CARPATHIANS FORELAND BASIN(S): regional setting

Carpathians arc, a single orogen, but 2 quite different foreland basins: Southversus East Carpathians

Lower plate made up of 2 different major lithospheric blocks: East European/Scythian/North Dobrogea orogen versus Moesia

Question:

is the subsidencein the Carpathians forelandbasin the result of a single,

long-lasting process or,it rather represents a sum

of different tectonic mechanisms?

Wedge-shaped basin

Subsidence coeval with thrusting

Minor post-thrusting tectonic deformation (contractional structures if mechanicallycoupled with the orogenic wedge)

Minor erosion (max. a few hundreds of meters) on the forebulge

Positive vertical movements of both foreland basin and orogenic wedge once thelatter is significantly eroded (isostatic rebound)

FORELAND BASINS: typical features

GENERAL FEATURES(South Carpathiansforeland basin)

BASE NEOGENE CONTOUR MAP

Eastern part deepens NNE-wardsOverthrust by the Subcarpathian nappeAffected by mainly Sarmatian and youngerfaults with relatively minor offsetsMinor erosional features at the Neogene base

Western part is thedeepest (>3.5 km)Large-scale erosionalfeaturesTo the N, erosion replaced by Pg-Micontinuous sedimentationEarly Miocene large normal faultsNo contractional deformation in theNW corner

SOUTH CARPATHIANS FORELAND

PALEOGENE - BURDIGALIAN

DEEP EROSION INDICATING MAJOR UPLIFT IN ORDER FOR THE FLUVIALNETWEORK TO INCISE!

SOUTH CARPATHIANS FORELAND

BURDIGALIAN

EXTENSION in the (at least) NW part of Moesia!ENE-trending normal faults farther to the S can be attributed to the same extensionalevent (they clearly pre-dates the erosional unconformity and are younger that UpperCretaceous)

Note that the amount of erosion increases N-wards instead of S-wards as it would beexpected in a typical foreland flexure (erosion occurring on the forebulge)

SOUTH CARPATHIANS FORELAND

BADENIAN-to-PLIOCENE

Since the Badenian (mostly since Sarmatian): FOREDEEP STAGE

Neogene sequence is wedge-shaped and thickens towards the belt; apparently, it indicatesa typical foreland basin. However, subsidence continues long time after the emplacementof the Subcarpathian nappe!

GENERAL FEATURES(East Carpathians foreland basin)

BASE NEOGENE CONTOUR MAP

Basin generally deepens towards East Carpathians

However, the deepest part (~13 km in FocsaniDepression!) lies out of the belt

Focsani basin prolongs to the W and NW beneaththe Carpathians front

Structural grain NW-SE oriented, parallel tothe central-northern segment of the belt, butoblique from the Trotus fault to the S

Intense faulting of the E and SE margin of theFocsani Depression

EAST CARPATHIANS FORELAND

BADENIAN

BADENIAN THICKNESS MAP

Large variations in the subsidence pattern fromEast European platform to Moesia

Most important, in Moesia (to the S of Trotus fault)subsidence is controlled by NW-SE trending riftbasins

Maximum subsidence occurs in Focsani Depression,which prolongs beneath the present Carpathians

East European platform was only gently subsiding

EAST CARPATHIANS FORELAND

EAST EUROPEANPLATFORM

MOESIA

Contrasting deformations:

THRUSTING in East European platform

EXTENSION in Moesia

BADENIAN

EAST CARPATHIANS FORELAND

Major shortening event (thrusting of Subcarpathiannappe)

Maximum subsidence in Focsani Depression and veryimportantly, the depocenter moves out of the belt

Generally, the basin deepens towards the belt indicatinga flexural mechanism for subsidence

However, different structural styles from East Europeanplatform to Moesia: normal faulting trending roughlyparallel with the orogen in the former and strike-slip inthe latter

SARMATIAN

SARMATIAN THICKNESS MAP

EAST CARPATHIANS FORELAND

EAST EUROPEANPLATFORM

MOESIA

Both foreland basins infill are wedge-shaped thickening towards the belt; however,significant differences can be noted:1) Sarmatian sequence is much thicker in the East European platf. than in Moesia;2) Post-Sarmatian sequence is generally missing in the East European platf.; instead,thick, wedge-shaped post-Sarmatian sequence is documented in Moesia (the Pliocenesequence is roughly as thick as the Miocene one)

EAST CARPATHIANS FORELAND THICKNESS MAPS

Subsidence is basically confined to Moesian domainMaximum subsidence is always in the Focsani Depression, which becomes clearly “detached”from the belt since Pontian onwardsNormal faulting along the eastern margin of FD and strike-slip during the Meotian andmainly during the Pliocene

SUMMARY OF SUBSIDENCE STAGES:

1) EXTENSION (ALL) ALONG THE NORTHERN MARGIN OF MOESIA(BURDIGALIAN TO BADENIAN): accommodation space mostly provided byrift basins

2) LAST MAJOR THRUSTING EVENT (SARMATIAN) EMPLACING THESUBCARPATHIAN NAPPE OVER (ALMOST) THE ENTIRE FORELAND(BOTH EAST EUROPEAN PLATFORM AND MOESIA): accommodation spacerelated to flexural tilting lasting apparently till the end of Miocene/beginning of Pliocene

3) ALTHOUGH PLIOCENE CONTRACTION IS MINOR BY COMPARING WITHTHE SARMATIAN ONE, SIGNIFICANT SUBSIDENCE IS RECORDED, BUT ONLYIN MOESIA (here, it roughly equals the Sarmatian-to-Pontian one!)

DIFFERENCES BETWEEN (A) EAST EUROPEAN PLATFORM AND (B) MOESIABECOME CLEAR (not only in terms of crustal/lithospheric thicknesses):

Typical foreland basinSubsidence coeval with thrustingBasin deepens towards the belt

Unusual deep basin out of the loadSignificant subsidence and tectonicdeformation post-thrustingBasin evolved initially as extensional

A) B)

SUBSIDENCE CAUSE: previous proposed models

(after Wortel and Spakman, 2000)

AIMS:

1) UNUSUAL LARGE DEPTH IN FOCSANI DEPRESSION (FD)2) TIMING OF SUBSIDENCE IN FD, WHICH SIGNIFICANTLY POST-DATES THE MAIN THRUSTING EVENT IN THE OROGEN3) POSITION RELATIVE TO THE BELT4) SEISMICITY IN THE NEIGHBORING VRANCEA ZONE5) NOEGENE VOLCANIC ARC

THERE IS NO MIGRATION OF THE FOREDEEP DEPOCENTER ALONG THE ARC!

ALSO, THE PATTERN OF YOUNG VERTICAL MOVEMENTS CONTRADICTS THEINFERRED “SLAB” LOCATION: i.e. SUBSIDENCE OVER THE ENTIRE MOESIA andABOVE THE “SLAB” UPLIFT HAS BEEN OCCURRING RATHER THAN SUBSIDENCE!

HOWEVER,

EXTENSIONAL MODELLING

INFERENCES:

Low magnitude of extension

Almost no post-rift subsidence

Good correlation with similar modelling in the Black Sea (Spadini et al., 1996)

PARAMETERS OF THEBEST-FIT MODEL:

Initial crustal thick. 32 kmEET 35 kmNecking depth 25 kmUniform thinning (McKenzie, 1978)

THINNING FACTORS

REAL CROSS-SECT.(TOP BADENIANFLATTEN)

MODELLEDCROSS-SECT.(BEST-FIT)

ASSUMPTIONS:

Paleowater depth 0 mDuration of rifting 3 MaInitial lithosph. thick. 180 km(e.g. Nemcok et al, 1998)

(numerical code of Kooi, 1991)

SYN-RIFT

POST-RIFT

FLEXURAL MODELLING

2D TEST – DEPOCENTER POSITIONConstant (or gently varying) EET determines a basindeepening towards the load (max. deflection is moreor less centered beneath the load)

High and abrupt reduction in foreland strength implies:THE DEPOCENTER SHIFTS OUT OF THE LOAD

In our case (eastern Moesia inclusive of FD), weakeningis promoted by extension and mainly crustal-scale faults(e.g., Burov et al., 1998)

BASICS DEFLECTION = f (LOAD; INTR. STRESS;PLATE STRENGTH)

PLATE BEHAVIOR:PURE ELASTICVISCOELASTICDEPTH-DEPENDENT RHEOLOGY

(numerical code ofGarcia-Castellanos et al., 1997)

PARAMETER DESCRIBING THE PLATEBEHAVIOR: EFECTIVE ELASTICTHICKNESS (EET)

FLEXURAL MODELLING

3D MODELLING: INPUT SETTING (numerical code of Garcia-Castellanos, 2002)

Pure elastic rheologyLoad provided by the present-day topographyInstantaneous loadingEET constants inside the domains, linearly interpolated in the bounding polygonsModeling “starts” at the Badenian/Sarmatian boundary assuming 0 m paleobathimetry

A local flexural basin is predicted in the Bend area out of the load; depth ~ 3.3 kmTypical foredeep in front of East and South CarpathiansMaximum deflection is predicted to the inner part of the beltForebulge develops in the uplifted Dobrogea area

The model is satisfactory for Transylvanian basin, East – European platform andeastern half of Western Moesia. However, there are 2 notable misfits:

FLEXURAL MODELLING

MISFITS: A. Focsani Depression;B. NW Moesia

Some correlations between these regions andthe overall tectonic and vertical movementshistories:

1) Both lie in Moesia in places associated withstrong orogen bending

2) Both were affected by extension prior flexure

3) Both experienced large subsidence that wasgoing on after the Subcarpathian nappe had beenemplaced

4) Both are affected by young to active tectonic deformations

5) Both are associated with important positive vertical movementsin the neighboring orogen

AB

(exhumation data from Sanders et al., 1999)

FLEXURAL MODELLING

Apparently, topography in the Bend zone reflects only the last deformations (“Walachian”)

Did the Sarmatian shortening create no topography in the Bend zone? Or, is this simply theeffect of using a 3D “static” modeling instead of a kinematic one?

An additional load has been added to the present topography in the Bend zone

In tems of force: 1.7 x 1010 N Keep in mind that a 400 km-long “slab” would exert12.7 x 1010 N (minimum density contrast of 25 kg/m3)

N – S INTRAPLATECOMPRESSIVE STRESS(2 1012 Pa m)

NO INTRAPLATESTRESS

FLEXURAL MODELLING

STILL, THERE ARE DIFFERENCES IN ORDER OF ~2 km IN THE 2 REGIONSBETWEEN PREDICTED AND ACTUAL DEPTHS

B

B

However, it should be kept in mind that the flexuralmodeling reflects only the post-Badenian subsidenceand not the entire Neogene stage!

As in the case of Focsani Depression, the Burdigalian extension accounts probably for thefirst subsidence stage in NW Moesia!A notable difference can be deduced: Burdigalian extension was larger and the post-riftcooling subsidence was significant in NW Moesia because this region had had a highelevation prior the onset of sedimentation (remember the canyons!)

The remaining 2 km of subsidence are most likely Pliocene-Quaternary and could be theresult of:

LITHOSPHERIC BUCKLING (see e.g. Cloetingh et al., 1999)

(from Bertotti et al., 2003)

INDICATORS (Pliocene to Recent times span):

1) Subsidence in foreland (only in Moesia) coeval with uplift in neighboring orogen2) Regional stress: compressive, roughly N-S oriented (e.g. Hippolyte and Sandulescu,1996; Matenco and Bertotti, 2000)

Buckling would be responsible for a max. of 2 km subsidence during Pliocene-Quaternary,that is, an overall rate of 0.4 mm/yr, which seems reasonable!

1) THE AMOUNT AND TIMING OF THE SUBSIDENCE IN THE CARPATHIANSFORELAND BASIN PROVIDE EVIDENCES OF TOTALLY DIFFERENT BEHAVIORSBETWEEN THE SECTORS OVERLYING THE EAST EUROPEAN PLATFORM ANDMOESIA, RESPECTIVELY

2) SUBSIDENCE IS A SUM OF THE EFECTS OF DIFFERENT TECTONIC PROCESSESRANGING FROM EXTENSION TO FLEXURAL LOADING AND FINALLY BUCKLINGRATHER THAN THE CONSEQUENCE OF A SINGLE, LONG-LASTING MECHANISMWITH DEEP-SEATED CONTRIBUTION

3) MAIN FACTOR INVOLVED IN THE SUBSIDENCE PATTERN IS REPRESENTEDBY LATERAL VARIATIONS IN LITHOSPHERIC STRENGTH, THAT IS, STRONGEAST EUROPEAN PLATFORM VS. WEAK MOESIA

4) MOST PROBABLY, THE MORE CURVED A PLATE BOUNDARY (AN OROGEN) IS,THE MORE IMPORTANT THE RHEOLOGICAL CHANGES ARE IN SHAPING THEARCHITECTURE OF A FORELAND BASIN

CONCLUSIONS

PhD THESIS CARRIED OUT AT:

ISES

BUCHARESTUNIVERSITY

VRIJE UNIVERSITEITAMSTERDAM(Integrated Solid Earth Sciences

Netherlands research program)

M. Tarapoanca, C. Dinu, D. Ciulavu: Neogene kinematics of the northeastern sector ofMoesian platform (Romania); AAPG Bulletin, in press

M. Tarapoanca, G. Bertotti, L. Matenco, C. Dinu, S. Cloetingh: Architecture of the FocsaniDepression: a 13 km deep basin in the Carpathians Bend zone (Romania); Tectonics, in press

M. Tarapoanca, D. Garcia-Castellanos, G. Bertotti, L. Matenco, S. Cloetingh, C. Dinu:Role of the 3D distributions of load and lithospheric strengths in arcuate orogenic arcs:poly-stage subsidence in the Carpathians foredeep; Earth and Planetary Science Letters,submitted

M. Tarapoanca, G. Bertotti, L. Matenco: Controls on the vertical movements in theCarpathians foredeep; Tectonophysics, submitted

M. Tarapoanca, G. Bertotti, L. Matenco, J. Matresu, S. Cloetingh, C. Dinu: Deep incisionin forelands: the case of Moesia; Geology, in preparation