axial-Belt
Provenance
(low-grade covers)
50
(low-grade covers)
40
40
30
Clastic-Wedge
Provenance
DissectedContinental-Block
Provenance
20
Clastic-Wedge
Provenance
30
20
Axial-Belt Provenance
Axial-Belt
Provenance
(metaophiolites)
(metaophiolites)
10
10
Undissected-Continental-Block Provenance
Undissected-Continental-Block Provenance
L Lv+Lu
Unmetamorphosed sedimentary covers
Paleometamorphic basements
D
E
pro
foreland
basin
B
E
F
axial belt
forebelt
G
Ls
PALEOMARGIN REMNANTS (Helvetic and Southalpine Domains)
MAGMATIC ARC (Periadriatic Zone)
Tonalites and granodiorites
major
rivers
retrobelt
retro
foreland
basin
REMNANT-OCEAN TURBIDITES
(Liguride Units)
F
C
A D
R
PE
UP
ER
W
LO
E
AT
PL
E
AT
PL
G
Figure 4 - Garzanti et al.
12
Metasedimentary cover nappes shed lithic to quartzolithic detritus, including metapelite,
metapsammite, and metacarbonate grains of various ranks (fig. 3A, 4B, 5A); only amphibolite-facies
metasediments supply abundant heavy minerals (e.g., almandine garnet, staurolite, kyanite,
sillimanite, and diopsidic clinopyroxene). Continental-basement nappes shed hornblende-rich
quartzofeldspathic detritus (fig. 3B, 4A). Largely retrogressed blueschist to eclogite-facies
metaophiolites supply albite, metabasite and foliated antigorite-serpentinite (serpentine-schist)
grains (fig. 4C), along with abundant heavy minerals (e.g., epidote, zoisite, clinozoisite, actinolitic
to barroisitic amphiboles, glaucophane, omphacitic clinopyroxene, and lawsonite).
Increasing metamorphic grade and/or deeper tectonostratigraphic level of source rocks may be
reflected by: a) increasing rank of metamorphic rock fragments (as indicated by progressive
development of schistosity and growth of micas and other index minerals; MI index of Garzanti and
Vezzoli 2003); b) increasing feldspars; c) increasing heavy-mineral concentration (HMC index of
Garzanti and Andò 2006b); d) increasing hornblende, changing progressively in color from
blue/green to green/brown (HCI index of Garzanti et al. 2004b); e) successive appearance of
chloritoid, staurolite, kyanite, fibrolitic and prismatic sillimanite (MMI index of Garzanti and Andò
2006a).
Continental-Block Provenance. Allochthonous platform to pelagic strata, representing the
tectonically dismembered remnants of sedimentary successions originally deposited on a
continental paleomargin, supply diverse sedimentary to low-rank metasedimentary grains (e.g.,
limestone, dolostone, chert, shale, slate, and metacarbonate; fig. 3H, 4F, 5B, 5C), locally associated
with quartz, feldspars, or volcanic/metavolcanic rock fragments recycled from interbedded
siliciclastic or volcaniclastic units (“Undissected-Continental-Block Provenance”). Tectonically
imbricated basement units shed quartzolithic to quartzofeldspathic sands with micas, garnet,
staurolite, kyanite, sillimanite, hornblende, epidote or pyroxenes (fig. 4E; “Dissected-ContinentalBlock Provenance”).
Detritus supplied by paleomargin remnants incorporated within thick-skinned thrust belts vary
markedly during unroofing of deep-seated basement rocks. Heavy-mineral concentration
progressively increases, and detrital modes ideally change from lithic sedimentaclastic or locally
sedimentaclastic-volcaniclastic (cover sequences), to quartzolithic, quartzofeldspathic, and
feldspathic metamorphiclastic signatures (greenschist-facies to granulite-facies basement units;
Garzanti et al. 2006). Instead, thin-skinned orogens associated with westward subduction zones
mostly incorporate cover strata, which invariably supply lithic sedimentary detritus (with various
amounts of recycled quartz).
CONTINENTAL LOWER PLATE (SUBDUCTED ARABIAN MARGIN)
Lawsonite schists & marbles
Outer margin cherts and turbiditic limestones
Inner margin carbonate platform
B
A
C
Ls+Lm
Q 40
High-pressure covers
90
Inner margin platform
Continental-Block
Provenance
Outer margin sequences
Outer margin
80
Oceanic crust
30
Inner margin
Oceanic mantle
Axial-Belt
Provenance
70
Axial-Belt
Provenance
60
20
50
Continental-Block
Provenance
Inner margin
40
30
DissectedOphiolite Provenance
Outer margin
10
20
UndissectedOphiolite Provenance
10
DissectedOphiolite Provenance
F 40
L
UndissectedOphiolite Provenance
Lv
Lu
OCEANIC UPPER PLATE (OBDUCTED OPHIOLITE)
Boninitic lavas
Crust
OPH
IOLIT
E
Man
tle
INNER
MARGIN
PLATFORM
B
OU
T
SE E R M
QU AR
EN GI
CE N
S
D
E
C
OPHIOLITE
ARABIAN
MARGIN
Mantle harzburgites
Blueschist
A
Eclogite
E
D
Figure 5 - Garzanti et al.
13
Clastic-Wedge Provenance. Sand recycled from fluvial to turbiditic foreland-basin, foredeep, or
remnant-ocean-basin clastic sequences tend to reproduce the composition of orogen-derived (and
thus typically quartzolithic; Dickinson 1985) parent sandstones, generally with significant addition
of labile mudrock grains (fig. 3C, 3D, 4G; Cavazza et al. 1993; Fontana et al. 2003). Because
unstable minerals are extensively dissolved during diagenesis of parent sandstones, recycled heavymineral assemblages only include stable to ultrastable species and have very low concentrations
(Garzanti et al. 2002a; Garzanti and Andò 2006b).
Composite Orogenic Provenances
“Complex orogenic belts may include all three kinds of provenance in subparallel linear belts which may
jointly contribute mixed detritus to varied successor basins. Arc-derived detritus may also be incorporated
into such mixed suites …” W.R. Dickinson and C.A. Suczek (1979, p.2176)
In order to illustrate how primary provenances may combine in different scenarios of plate
convergence and give rise to composite orogenic provenances, we follow an exemplary rather than
exhaustive approach. We describe the signatures of sediments supplied by a large subduction
complex (Indo-Burman Ranges and Andaman Islands), by a thin-skinned orogen produced by
westward subduction (Apennines), and by three types of “thick-skinned” composite orogens
produced by eastward subduction of continental-beneath-oceanic (Oman “obduction orogen”),
oceanic-beneath-continental (Andean cordillera), and continental-beneath-continental lithosphere
(Alpine and Himalayan “collision orogens”).
Detritus from the Indo-Burman-Andaman Subduction Complex. Subduction complexes large
enough to be exposed subaerially and to become significant sources of terrigenous detritus are
typically formed by tectonic accretion above trenches choked with thick sections of remnant-ocean
turbidites (Ingersoll et al. 2003). This is the case of the outer ridge extending from the Indo-Burman
Ranges to offshore Sumatra, which largely consists of accreted abyssal-plain sediments ultimately
derived from the rising Himalaya and locally overthrust by volcaniclastic and ophiolitic forearc
sequences (Curray 2005; Allen et al. in press). Modern sands from the Indo-Burman Ranges and
Andaman Islands consist of quartz and feldspars recycled from turbiditic sandstones, with various
amounts of shale/slate grains shed from turbiditic mudrocks (“Clastic-Wedge Provenance”).
Ultramafic and mafic detritus is supplied locally from accreted forearc ophiolites (“Ophiolite
Provenance”). Additional volcanic detritus and chert grains are recycled from arc-derived and deepwater sediments of the forearc basin (fig. 6).
Detritus from the Apenninic Thin-skinned Orogen. Modern sands from the Apennines are
derived from diverse source rocks, including pelagic cherty limestones and carbonate platforms of
Lms + Lmf
Q
90
REMNANT-OCEAN TURBIDITES
Recycled beach sands
Recycled river sands
90
OBDUCTED OPHIOLITES
80
80
Upper oceanic crust
Lower oceanic crust
Oceanic mantle
70
70
60
Clastic-Wedge
Provenance
(remnant-ocean
turbidites)
50
50
40
40
30
30
20
10
Clastic-Wedge
Provenance
(remnant-ocean
turbidites)
60
20
Ophiolite
Provenance
Ophiolite
Provenance
10
minor
F
L
Lv + Lu
Figure 6 - Garzanti et al.
ophiolitic
detritus
Ls
14
the Apulian paleomargin (“Undissected-Continental-Block Provenance”), foredeep turbidites
accreted along the pro-side of the orogen (“Clastic-Wedge Provenance”), and volcanic or locally
plutonic arc rocks exposed along its retro-side (“Magmatic-Arc Provenance”). Because westward
Apenninic subduction started in the late Paleogene along the retro-side of eastward Alpine
subduction (Doglioni et al. 1998), the composite Apenninic orogen is capped by the proto-Alpine
subduction complex, including remnant-ocean turbidites and accreted ophiolites, and incorporates
boudinaged greenschist-facies to amphibolite-facies remnants of the Alpine axial belt (fig. 3).
Because of modest tectonic uplift, erosional unroofing is limited and the spatial distribution of
detrital signatures is largely controlled by geological inheritance (Garzanti et al. 2002a).
Detritus from the Oman Obduction Orogen. Modern sands from the Oman obduction orogen
are mainly derived from mafic and ultramafic rocks occupying the highest structural position of the
tectonic pile. Sedimentary rock fragments, subordinate quartz, and metamorphic detritus are
supplied by frontally accreted to deeply subducted continental-margin rocks, exposed in tectonic
windows (fig. 5; Garzanti et al. 2002b). Where and when erosion bites into deeper structural levels,
detritus of “Ophiolite Provenance” is thus mixed with, and finally ideally replaced by, detritus of
“Continental-Block” and “Axial-Belt” provenances.
Detritus from the Andean Cordillera. Modern sands from the Andes show a marked
asymmetry. Volcano-plutonic arc detritus is dominant along the pro-side of the cordillera, whereas
quartzolithic to quartzose detritus with abundant metamorphic lithic grains characterize its retroside (fig. 7; Potter 1994). Sediments in the Peru-Chile Trench range from “Undissected-Arc
Provenance” adjacent to areas of active volcanism, to “Transitional-Arc” and “Dissected-Arc”
provenances where the batholithic roots of the inactive arc massif have been uplifted and widely
exposed along the Cordillera Occidental (Yerino and Maynard 1984; Thornburg and Kulm 1987).
Sediments shed by metamorphic to granitoid basement rocks and Paleozoic to Mesozoic strata of
the Cordillera Oriental and Subandean thrust-belt display “Continental-Block Provenance”, and are
invariably mixed with subordinate volcano-plutonic detritus from the arc massif (DeCelles and
Hertel 1989). Recycling of weathered orogenic detritus (“Clastic-Wedge Provenance”) leads to a
marked increase in quartz across subequatorial lowlands (Johnsson et al. 1988).
Detritus from the Alpine and Himalayan Collision Orogens. Modern sands from the Alps and
the Himalaya chiefly include quartz, feldspars, metamorphic rock fragments, micas, and amphibolegarnet-epidote heavy-mineral assemblages, reflecting major supply from partially retrogressed highpressure (e.g., Penninic Domain) to high-temperature (e.g., Greater Himalaya) neometamorphic
rocks of the high-topography and rapidly exhumed axial belt. Similar signatures, however, may
characterize first-cycle detritus from paleometamorphic basements representing remnants of the
Q
PERU-CHILE TRENCH
90
field of recycling &
chemical weathering
Continental-Block
Provenance
60
40
30
RA
20
20
10
F
Continental-Block
Provenance
40
L
E
Magmatic-Arc
Provenance
50
I L
RD
CO
Johnsson et al. 1988
30
70
DeCelles & Hertel 1989
HE
FT
50
80
³ 50% recycled
first-cycle
EO
60
Clastic-Wedge
Provenance
SID
OTR
RE
80
90
E RA
SID E
O- ILL
TR RD
RE CO
E
TH
OF
Yerino & Maynard 1984
Y & M 1984 average
Thornburg & Kulm 1987
Lm
FORELAND BASIN
100% recycled
Bolivia
Peru
SUBANDEAN THRUST BELT
> 90% recycled
Recycling of weathered
foreland-basin clastics
Magmatic-Arc
Provenance
10
PRO-SIDE OF THE CORDILLERA
L
Lv PRO-SIDE OF THE CORDILLERA
Figure 7 - Garzanti et al.
Ls
15
continental margins caught in collision, as well as polycyclic detritus recycled from orogen-derived
clastic wedges accreted along the mountain front (fig. 4; Garzanti et al. 2004b, 2006).
In foreland-basin to remnant-ocean-basin successions, neometamorphic detritus of “Axial-Belt
Provenance” can be differentiated from paleometamorphic detritus of “Continental-Block
Provenance” only by using appropriate detrital-geochronology techniques (Najman 2006). As a
further complexity, allochthonous remnants of continental paleomargins not only are commonly
overthrust by the axial belt and confined to external parts of the orogen (Helvetic Domain, Lesser
Himalaya), but may as well lie structurally above it as a “tectonic lid” (Austroalpine Domain,
Tethys Himalaya). External belts may directly face foreland basins on both sides of the orogen,
where the axial belt is narrow and topographically subdued (e.g., Maritime and Eastern Alps).
The composition of foreland-basin sediments may change from volcaniclastic, ophioliticlastic or
sedimentaclastic/low-rank metasedimentaclastic in early syn-collisional stages, when detritus from
volcanic arcs and subduction complexes may be dominant (“Taiwan stage”; Dorsey 1988; Garzanti
et al. 1996; Najman and Garzanti 2000), to high-rank neometamorphiclastic at later collisional
stages, when the axial metamorphic core of the orogen starts to be rapidly exhumed (White et al.
2002). Focused erosion of rapidly uplifted gneiss domes may then produce huge volumes of
hornblende-rich quartzofeldspathic detritus, which typically exceed the storage capacity of
associated foreland basins and can even reach totally unrelated sedimentary basins thousands of
kilometers away (Ingersoll et al. 2003; Garzanti et al. 2004a, 2004b). Volcanic or ophiolitic detritus
becomes volumetrically insignificant, but contributions from dissected-arc massifs may remain
locally prominent (Garzanti et al. 2005). Because of the progressive lateral growth of external belts,
which shield the foreland basin from axial-belt detritus, compositional trends may revert in time to
sedimentaclastic/low-rank metasedimentaclastic (White et al. 2002). Detrital signatures of forelandbasin sediments are controlled by the entry points of high-rank neometamorphiclastic detritus
carried by major rivers draining the axial belt, and thus vary irregularly along strike and are strongly
dependent on drainage changes (Muttoni et al. 2003; Najman et al. 2003).
Conclusions
“Provenance interpretations for sedimentary assemblages can be addressed most effectively in the context
of global paleogeographic patterns inferred from paleotectonic reconstructions and can be used to test
such reconstructions.” W.R.Dickinson (1988, p.22)
Orogens formed at convergent plate margins, representing topographically elevated sources of
detritus, are composite geological structures of great complexity, including diverse rock units
16
assembled in various ways by geodynamic processes. Orogenic sediments thus embrace a large
range of mixed signatures, including variable proportions of both first-cycle and multiciycle detritus
from neometamorphic, paleometamorphic, sedimentary, and igneous rocks. Unravelling provenance
of orogen-derived terrigenous successions is consequently an arduous task, which requires a
detailed, but at the same time simple and flexible, reference model.
In order to establish a classification of orogenic belts and orogenic sediment provenances, we
reduce the numerous possible plate interactions observed along subduction zones by selecting a
limited number of variables (westward vs. eastward subduction polarity; oceanic vs. continental
downgoing and overriding plates). Eight possible scenarios of plate convergence are thus
recognized, each characterized by the tectonic assembly of a distinct type of composite orogen (fig.
1). As a further and most important simplification, we represent the structure of orogenic belts as a
hierarchy of genetically associated rock complexes generated by tectonic and magmatic processes
along subduction zones. The diversity of composite orogens is thus seen as resulting from
juxtaposition and superposition of a limited number of sub-parallel geological domains.
Five types of such elongated orogenic domains are identified as the primary building blocks of
composite orogenic prisms : 1) magmatic arcs (autochthonous or allochthonous arc crust); 2)
obducted or accreted ophiolites (allochthonous oceanic lithosphere); 3) neometamorphic axial belts
(subducted continental-margin crust or adjacent oceanic lithosphere); 4) paleomargin remnants
(allochtonous continental crust); and 5) orogenic clastic wedges (allochthonous foreland-basin,
foredeep, or remnant-ocean-basin fills).
Detritus produced by erosion of each of these primary orogenic domains is characterized by
specific detrital modes, heavy-mineral assemblages and unroofing trends, which can be predicted
and modelled. The five primary orogenic domains thus correspond to five (four chiefly first-cycle
and one polycyclic) primary types of sediment provenances, that refine and expand on the classic
model proposed by the Dickinson school since the late Seventies (fig. 2). As a final step, the
complexity of detrital signatures produced by each type of composite orogen as a whole may be
represented as resulting from a limited number of combinations of the five primary provenances in
various proportions (fig. 3 to 7).
This relatively simple scheme is flexible enough to describe the complete range of mixed detrital
signatures observed along modern subduction zones from the Mediterranean Sea to the Indian and
Pacific Oceans. It is thus proposed here as a conceptual tool to model the composition of sediments
produced by erosional denudation of composite orogenic systems, and to predict the evolution of
detrital modes and heavy-mineral assemblages as recorded in space and time by clastic successions
deposited in arc-related, foreland, foredeep, and remnant-ocean sedimentary basins.
17
ACKNOWLEDGMENTS
This work benefited through the years from numerous fruitful discussions with, and precious advice
by, Bill Dickinson, Ray Ingersoll, Yani Najman, Abhijit Basu, Salvatore Critelli, Peter DeCelles,
Andrea Di Giulio, Daniela Fontana, Bruno Lombardo, Curzio Malinverno, Maria Mange, Magda
Minoli, Renzo Valloni, Hilmar von Eynatten, Gert Weltje, and Gianni Zuffa. Careful stimulating
reviews by Raymond Ingersoll and Peter Cawood are greatfully acknowledged.
18
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25
FIGURE CAPTIONS
Figure 1. The eight true-scale schematic diagrams illustrate different styles of orogenic deformation
for the eight identified scenarios of plate convergence. Orogens are made of rocks accreted from the
lower and/or upper plates; shades of continental crust highlight upper plate (dark brown) versus
lower plate (light brown) contributions. East-facing, singly vergent and low-relief prisms largely
consist of deformed lower-plate rocks (panels C and D). West-facing, doubly vergent and highrelief orogens mostly consist of deformed upper-plate rocks in pre-collisional stages (panel F),
lower-plate rocks being massively involved only during final continental collision (panels G and H).
High-pressure neometamorphic rocks, exhumed in west-facing orogens, are light blue. The
profound global asymmetry between east-facing versus west-facing orogens and subduction zones
can be fully appreciated only when the mainstream of plate motions is recognized.
Figure 2. Detrital modes, heavy-mineral assemblages and compositional trends for the five
identified types of primary provenances. Plotted are sample means of modern sand provinces in
areas with arid climate and/or high relief (sources cited in text); arc-derived volcano-plutonic sands
include 32 Quaternary circum-Pacific suites compiled by Marsaglia and Ingersoll (1992). Samples
with transitional sub-provenance are grey in all diagrams. Q= quartz, F= feldspars, L= lithic grains
(including Lc carbonate lithics; symbols have thick and very thick outline if Lc/QFL% ≥ 20 and ≥
50, respectively). Lm= metasedimentary and felsic metaigneous lithic grains; Lv= volcanic,
metavolcanic and mafic metaigneous lithic grains; Lu = ultramafic lithic grains (serpentinite,
serpentine-schist); Ls= sedimentary lithic grains. A= amphiboles; P+O+S= pyroxenes + olivine +
spinel; &tHM= other transparent heavy minerals. 90% confidence regions about the mean
calculated after Weltje (2002). Provenance fields after Dickinson (1985; MA= Magmatic Arc; CB=
Continental-Block; RO= Recycled Orogen); field of recycling after Garzanti et al. (2006).
Figure 3. Detrital modes of modern sands from the Apenninic thin-skinned orogen. Plotted are river
or beach samples derived from one single geological domain or sub-domain (Garzanti et al. 2002a;
first-order sampling scale of Ingersoll 1990). Also shown are fields for the ten largest rivers on each
side of the orogen (own unpublished data; second-order sampling scale of Ingersoll 1990). All of
these carry recycled detritus of “Clastic-Wedge Provenance”, associated with lithic sedimentary
detritus of “Undissected-Continental-Block Provenance” (Adriatic rivers draining the pro-side of
the orogen) or with feldspatholithic to arkosic detritus of “Magmatic-Arc Provenance” (Thyrrhenian
rivers draining the retro-side of the orogen). Detritus from remnants of the Alpine subduction
26
complex and axial belt are locally dominant (Northern Apennines and Calabria, respectively).
Petrographic parameters, symbols, provenance fields and confidence regions about the mean as in
fig. 2. Scale bar = 250 microns; all photos with crossed polars.
Figure 4. Detrital modes of modern sands derived from the Alpine collision orogen. Plotted are
river samples derived from one single geological domain or sub-domain (Garzanti et al. 2004b;
2006). Major rivers (Rhône, Rhein, Inn, Salzach, Mur, Drau, Adige, and Po) carry mixed detritus
mostly supplied by Axial-Belt neometamorphic rocks and by Continental-Block paleometamorphic
and sedimentary rocks in various proportions. Petrographic parameters, symbols, provenance fields
and confidence regions about the mean as in fig. 2. Profile modified after Polino et al. (2002). Scale
bar = 250 microns; all photos with crossed polars.
Figure 5. Detrital modes of modern sands from peri-Arabian obduction orogens. Plotted are river or
beach samples derived from one single geological domain or sub-domain (Garzanti et al. 2000;
2002a). Crust-derived feldspatholithic detritus to mantle-derived lithic detritus of “Ophiolite
Provenance” is shed by the oceanic upper plate, whereas sedimentary to neometamorphic
metasedimentary and metavolcanic detritus of “Continental-Block” to “Axial-Belt” Provenances is
shed by the continental lower plate. Petrographic parameters, symbols and confidence regions about
the mean as in fig. 2. Scale bar = 250 microns; all photos with crossed polars.
Figure 6. Detrital modes of modern sands from the Indo-Burman-Andaman subduction complex.
Plotted are single samples from major rivers and beaches (own unpublished data). Recycled detritus
of “Clastic-Wedge Provenance” includes various amounts of shale/slate lithic grains eroded from
turbiditic mudrocks. Sands from the Indo-Burman Ranges include minor volcanic-arc detritus and
invariably negligible chert (Lv/QFL% 3±2, Lch/QFL% 0.5±0.5). Ophiolitic detritus is shed locally
from forearc slivers found at the top of the orogenic stack in the Andaman Islands, where beach
sands may locally contain chert (symbols have thick outline if Lch/QFL% 5÷10 and Lch/L%
10÷15; and very thick outline for the only chert-rich sample where Lch/QFL = 38 and Lch/L% =
68). Petrographic parameters, symbols, provenance fields and confidence regions about the mean as
in fig. 2.
Figure 7. Detrital modes of modern sands from the Andean cordillera. Feldspatholithic suites of
“Magmatic-Arc Provenance” characterize the west-facing pro-side of the orogen (modes of deepsea samples after Yerino and Maynard 1984, and Thornburg and Kulm 1987). Instead, quartzolithic
27
sands of “Continental-Block” to “Clastic-Wedge” Provenance, showing increasing degree of
chemical weathering towards lower equatorial latitudes (Johnsson et al. 1988), characterize its eastfacing retro-side (modes of river samples after DeCelles and Hertel 1989). Petrographic parameters,
symbols, provenance fields and confidence regions about the mean as in fig. 2.
Table 1. Structural features of orogenic belts formed in the eight identified scenarios of plate
convergence (O= oceanic lithosphere; C= continental lithosphere). Topographic relief, deformation
style, and rock units involved are largely controlled by subduction polarity. The illustrated endmember orogens may evolve dynamically through geologic time. When a continental margin
arrives at the oceanic trench, pull-arc orogens (Laubscher 1988) evolve from Northern-NewZealand-type to Banda-type (or Carpathian-type), “ophiolite-capped” push-arc orogens from
Andaman-type to Oman-type, and higher-topography push-arc orogens involving continental rocks
in the hangingwall of the subduction zone from Andean-type to Himalayan-type.
Table 2. The complex problem of modelling orogenic provenance is handled by operating in two
steps. The diagnostic detrital modes produced by each distinct orogenic domain are first identified
(“primary orogenic provenances”), and next appropriately recombined to describe detrital suites
recorded by arc-related, foreland, foredeep, and remnant-ocean basin fills (“composite orogenic
provenances”). Locally significant to locally dominant provenances may characterize first- to
second-order-scale samples, whereas detritus of mixed provenance is invariably recorded at thirdorder scale (e.g., big rivers; Ingersoll 1990).