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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: eduardo.garzanti@unimib.it)

1 Author for correspondence

2 Dipartimento di Scienze della Terra, Università “La Sapienza”, Piazzale Aldo Moro 5, 00185

Roma, Italy (e-mail: carlo.doglioni@uniroma1.it)

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 (Indo-

Burman-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, thick-

skinned, 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 thick-

skinned 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, high-

topography 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).

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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 westward-

subducting 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 Sumatra-

Java 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

MARIANAS oceanic intraoceanic SANDWICH offscraping arc

N. NEW ZEALAND Inherited upper plate oceanic boudinagedCALABRIA + Lower plate offscraping belt & prism

BANDA Inherited upper plate continental boudinagedS. APENNINES + Lower plate offscraping belt & prism

CARPATHIANS Inherited upper plate continental boudinagedN. APENNINES + Lower plate offscraping belt & prism

ANDAMANS Upper plate oceanic intraoceanicVANUATU + Lower plate collision prism

ANDES continentalCASCADES buckling

OMAN Upper plate obductionPAPUA + Lower plate orogen

HIMALAYA Upper plate continental collisionALPS + Lower plate collision orogen

Table 1 - Garzanti et al.

cordillera

C\C

C\O obduction

O\C Mostly upper plate

C/C

EASTWARD SUBDUCTION HIGH RELIEF, THICK-SKINNED, DOUBLY VERGENT "PUSH-ARC" OROGENS

SLOWER SUBSIDENCE OF TRENCH OR FORELAND BASIN NO TRUE BACKARC BASIN

O\O

WESTWARD SUBDUCTION LOW RELIEF, THIN-SKINNED, SINGLY VERGENT, "PULL-ARC" OROGENS

RAPID SUBSIDENCE OF TRENCH OR FOREDEEP OCEANIC (OR ENSIALIC) BACKARC BASIN

O/O Lower plate

O/C

C/O

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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 thick-

skinned 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., Hindukush-

Karakorum, 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 remnant-

ocean 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 abyssal-

plain 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

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relationship with relief distribution and drainage patterns (Dickinson and Suczek 1979, p.2176-

2178; 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 first-

cycle 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

Indo-Burman-type Subduction Complexes

Apennine-type Thin-skinned Orogens

Oman-type Obduction Orogens Andean-type Cordilleras Alpine-type

Collision Orogens

MAGMATIC-ARC PROVENANCE

Minor (largely recycled)

Dominant locally (retro-side)

Minor locally (retro-side)

Dominant (pro-side)

Significant locally (early stage -> undissected arc; late stage ->

dissected arc)

OPHIOLITE PROVENANCE

Significant locally (retro-side)

Dominant locally (subduction-complex remnants)

Dominant (early stage)

Significant locally (pro-side)

Significant locally (early stage)

AXIAL-BELT PROVENANCE Insignificant Dominant locally

(boudinated remnants)Significant locally (late

stage)Significant locally (pro

side) Dominant (late

stage)

CONTINENTAL-BLOCK PROVENANCE

Significant locally (incorporated terranes)

Major Dominant locally (late stage)

Dominant (retro-side)

Major (late stage)

CLASTIC-WEDGE PROVENANCE

Dominant (remnant-ocean turbidites)

Major (foredeep turbidites)

Minor Major (retro-side)

Major (foreland-basin clastics)

Table 2 - Garzanti et al.

COMPOSITE OROGENS

PRIMARYOROGENIC

PROVENANCES

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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 “Collision-

Orogen 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.

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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.

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-Continental-

Block 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).

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 heavy-

mineral 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 deep-

water 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

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 retro-

side (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 amphibole-

garnet-epidote heavy-mineral assemblages, reflecting major supply from partially retrogressed high-

pressure (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

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 foreland-

basin 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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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 high-

relief 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 deep-

sea 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 east-

facing 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 end-

member orogens may evolve dynamically through geologic time. When a continental margin

arrives at the oceanic trench, pull-arc orogens (Laubscher 1988) evolve from Northern-New-

Zealand-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 third-

order scale (e.g., big rivers; Ingersoll 1990).