Japanese Journal of Structural Geology, No. .1, ,-ῌ-0, ,**-

ArticleDeformation pattern of convergent margin within
accretionary prism, Aleutian arc-trench system :,D ﬁnite element modeling
Handi Lu*
Abstract : A series of ﬁnite element models simulate the paleostress ﬁeld in theaccretionary prism at di#erent stages during the Late Cretaceous accretion of Mesozoic
melange belts (McHugh Complex and Iceworm melamges) in the Chugach convergent
margin, central-south Alaska. The accretionary prism is assumed to be elastic continuum. The subduction of the Paciﬁc plate beneath the North American plate and theunderplating of the oceanic crust is subjected as the boundary condition. The ﬁrst
episode of the Late Cretaceous accretion is modeled with two types of accretionary
wedge of di#erent length and width. From the ﬁnite element modeling, at initiation
stage of accretion the paleostress s+ is compressive and nearly horizontal and thrustfaults develop as the Paciﬁc plate subducts beneath the North American plate. The
underplating of the oceanic crust reoriented the paleostress ﬁeld in the prism. Normal
faults develop at the base of the accretionary wedge. Strike-slip faults develop in the
deep portion of the oceanic crust. The underplating process might cause the lateralcrustal growth and thickening of the wedge. At late stages of accretion, the paleostresss+ is compressive and nearly vertical at the toe of the wedge as the Late Cretaceous
accreted sediments are poorly consolidated. However s+ is compressive and nearlyhorizontal as the accreted sediments are well consolidated. The consolidation state of
the Late Cretaceous acreted sediments reguates the paleostress distribution at the toeof the wedge. The fault development at initiate stages of the Late Cretaceous accretion
would result in tectonic and stratal disruption within the accretionary prism and leads
to the formation of melamge belts. The formation of the melange belts is relative withthe subduction of the Paciﬁc plate beneath the North American plate and the underplating of the oceanic crust.
Key words : accretionary prism, Aleutian arc-trench system, ﬁnite element method, Late
Cretaceous accretion, subduction, underplating.

+.

Introduction

The eastern Aleutian arc-trench system has
been studied intensively during the last four
decades since the great +30. Alaskan earthquake. The Gulf of Alaska is tectonically one of
the most active regions in the world. The convergent margin of the North American plate isone of a few margins where older accretionary
history is extensively exposed on land andReceived . June ,**, ; accepted +0 January ,*** Graduate School of Engineering and Science, Universityof the Ryukyus, Nishihara, Okinawa 3*-ῌ*,+-where crustal growth is still active. Along the
Aleutian trench, the Paciﬁc plate moves northnorthwest at a rate of /ῌ1 cm/y relative to theNorth American plate as shown in Fig. + (Jacob,+320).
Although the subduction along the southern
Alaska margin has continued since Early Jurassic, the growth of the Kodiak segment of thecontinental margin has been dominated by two
relatively short episodes of accretion or crustal
growth. The ﬁrst episode of accretion is in Late
Cretaceous and the second in Early Eocene(Byrne, +320 ; Byrne and Fisher, +321).

ῌ 23 ῌ

Handi Lu

Fig. +
Plate-tectonic map of the Gulf of Alaska showing the Paciﬁc, North American, and Juan de Fuca plates, theirabsolute plate-motion vectors (solid arrows), and vectors of relative plate motions (open arrow). Isodepth contours of the
deep seismic zone are displayed. Large numbers next to arrows indicate plate motion rates in cm/y (Jacob, +320).

The ﬁnite element method is an e#ective tool
to recover the regional paleostress ﬁeld and deformation pattern if the structural and tectonic
evolutionary history is clariﬁed. Lundgren et al.
(+33/) used ﬁnite element modelling technique tostudy crustal deformation in Alaska arc-trenchsystem. Hassani et al. (+331) proposed threeﬁnite element models (linear elastic, Maxwell
viscoelastic and elastoplastic) to simulate platedeformation and stress during subduction process. Toth and Gurnis (+332) utilized a viscoelastic medium to study the dynamics of subduction
initiation at preexisting fault zones in order tohave a greater understanding of subduction process. Wang and Wang (+333) studied the kinematics and dynamics of the North American
plate assuming a Newtonian rheology. Tang andChemenda (,***) utilized the elastoplastic rheology to study the deformation and failure of theoverriding plate during arc-continent collisionin Taiwan.

In order to determine the deformation patternof convergent margin within the accretionary
prism, Aleutian arc-trench system, a numericalexperiment is carried out with two-dimensional(,D) ﬁnite element method.
(+) The ﬁrst episode of accretion or crustal
growth in Late Cretaceous is modeled with two
types of accretionary wedge of di#erent length
and width. The accretionary prism is simply
treated as elastic continuum.
(,) A series of ﬁnite element models simulate
the paleostress ﬁeld in the accretionary prism at
di#erent stages of accretion. Fault developments is predicted according to the MohrCoulomb criterion.
The subduction of the
Paciﬁc plate beneath the North American plate
and the underplating of the oceanic crust is
simpliﬁed to be the boundary condition. Increasing consolidation of the Late Cretaceous accreted sediments are taken into considerationby using di#erent values of physical properties.

ῌ 24 ῌ

Structural Geology No. .1

Deformation pattern of convergent margin within accretionary prism, Aleutian arc-trench system : ,D ﬁnite element modeling(-) The simulated results are compared withthe deformation mechanism of melange formation by Moore and Byrne (+321), and the kinematic model of melange of the McHugh Complex by Kusky and Bradley (+333).
,.

Geological setting

Alaskan Paciﬁc margin is composed of twoparallel composite terranes or ‘superterranes’-theWrangellian superterrane (consisting of the Peninsular, Wrangellia, and Alexander Terranes),
and farther outboard, the Chugach-Prince William superterrane (Nokleberg et al., +323, +33. ;Plafker et al., +33.).
Chugach terrane in southern Alaska forms theoutboard accretionary margin of the Wrangellian composite terrane, and consists of two
major lithotectonic units, including TriassicCretaceous melange of the McHugh Complex
and Late Cretaceous ﬂysch of the Valdez Group(Kusky et al., +331 b).
The McHugh Complex of central-south
Alaska (Clark, +31-) and the Uyak Complex of
Kodiak (Connelly, +312), are melanges constituting part of the Chugach terrane (Fig. ,). It isinterpreted as a Mesozoic-Cenozoic accretionary
prism formed by o#scraping (and/or underplating) outboard of the seaward margin of the com-posite Peninsular-Wrangellian-Alexander superterrane (Plafker et al., +323 ; Bradley and Kusky,+33, ; Kusky et al., +331 a).
The Valdez Group is a less chaotically deformed ﬂysch of argillite, and graywacke(Kusky et al., +331 b). The Iceworm melange isinterpreted as a contractional fault zone (Chugach Bay Thrust) along which the Valdez Groupwas emplaced beneath the McHugh Complex.
-.

Description of ﬁnite element method

The ﬁnite element method is an extensivelyused and helpful technique for numerical experiment of structural deformation in nature(Ramsay and Lisle, ,***). One example is theapplication of elastic rheological law underplane stress or plane strain condition. The elastic rheology, plane strain condition and prediction of fault development are discussed as follows.
-. +. Elasticity : the elas.f code
Elastic materials deform when a force is applied and return to their original shape when theforce is removed. In isotropic continuum theelastic properties are independent of direction.
Turcotte and Schubert (+32,) have pointed outthat the upper half of the lithosphere is referredFig. ,
Geological map of southern Alaska
showing major tectonic elements (Kusky
and Bradley, +333).

ῌ 25 ῌ

Handi Lu

to as the elastic lithosphere on the basis of thetheory of plate tectonics. One of the fundamental postulates of plate tectonics is that the surface plates constituting the lithosphere do not
deform signiﬁcantly on geological time scales.
Bott (+33*) suggested that the uppermost ,* kmof the continental crust was elastic, representing
the strong and cool layer of the upper lithosphere. Hassani et al. (+331) also ﬁxed the elastic
thickness to -* km for the oceanic lithosphere in
their numerical models.
One important reason for studying the elastic
behavior of the lithosphere is to determine thestate of stress in the lithosphere. In this study
linear elastic rheology is adopted under planestrain condition to recover the paleostress ﬁeld
in the accretionary prism during Late Cretaceous accretion in the eastern Aleutian arctrench region. The paleostress ﬁeld and fault
development associated with the tectonic andstratal disruption which leads to the formationof melange structures can be simulated withnumerical model in which treats the lithosphereas elastic, homogeneous and isotropic continuum. The elas.f code developed by Prof. D. Hayashi is used to calculate the displacements andstresses of ﬁnite element model. Gravitational
body force is included in ﬁnite element models.
-. ,. Plane strain condition
The plane strain condition exists when there
is one and only one zero component of principal
strain. For example, the stain in a direction
perpendicular to the x-y axis plane is zero, andnone of the other two principal strains are zero.
Since the strike of tectonic units (Peninsularterrane, Chugach terrane) in the eastern Aleutian arc-trench system are nearly perpendicular
to the convergent direction of the Paciﬁc plate,the plane strain condition can be applied tosimulate the paleostress ﬁeld in the accretionary
prism.
All the ﬁnite element models are in ,D elastic
state under plane strain condition. The thirdprincipal stress s*, which acts over the section
plane of the prism, can be obtained from theplane strain condition asῌsῒῐnῌ῍s+῏s,῎where n is Poisson’s ratio (Timoshenko and Goodier, +31* ; Hayashi and Kizaki, +31,). According

to the values of s+, s, and s* of each ﬁnite element, the maximum, intermediate and minimum principal stresses are deﬁned. The semi--D
stress ﬁeld of each model is envisaged with the
newly obtained principal stresses s+, s, and s-.
-. - Prediction of fault development
As the semi--D stress ﬁeld of each model isavailable, it is possible to describe in whichﬁnite element a fault develops according to theMohr-Coulomb criterion. The proximity to failure (PF) is calculated as the ratio of the current
di#erential stress to the maximum di#erential
stress predicted by the Mohr-Coulomb criterion(Melosh and Williams, +323). Faulting occurs
whenever the value of PF exceeds +.*.
..

Models of accretion

A series of ﬁnite element models simulate forthe paleostress ﬁeld and fault development with
di#erent boundary conditions. s+ and s- represent the maximum and minimum principal
stress, and compression is taken as positive.
For the sake of simpliﬁcation, Two criticaltapers are applied for the accretionary wedge at
di#erent stages of the late Cretaceous accretion(see Moore et al., +33+). The oceanic crust isoverlain by the accretionary wedge. The crosssections are oriented in NW-SE direction in theeastern Aleutian arc-trench region. The underplating of the oceanic crust along the base of theaccretionary complex has played an importantrole in the Late Cretaceous accretion (Kusky andBradley, +331 b).
The underplating processcauses tectonic and stratal disruption within theaccretionary prism. Rapid lateral accretion
would cause the wedge to thicken to maintainthe critical taper. The decollement dip of theaccretionary wedge changes to be relatively
gentle.
.. + Experiment I (initiation stage of accretion)
Models A+, A, and A- simulate for the paleostress ﬁeld in the accretionary prism at initiation
stages of the Late Cretaceous accretion as the
Paciﬁc plate subducts beneath the North American plate. The accretionary wedge in model A+
has a critical taper of +-ῑ as shown in Fig. - a. It
is about 3* km long and ,* km deep. The thick-ῌ 26 ῌ

Structural Geology No. .1

Deformation pattern of convergent margin within accretionary prism, Aleutian arc-trench system : ,D ﬁnite element modelingFig. (a) Model geometry of the Mesozoic
accretionary wedge and oceanic crust atinitiation stages of Late Cretaceous accretion,q῍+-῎. (b) Finite element mesh of models A+,
A, and A-.

ness of the oceanic crust is 0 km. The dip of theoceanic crust is +*῎ to the horizon while the dipof the surface slope is -῎.
The ﬁnite element grid of models A+ consistsof 2- nodes and +,3 linear triangular elements(Fig. - b). The grid of models A, and A- is thesame as model A+. As listed in Table +, Young’s
modulus 1* GPa, Poisson’s ratio *.,/ and density,/** kg/m- are assigned to the accretionary
complex, based on Clark (+330). Young’s modulus /* GPa, Poisson’s ratio *.,/ and density ,2**kg/m- are assigned to the oceanic crust (seeCondie, +331).
The boundary condition of models A+, A, andA- is mentioned below. The left-side wall (OA)of the accretionary prism is horizontally constrained but vertically free. The convergent
displacement, varing from the shallow node (1**m) to the deep node (+0* m) with a decrement of,2 m, is subjected along the bottom of the oceanic crust (BA) in the three models. The convergent displacement of 1** m is subjected to
the right-side wall of the prism (DB). Node A is
ﬁxed. The underthrusting of o#scraped sediments is taken into account as the boundary
condition in model A,. The underthrusting displacement of 1** m is subjected along thebottom of the overriding wedge (DF). The underplating displacement of +** m is subjectedalong the interface (CE) between the accretionary complex and the oceanic crust inmodel A- brcause the boundary between theoceanic crust and the overriding wedge is taken
as free-slip condition.
The purpose for applying a varying convergent displacement in ﬁnite element models is
to obtain a simulated result as that the paleostress s+ is nearly horizontal at the toe of thewedge. Hassani et al. (+331) proposed that thefriction coe$cient m along the interface between
two plates is a very important parameter for the
stress regime of the overriding plate during the
subduction process. If the friction coe$cient m
increases from the shallow to deep portions
along the subduction slope, the resistant force tothe subduction of the underriding lithosphere
increases greatly in the deep place. The subduction of the Paciﬁc plate beneath the North American plate seems to be more di$cult towards thedeep place. It can be regarded as that the convergent rate along the subduction slope decreases from the shallow to the deep portion.
The semi--D stress ﬁeld of model A+ is illustrated in Fig. .. s+ is compressive and nearlyhorizontal while s- is compressive and nearlyvertical. Cohesion strength +, MPa and internal
friction coe$cient +.* are assigned to the accretionary complex and oceanic crust (Table +).

ῌ 27 ῌ

Handi Lu

Semi--D stress ﬁeld of models A+, A, and A-, indicating magnitude and orientation of the principal stresses s+, s-.
Fig. .
Straight lines represent compression, and lines with arrow represent tension.

As shown in Fig. / there are some ﬁnite elements with PF῍+.* in the vicinity of the surface
slope within the accretionary complex, where
thrust faults develop.
The paleostress ﬁeld of model A, is similar tomodel A+ except that s+ is sub-parallel to the dipof the subduction slope within the shallow portion of the oceanic crust. Fault development inmodel A, is similar to that in model A+.
The paleostress ﬁeld of model A- is di#erent
from model A+ in the deep portion of the accretionary complex and the oceanic crust. The
paleostress s+ is reoriented to be oblique in theleft half of the prism. And tensional stresses

appear in the wedge and the oceanic crust.
Fault development in model A- is di#erent fromthat in model A+. In model A- thrust faults
develop at the toe of the wedge and in some partof the oceanic crust. Strike-slip faults develop in
the deep portion of the oceanic crust. Normal
faults develop in the deep place of the accretionary complex.
The simulated results of experiment I aresummarized as follows.
(+) The simulated results show that the paleostress s+ within the accretionary prism is compressive and nearly horizontal as the Paciﬁcplate subducts beneath the North Americanῌ 28 ῌ

Structural Geology No. .1

Deformation pattern of convergent margin within accretionary prism, Aleutian arc-trench system : ,D ﬁnite element modeling

Table +

Physical properties applied for di#erent geological units.

plate. Thrust faults develop at shallow place
within the accretionary complex.
(,) The simulated results show that the underthrusting of o#scraped sediments seems not inﬂuence the paleostress distribution of the wedge
very much. However, the paleostress s+ is reoriented as being sub-parallel to the subduction
slope in the shallow part of the oceanic crust.
(-) The simulated results show that the underplating of the oceanic crust reorients the paleostress axes and regulates fault development in
the accretionary prism. Tensional stress appears and normal faults develop at deep place of
the wedge. Strike-slip faulting occurs in the
deep portion of the oceanic crust. Thrust faults
develop at the toe of the wedge and in some partof the oceanic crust.
.. ,. Experiment II (late stage of accretion)
Models B+, B, and B- simulate for the paleostress ﬁeld within the accretionary prism at latestages of the Late Cretaceous accretion. The
accretionary wedge in model B+ has a criticaltaper of +*῍ (Fig. 0 a). It is ,// km long and -, km
deep. The oceanic crust is 0 km thick. The dip of
the subduction slope is 1῍ to the horizon and thedip of the surface slope is -῍. The Late Cretaceous accreted sediments enclosed by quadrilat-eral EFDC is somewhat arbitrary (see Moore etal., +33+).
The ﬁnite element grid of model B+ has +3*
nodes and -+1 elements (Fig. 0 b). The grid ofmodels B, and B- is the same as model B+.
Physical properties assigned to the Mesozoic accretionary complex and the oceanic crust are
the same in the three models. However di#erent
mechanic properties are applied to the Late Cretaceous accreted sediments of being poorly, half
and well consolidated (Table +).
The boundary conditions of models B+, B, andB- are the same as each other. The left-side wall
(OA) of the accretionary prism is horizontally
constrained but vertically free. The convergent
displacement, varying from 1** to -1 m with adecrement of +. m, is subjected along the bottomof the oceanic crust (BA). The convergent displacement of 1** m is subjected to the right-sidewall of the prism (DB). Node A is ﬁxed.
Model B+ simulates the paleostress ﬁeld in theaccretionary prism as the accreted sediments
are poorly consolidated. Young’s modulus /GPa, Poisson’s ratio *.,* and cohesion strength /MPa are adopted for the accreted sediments as
being poorly consolidated (see Clark, +300). The
stress ﬁeld of model B+ is shown in Fig. 1.
Within the Mesozoic accretionary complex, s+ isῌ 29 ῌ

Handi Lu

Fig. /
Fault development in models A+, A, and A-.
The principal stresses s+, s- are shown just for the ﬁnite
elements with PF῎+.*.

Fig. 0 (a) Model geometry of the Mesozoic
accretionary wedge and oceanic crust at
late stages of Late Cretaceous accretion,q῍+*῏. (b) Finite element mesh of models
B+, B, and B-.

ῌ 30 ῌ

Structural Geology No. .1

Deformation pattern of convergent margin within accretionary prism, Aleutian arc-trench system : ,D ﬁnite element modelingSemi--D stress ﬁeld of models B+, B, and B-, showing magnitude and orientation of the principal stresses s+, s-.
Fig. 1
Straight lines indicate compression, and lines with arrow indicate tension.

compressive and nearly horizontal in the vicinity of the surface slope. Within the accreted
sediments s+ is compressive and nearly verticalat the toe of the wedge. As illustrated in Fig. 2,
there is no ﬁnite elements with PF῍+.* in theaccreted sediments and no fault development
occurs.
Model B, simulates the paleostress ﬁeld in theaccretionary prism as the accreted sediments
are partially or haif-consolidated. Young’s modulus ,/ GPa, Poisson’s ratio *.,/ and cohesion
strength 2 MPa are assigned to the halfconsolidated sediments (see Clark, +300). Within
the Mesozoic accretionary complex s+ is compressive and nearly horizontal (Fig. 1). Within
the accreted sediments s+ is compressive andnearly horizontal in the vicinity of the surface

slope. There are few ﬁnite elements with PF῍+.* in the accreted sediments, where thrust faults
develop (Fig. 2).
Model B- simulates the paleostress ﬁeld in theaccretionary prism as the accreted sediments
are well consolidated. Young’s modulus 0* GPa,
Poisson’s ratio *.,/ and cohesion +, MPa are assigned to the accreted sediments as being wellconsolidated (see Clark, +300). The stress ﬁeld ofmodel B- shows that s+ is compressive andnearly horizontal at the toe of the wedge (Fig. 1).
There are some ﬁnite elements with PF῍+.* atthe toe of the accretionary wedge, where thrustfaults develop (Fig. 2).
The simulated results in experiment II aresummarized as follows.
(+) The simulated results show that the consol-ῌ 31 ῌ

Handi Lu

Fig. 2
Fault development in models B+, B, and B-. The principal stresses s+, s- are shown just for the ﬁnite elements withPF῍+.*.

idation state of the late Cretaceous accreted sediments regulates the paleostress distribution at
the toe of the wedge. As the accreted sediments
are poorly consolidated s+ is compressive andnearly vertical. However s+ is compressive andnearly horizontal as the accreted sediments are
partially and well consolidate.
(,) The simulated results show that the consolidation state of the late Cretaceous accreted sediments controls the occurrence of thrust development at the toe of the wedge. No fault development occurs if the accreted sediments arepoorly consolidated. In contrast thrust faults
develop at the toe of the wedge as the accreted
sediments are well consolidated.

/.

Discussion/. +. Comparison with Moore and Bynre’s mechanism
The simulated results are in agreement withthe mechanism of melange formation in accreting sediments-thickening of fault zones by
Moore and Bynre (+321). In the opinion of Moore
and Byrne (+321), sediments accreted at subduction zones undergo stratal disruption and form atype of melange (Fig. 3). The thickness of the
disrupted zones grows with progressive deformation. Initial fault surfaces are abandoned andreorientation of fault surfaces occurs as deformation propagates into adjacent undeformed
sediments. The disruption processes occurringῌ 32 ῌ

Structural Geology No. .1

Deformation pattern of convergent margin within accretionary prism, Aleutian arc-trench system : ,D ﬁnite element modelingFig. 3
Cross section of accretionary prism showing progeressive stratal disruption during o#scraping and underthrusting(Moore and Byrne, +321). In accreting sediments the initial fault surfaces are abandoned and deformation propagates into
adjacent undeformed sediment.

in the accretionary wedge result principallyfrom the deformation of the consolidating sedimentary mass. From the simulated results, thepaleostress s+ is compressive and nearly horizontal and thrust faults develop at the toe of theaccretionary wedge as the Paciﬁc plate sbducts
beneath the North American plate (Fig. /). The
fault development causes tectonic and stratal
disruption in the accretionary complex andleads to the melange formation.
/. ,. Comparison with Kusky and Bradley’s
medel
The simulated results coincide with the schematic deformation medel of the McHugh Complex by Kusky and Bradley (+333). Firstly theirschematic deformation model indicates that the
paleostress s+ is slightly seaward-dipping to subhorizontal on a regional scale in the accretionary wedge (Fig. +*). The simulated results show that s+ is compressive and nearly horizontal at the toe of the wedge during the subduction process (Fig. .). Secondly Kusky andBradley (+333) proposed that melange of the
McHugh Complex is generated by tectonic andstratal disruption at the toe of the accretionary
wedge (Fig. +*). The simulated results show thatfaulting occurs in the accretionary wedge andthe oceanic crust for the subduction of the
Paciﬁc plate beneath the North American plate
and the underplating of the oceanic crust (Fig.
/). The fault development would cause tectonic
and stratal disruption in the accretionary prism.
The tectonic and stratal disruption is relative tothe formation of melange structures in convergent margin.
/. -. Underplating and accretion
The growth of the Kodiak segment of thecontinental margin has been dominated by two
episodes of accretion, one in Late Cretaceousand the other in Early Eocene (Byrne +320 ;Byrne and Fisher +321). Two critical tapers with
di#erent length and thickness are applied tomodel the Late Cretaceous accretion in Chugach
convergent margin. The oceanic plate is thinly
sedimented at initiation stages of accretion but
thickly sedimented at late stages of accretion.
During the Late Cretaceous accretion, thePaciﬁc plate subducts beneath the North American plate. The paleostress s+ is compressive andnearly horizontal. Thrust faults develop in theaccretionary complex. The Mesozoic rock layers
are disrupted and o#scraped at the toe of thewedge. The underthrusting of o#scraped sediments does not greatly inﬂuence the paleostressῌ 33 ῌ

Handi Lu

Fig. +*(a) Schematic conditions of deformationin an accretionary wedge where s+ is slightly
dipping to subhorizontal on a regional scale. (b)Schematic graph shows the di#erent rheologicalunits in the rock sequence experiencing bothbrittle and ductile extension (layers῎./῏ to s+),brittle extension and ductile contraction (layers-*ῌ./῏ to s+), and dominantly brittle and ductile
contraction (layers῍-*῏ to s+). Early extensionalmelange formation at a low angle to s+ is related
to sub-simple shear (Kusky and Bradley, +333).

ﬁeld in the accretionary prism. However theunderplating process has played a major role in
the accretion of the prism. The underplating of
the oceanic crust reorients the paleostress axes
and regulates the fault development in the accretionary prism. It generates normal faults
around the base of the accretionary complex
and causes strike-slip faulting in the deep portion of the oceanic crust. As the sediments areaccreted in the wedge, the thickening of thewedge would happen and the decollement dip of
the wedge changes to be relatively gentle.
/. .. Role of sedimentation and consolidation
The most persistent question relative to melange formation is the roles of soft-sedimentary
vstectonic process on disruption and mixing. At
late stages of the Late Cretaceous accretion, thewedge is in a rapid depositional environment
and the accreted sediments are poorly consolidated at initiation. The simulated results
show that the paleostress s+ is compressive andnearly vertical at the toe of the wedge and nofault develops as the accreted sediments arepoorly consolidated. However as the poorly
consolidated sediments are dewatered and hardened and well consolidated, compressive s+ becomes nearly horizontal and thrust faults develop at the toe of the wedge.
The increasing consolidation of the accreted
sediments regulates the orientation of paleostress axes and controls the occurrence of thrustdevelopment at the toe of the wedge as the
Paciﬁc plate subducts beneath the North American plate. The gravity force only can not resultin faulting within the accretionary wedge if noconvergent displacement is subjected as theboundary condition. From this point, the sedimentary process has played little role in theformation of melange belts within the accretionary prism.
0.

Conclusion

A series of ﬁnite element models simulate thepaleostress ﬁeld and fault development in theaccretionary prism during the Late Cretaceous
accretion in the eastern Aleutian are-trenchῌ 34 ῌ

Structural Geology No. .1

Deformation pattern of convergent margin within accretionary prism, Aleutian arc-trench system : ,D ﬁnite element modelingregion. All the simulated results obtained fromthe numerical modeling are summarized below.
(+) The simulated results show that the paleostress ﬁeld in the accretionary prism evolves
during the Late Cretaceous accretion. At initiate stages of accretion, the paleostress s+ is compressive and nearly horizontal at the toe of theaccretionary wedge for the subduction of the
Paciﬁc plate beneath the North American plate.
The paleostress axes are reoriented to be oblique
for the underplating of the oceanic crust. At latestages of accretion the paleostress s+ is compressive and nearly vertical at the toe of the wedge
as the accreted sediments are poorly consolidated. However the paleostress s+ is compressive and nearly horizontal at the toe of thewedge as the accreted sediments are well consolidated.
(,) The simulated results show that thrustfaults develops within the accretionary complex
at initial stages of the Cretaceous accretion forthe subduction of the Paciﬁc plate beneath theNorth American plate. The underplating of theoceanic crust generates normal faults aroundthe base of the accretionary complex and strikeslip faults in the deep portion of the oceanic
crust. At late stages fo the accetion no fault
develops within the accreted sediments of being
poorly cosolidated. However thrust faults develop at the toe of the wedge as the accreted sediments are well consolidated.
(-) The formation of melange belts in the convergent margin is relative to the subduction of
the Paciﬁc plate beneath the North American
plate and the underplating of the oceanic crust.
As the Paciﬁc plate subducts beneath the North
American plate, thrust faults developed within
the accretionary complex. The underplating of
the oceanic crust generates normal faulting
around the base of the accretionary complex
and strike-slip faulting and thrust developmentin the oceanic crust. The fault development
causes tectonic and stratal disruption in the accretionary prism and leads to the formation of
the McHugh Complex and Iceworm melange.
(.) The sedimentary process has played little
role in the formation of melange belts in convergent margin. As the late Cretaceous accreted
sediments are poorly consolidated and no faulting occurs at the toe of the accretionary wedge.

However thrust faults develop within few ﬁnite
elements with PF῍+.* at the toe of the wedge as
the accreted sediments are well consolidated.
The fault development and stratal disruption
does not intensively occur within the accreted
sediments. The consolidation process contributes less to the formation of melange belts in theaccretionary prism.
Acknowledgements
The author gratefully acknowledges the Ministry of Education, Culture, Sports, Science andTechnology, Japan for the support of Monbusho
scholarship during his study period at University of the Ryukyus, Okinawa. Prof. D. Hayashi isthanked for critically reviewing of the manuscript. Other anonymous reviewers are thanked
for their comments improved the manuscript
greatly.
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