arcarc--continent continent collision (Luzon and collision (Luzon and

Chinese mainland)Chinese mainland)

oblique convergenceoblique convergence

eroding version eroding version of an of an accretionaryaccretionary

prismprism

strain is strongly strain is strongly partitionedpartitioned

TimeTime--transgressivetransgressivecollision progressescollision progresses

north to south as north to south as arc is accreted.arc is accreted.

TimeTime--transgressivetransgressivecollision progressescollision progresses

north to south as north to south as arc is accreted.arc is accreted.

Trade Distance for Time…Trade Distance for Time…

Chang, 2000

Taiwan is a classic, doubly Taiwan is a classic, doubly vergentvergent critical wedgecritical wedge

•• Material is accreted onto front of thrust belt Material is accreted onto front of thrust belt by plate convergenceby plate convergence

•• Material is Material is advectedadvected out the top of mountain out the top of mountain belt by erosionbelt by erosion

•• Relative amount of input versus output dictates Relative amount of input versus output dictates mass balance/wedge shape and reorganizes mass balance/wedge shape and reorganizes faulting/folding + earthquakesfaulting/folding + earthquakes

Anatomy of the Taiwan Anatomy of the Taiwan orogenorogen

Powell, 2003Powell, 2003

Frontal accretionFrontal accretionAdvection from erosionAdvection from erosion

SeismicitySeismicity & Thermal Structure& Thermal Structure

Powell, 2003Powell, 2003

2400 lives lost2400 lives lost14 Billion US$ in economic losses14 Billion US$ in economic lossesFocussedFocussed work on Taiwanwork on Taiwan

1999 Mw 7.6 Chi Chi Earthquake1999 Mw 7.6 Chi Chi Earthquake

70+ km rupture of 70+ km rupture of ChelungpuChelungpu thrustthrust 1999 Mw7.6 Chi Chi EQ1999 Mw7.6 Chi Chi EQ

Cross Section Cross Section -- ChelungpuChelungpu ThrustThrust

12km of total slip suggests long history of shortening (~1Ma)12km of total slip suggests long history of shortening (~1Ma)

Surface offset from 4Surface offset from 4--10 m10 m

Offset running track Offset running track -- KungfuKungfu high schoolhigh school

CloseupCloseup of Running Track of Running Track -- 5m of slip5m of slip

Surface “rupture” was often a passive fold, Surface “rupture” was often a passive fold, rather than a fault scarprather than a fault scarp

Secondary extensional faultingSecondary extensional faultingand surface foldingand surface folding

Sedimentation rate is high Sedimentation rate is high compared to fault tip propagationcompared to fault tip propagation

WufengWufeng (bean field) trench site excavation(bean field) trench site excavation

WufengWufeng (bean field) trench log(bean field) trench log

Line length Line length balancingbalancing

Area balancingArea balancing

Geometric patterns of faultGeometric patterns of fault--tip deformationtip deformationsurface foldingsurface folding (blind fault)(blind fault) surface faultingsurface faulting (breakthrough fault)(breakthrough fault)

ChungchengChungcheng--park trench park trench WenshanWenshan--farm trench farm trench

Surface foldingSurface folding

In the trenched profile, we can find In the trenched profile, we can find wedged deposits, wedged deposits, onlappedonlapped((unconformityunconformity), and ), and downwarddownward--steepeningsteepening bedbed--dips dips ((fanning fanning bedbed--dipsdips) ) structuresstructures which which indicate the growth folding.indicate the growth folding.

* * Each wedgeEach wedge--shaped deposits shaped deposits represents a represents a paleoearthquakepaleoearthquakeevent. event.

ChushanChushan trenchtrench

•• Northern wallNorthern wall

Southern wallSouthern wall

Northern wallNorthern wall

Northern wallNorthern wall

•• The northern wall shows a tight The northern wall shows a tight anticline with a steeply inclined anticline with a steeply inclined forelimb on the forelimb on the hangingwallhangingwall, and , and the footwall strata are unfolded. the footwall strata are unfolded. The deformation seems to be a The deformation seems to be a footwallfootwall--fixed fault propagation fixed fault propagation fold. fold.

Southern wallSouthern wallNorthern wallNorthern wall

Southern wall•• The deformation produced an open The deformation produced an open

anticline on the anticline on the hangingwallhangingwall and a and a tight syncline, that indicates a tight syncline, that indicates a hangingwallhangingwall--fixed fault propagation fixed fault propagation fold. fold.

• Trenches shows six paleoearthquakes in the past 3000 yrs.

vertical slip rate: 2.9-4.7 mm/year

• Based on the relationship between cumulative slip and the occurrence time, the series of earthquake seem to show a time-predictable earthquake mode, and slip rate of about 7 mm/yr.

How do fold and thrust belts grow?How do fold and thrust belts grow?SeismicitySeismicity & thermal structure& thermal structure

Powell, 2003Powell, 2003

They grow as wedges where erosion is linked to shorteningThey grow as wedges where erosion is linked to shortening

Thrust sheetsThrust sheetsverge away fromverge away from

backstopbackstop

Youngest thrustsYoungest thrustslocated at leading located at leading

edge of beltedge of belt

SteadySteady--state state produces selfproduces self--similar wedgesimilar wedge

Concept of a critical wedgeConcept of a critical wedge

Analogue model of thrust beltAnalogue model of thrust belt

(From Willett et al., Am Jour (From Willett et al., Am Jour SciSci. (2001) Modes of . (2001) Modes of orogenicorogenicwedge growth. wedge growth. OrogenicOrogenic wedge wedge (A) grows by mass added through (A) grows by mass added through frontal accretion. Wedge frontal accretion. Wedge deforms by pure shear such that deforms by pure shear such that a vertical column extends a vertical column extends vertically and shortens vertically and shortens horizontally. Vertical component horizontally. Vertical component of surface velocity is constant. of surface velocity is constant. Horizontal component of surface Horizontal component of surface velocity increases linearly in x velocity increases linearly in x from 0 to L reaching a maximum from 0 to L reaching a maximum value of value of VcVc, the convergence , the convergence velocity. B: velocity. B: OrogenicOrogenic growth by growth by underplatingunderplating. Wedge does not . Wedge does not deform internally, columns of deform internally, columns of rock move vertically in response rock move vertically in response to addition of new material at to addition of new material at base of wedge.base of wedge.

Map Area

CrossCrossSectionSection

ThroughThroughPuliPuli BasinBasin

MapMapTransectTransectThroughThrough

PuliPuli BasinBasin

Structural Architecture of WestStructural Architecture of West--Central TaiwanCentral Taiwan

Western Foothills minimum slip estimates:Changhua Fault = 3.8 kmChelungpu Fault = 18 km

Hsüehshan Belt slip estimates:Shuangtung Fault = 17 kmShuilikeng Fault = 5 kmTapingting Fault = 4 kmMeiyuan Fault = 12 kmTili Fault = ~19 km

Restoration of Cross SectionRestoration of Cross Section

69 km32 km

156 km83 km

Regional Shortening = 56%% Shortening = (lf – lo)/lo x 100

Hsüehshan Belt Shortening = 61%

Rates of erosion vary along the Taiwanese Rates of erosion vary along the Taiwanese OrogenOrogen

N

20 km

PuliPuli Basin is 1500Basin is 1500--2000 m lower than adjacent regions2000 m lower than adjacent regions

Map AreaMap Area PuliPuliBasinBasin

PuliPuli Basin is filled with 150Basin is filled with 150--300m of Late Quaternary strata300m of Late Quaternary strata

Record of InfillingRecord of Infilling

Structural damStructural damfaultfault--bend fold abovebend fold aboveShuangtungShuangtung thrustthrust

PuliPuli Basin Depositional RecordBasin Depositional Record

••150150-->300 m thick>300 m thick••Coarsen upwardCoarsen upward••Locally derived Locally derived clastsclasts••Cut by drainages w/ slate Cut by drainages w/ slate clastsclasts••Basal strata at ~75 Ka (OSL)Basal strata at ~75 Ka (OSL)

PuliPuli Basin Basin --> folded alluvial surface> folded alluvial surface

View of folded alluvial surface in View of folded alluvial surface in PuliPuli BasinBasin

Cross sections Cross sections thru thru PuliPuli Basin Basin

illustrate young illustrate young folding above folding above

older older reactivedreactivedfaultsfaults

Faults are blindFaults are blindwhere they are where they are buried beneath buried beneath

basin stratabasin strata

Horizontal GPS Horizontal GPS velocity fieldvelocity field

Contours in mm/yr Contours in mm/yr = convergence= convergence

relative to Asiarelative to Asia

Velocity field from Velocity field from GPS mimics pattern GPS mimics pattern

of recent strain of recent strain and topographyand topography

••PuliPuli embayment is embayment is being shortened being shortened

over its entire 50 over its entire 50 km widthkm width

••Shortening Shortening further south is further south is

focused at front of focused at front of thrust beltthrust belt

Swath topography Swath topography PuliPuli Basin and Basin and AlishanAlishan RangeRange

Topography in wedge variesTopography in wedge variesLower in Lower in PuliPuli

Even wedge shapeEven wedge shapein southin south

Strain steps Strain steps hindwardhindward to infill to infill mass lost by erosion in mass lost by erosion in PuliPuli regionregion

Erosion Controls Erosion Controls Tectonics!Tectonics!

LeadingLeadingEdgeEdge

Observed Observed backsteppingbackstepping

of of orogenicorogenicfrontfront

Width of active Width of active belt of shortenbelt of shorten--

ing is wider in ing is wider in region of lower region of lower

elevation and elevation and wedge thicknesswedge thickness

Models of Models of erosionalerosional forcing and thermal weakeningforcing and thermal weakening

Thrust beltThrust beltwith evenwith even

erosionerosion

Thrust beltThrust beltwith unevenwith uneven

erosionerosion

Thrust beltThrust beltwith unevenwith unevenerosion anderosion andand thermaland thermal

weakeningweakening

Continuum Physics ModelContinuum Physics ModelInput includes:Input includes:Code: FLACCode: FLAC**Rate of Convergence: from GPSRate of Convergence: from GPSCrustalCrustal Structure includes Structure includes decollementdecollement, but not other faults, but not other faultsRock Strength: as f(frictional angle 35Rock Strength: as f(frictional angle 35oo, , decollementdecollement weaker =14weaker =14oo)Erosion Rates: erosion = f(elevation across the entire basin)Erosion Rates: erosion = f(elevation across the entire basin)Topography: not included, but builds naturally in modelTopography: not included, but builds naturally in modelDuration: 500 KaDuration: 500 KaPore Fluid Pressure: Pore Fluid Pressure: notnot includedincludedHeat flow: Heat flow: notnot included, but is likely to enhance effects of strain included, but is likely to enhance effects of strain

localization in region of low relieflocalization in region of low relief

*FLAC results based on 3D solutions for *FLAC results based on 3D solutions for governing force and velocity governing force and velocity eqnseqns using a using a LagrangianLagrangian method that allows large strains, method that allows large strains, variation in erosion and nonvariation in erosion and non--linearitieslinearities that that arise in the solutionsarise in the solutions Movie created by Phaedra UptonMovie created by Phaedra Upton

Geodynamics Group, Univ. of MaineGeodynamics Group, Univ. of Maine

Numerical modelNumerical modelsuggests erosionsuggests erosion

at scale of at scale of PuliPuliregion should causeregion should cause

wedge to be upliftedwedge to be upliftedin this area in this area -- sooosooothrusts thrusts backstepbackstep, ,

and are reactivated.and are reactivated.

So distribution of So distribution of mass in mass in mtnmtn belt belt

affects where affects where new mass is addednew mass is added

Model with focused erosion within the Model with focused erosion within the PuliPuli EmbaymentEmbayment

Block diagram Block diagram showing vertical showing vertical displacement displacement over the entire over the entire modelmodel

ContractionalContractional strain (calc from strain (calc from displacements). The pattern displacements). The pattern of of contractionalcontractional strain is strain is perturbed by increased perturbed by increased erosion, concentrating this erosion, concentrating this component of strain at the component of strain at the western edge of the region western edge of the region subjected to focused erosionsubjected to focused erosion

Rotational strain, leftRotational strain, left--lateral is negative, rightlateral is negative, right--lateral is positive. lateral is positive. Rotational strain occurs Rotational strain occurs on the edges of the on the edges of the region of higher erosion. region of higher erosion. StrikeStrike--slip faulting would slip faulting would be expected there.be expected there.

Extremely fast Extremely fast exhumation causes exhumation causes high heat flow beneath high heat flow beneath Central RangeCentral Range

CrustalCrustal “aneurysm”“aneurysm”thermally weakens the thermally weakens the brittle crust, resulting brittle crust, resulting in smaller scale (<50 in smaller scale (<50 km) interaction betweenkm) interaction betweenerosion and rock uplift erosion and rock uplift (not usually recognized (not usually recognized in other active belts).in other active belts).