DOI: http://dx.doi.org/10.7306/gq.1366
Morphotectonic im pli ca tion of the Paleoproterozoic Mid-Lith u a nian Su ture Zone
Saulius ŠLIAUPA1, *, Jonas SATK¤NAS2, Gediminas MOTUZA3 and Rasa ŠLIAUPIEN#1
1 Na ture Re search Cen tre, Akademijos 2, LT-08412 Vilnius, Lith u a nia
2 Lith u a nian Geo log i cal Sur vey, Konarskio 35, LT-03123 Vilnius, Lith u a nia
3 Vilnius Uni ver sity, De part ment of Ge ol ogy and Min er al ogy, M.K. Èiurlionio 21/27, LT-03101 Vilnius, Lith u a nia
Šliaupa, S., Satkñnas, J., Motuza, G., ŠliaupienÅ, R., 2017. Morphotectonic im pli ca tion of the Paleoproterozoic Mid-Lith u a - nian Su ture Zone. Geo log i cal Quar terly, 61 (3): 590–601, doi: 10.7306/gq.1366
The Paleoproterozoic Mid-Lith u a nian Su ture Zone rep re sents one of the ma jor struc tures of the crys tal line base ment of Lith - u a nia, sep a rat ing the West Lith u a nian and the East Lith u a nian do mains. This zone has shown per sis tently low tec tonic ac tiv ity dur ing the Phanerozoic. The Mid-Lith u a nian Su ture Zone is marked by a dis tinct Mid dle Lith u a nian top o graphic low un der lain by a trough in the sub-Qua ter nary sur face that sug gests the morphotectonic na ture of this de pres sion. This is sup - ported by high-pre ci sion geo detic lev el ling data that has un rav elled the sub si dence trend of the Mid dle Lith u a nian trough.
The zone is also dis tinct in its pat tern of top o graphic lin ea ments. The per sis tence of the tec tonic ac tiv ity of the Mid-Lith u a nian Su ture Zone sug gests that it rep re sents a large-scale me chan i cal bound ary of the Earth’s crust, re sult ing in in creased ac cu - mu la tion of tec tonic strain.
Key words: morphotectonic, Mid-Lith u a nian su ture zone, Paleoproterozoic base ment, Qua ter nary, tec tonic in her i tance.
INTRODUCTION
The iden ti fi ca tion of con trol of the tec tonic struc tures on sur - face mor phol ogy is a com plex prob lem in plat form ar eas cov - ered by gla cial de pos its, due to low tec tonic ac tiv ity and a spe - cific sed i men tary en vi ron ment that re quires more com pre hen - sive ap proaches than those com monly used in palaeotectonic anal y sis. Lith u a nia can be con sid ered as a clas sic re gion of Qua ter nary cover formed dur ing con ti nen tal glaciations. The thick ness of the Qua ter nary suc ces sion var ies from 5 m in north ern Lith u a nia, an area where gla cial ero sion dom i nates, up to 300 m in mar ginal up lands and bur ied palaeoincisions.
How ever, lit tle ev i dence has been re ported that the for ma tion of the Qua ter nary cover and of re cent re lief was in flu enced by tec - tonic struc tures ex pressed in pre-Qua ter nary rocks (e.g., Baltrñnas et al., 2006; ÈyûienÅ and Satkñnas, 2008).
Lith u a nia is lo cated in the intracratonic Bal tic sed i men tary ba sin, which was ini ti ated in the Cam brian Pe riod. A sed i men - tary suc ces sion over lies Paleoproterozoic con ti nen tal crust that rep re sents a part of the Fennoscandian lithospheric seg ment (Bogdanova, et al., 2006).
Abun dant bore holes to gether with geo phys i cal sur veys (mainly gravimetric and magnetometric) al low map ping of tec - tonic struc tures in the base ment. The im pact of this tec tonic
grain on the Qua ter nary suc ces sion can be in ter preted through con sid er ing dif fer ent mech a nisms, e.g. (1) the in flu ence of the pre ex ist ing pre-Qua ter nary to pog ra phy on the dy nam ics of the ice sheets and melt wa ter streams, (2) the ac tiv ity of the tec tonic struc tures dur ing the Qua ter nary. The pres ent study is fo cused on the Mid dle Lith u a nian top o graphic low land that rep re sents one of the main top o graphic fea tures of Lith u a nia. It geo graph i - cally co in cides with one of the main struc tures in the crys tal line base ment re ferred to as the Mid-Lith u a nian Su ture Zone that also had a long-term in flu ence on the de vel op ment of the over - ly ing sed i men tary cover (Šliaupa et al., 2012).
MID-LITHUANIAN SUTURE ZONE
The thick ness of de pos its cov er ing the crys tal line crust of Lith u a nia var ies from 200 m in the south east to 2300 m in the west (Paškevièius, 1997). The tec tonic struc ture of the crust was un rav elled by ex ten sive deep drill ing, and po ten tial field and deep seis mic sound ing data. The base ment is com posed of two ma jor do mains – the West Lith u a nian Do main (WLD) and the East Lith u a nian Do main (ELD) dif fer ing in crustal thick ness, lithological com po si tion and tec tonic fab ric (Fig. 1). These do - mains rep re sent two tec tonic plates that col lided at around 1.84 Ga (Motuza, 2005; Linneman et al., 2008). At the time of col li sion the WLD ex isted as a newly formed con ti nen tal block (Motuza and Motuza, 2011), while the East Lith u a nian Plate, subducting to the north-west, was of oce anic type, in places pos si bly thick ened within intraoceanic vol ca nic arcs. This an - cient col li sion zone is iden ti fied as a spe cific struc tural unit re - ferred to as the Mid-Lith u a nian Su ture Zone (MLSZ). This zone
* Corresponding author, e-mail: sliaupa@geo.lt
Received: January 24, 2017; accepted: May 3, 2017; first published online: July 6, 2017
is dis tin guished as a tran si tional struc ture of the crust as it grad - u ally thick ens to the east from 40 km in the WLD up to 50–55 km in the ELD. The tran si tion zone is up to 35–40 km across.
A spe cific as so ci a tion of rocks show ing beltiform dis tri bu tion is de fined in the MLSZ (Fig. 1). A belt of am phi bo lites, orig i nally bas alts and diabases with sub or di nate fel sic supracrustal gneiss es, oc curs in the east ern part of the MLSZ. A belt of fel sic gneiss es, orig i nally lavas of andesitic and dacitic por phyry com - po si tion, rep re sents the west ern part of the MLSZ. The supracrustal rocks of the west ern belt are in truded by gra nitic and charnockitic plutons in the north. The width of the MLSZ is
~80–100 km (EUROBRIDGE’95 Seis mic Work ing Group and Yliniemi, 2001).
The in ter nal tec tonic grain of the crust of MLSZ is not well con strained. It is pre sumed that the east ern part of MLSZ rep re - sents part of an eroded accretionary prism com posed of a se - ries of wedges ex tend ing NNE–SSW. The bound ing faults are mapped by sharp gra di ents in the grav ity and mag netic fields.
Fur ther more, EUROBRIDGE deep seis mic sound ing data re - vealed sharp dis con ti nu ities in ter preted as shear zones. Some of these dis sect the whole crust down to the Moho bound ary (Giese, 1998; EUROBRIDGE’95 Seis mic Work ing Group and Yliniemi, 2001; Bogdanova et al., 2006; Linneman et al., 2008).
The ex is tence of such shear zones is sup ported by my lon ite bands doc u mented in deep boeholes in cen tral and south ern Lith u a nia and far ther south-south east in Belarus.
The MLSZ is traced into north east ern Po land as in di cated by a thick en ing of the crust de fined in the DSS pro files POLONAISSE P4 and P5 (Czuba et al., 2002; Grad et al., 2003). The beltiform lithological fab ric is less dis tinct in this part of the MLSZ be cause on the zone there are su per im posed large cratonic in tru sions such as those of Suwa³ki, Go³dap and Kêtrzyn, form ing a chain in an E–W di rec tion pos si bly con trolled by the large-scale fault struc ture (Wiszniewska et al., 2007). Far ther south-west is the the Mazowian gra nitic-migmatitic dome, which has also oblit er ated the struc tural fea tures of the supracrustal se quences of the su ture zone (Wiszniewska et al., 2005).
In the north, the MLSZ is trun cated along the Lith u a nian-Lat - vian bor der. Its con tin u a tion into Lat vian ter ri tory is sup ported nei ther by struc tural, nor by lithological data. The thick ness of the crust in south ern Lat via sharply in creases up to 65 km, as de - fined by the Kochtla–Jarve–Sovietsk DSS pro file (Ankundinov et al., 1994). Lithologically, the MLSZ is lim ited by an E–W trending belt of dif fer ent granulite fa cies rocks, pre dom i nantly migmatites (Bogatikov and Birkis, 1973; Vetrennikov, 1991).
Fig. 1. Geological scheme of the Mid-Lithuanian Suture Zone (modified after Motuza, 2005)
TECTONIC ACTIVITY OF THE MLSZ DURING PRE-QUATERNARY TIME
The MLSZ is con fined to the first-or der tec tonic bound ary of the Bal tic sed i men tary ba sin that sep a rates the Mazu ry– Bela rus anteclise in the east from the Bal tic syneclise in the west (Stirpeika, 1999).
The struc tural stud ies re vealed per sis tent tec tonic ac tiv ity of the MLSZ dur ing de po si tion of the over ly ing sed i men tary cover (Šliaupa et al., 2012). Ac tiv ity along the zone has been traced from as long ago as the Late Ediacaran to Ce no zoic, de spite the dra mat i cally chang ing geodynamic re gime in the Bal tic sed - i men tary ba sin through out the Ediacaran–Phanerozoic. The zone was char ac ter ized by pre vail ing rel a tive sub si dence from Cam brian (Fig. 2) to De vo nian times, as sug gested by in - creases in the thick nesses of the sed i men tary lay ers. The Mid - dle Lith u a nian de pres sion un der went in ver sion, show ing rel a - tive up lift, dur ing post-De vo nian time (Šliaupa et al., 2012). Per - sis tent tec tonic mo bil ity im plies spe cific me chan i cal prop er ties of the crust that cause ac cu mu la tion of tec tonic strain in this zone. The post-De vo nian tec tonic ac tiv ity of the MLSZ re mains am big u ous. Yet, the con trol of the MLSZ on the dis tri bu tion of Ju ras sic and Paleogene de pos its and some de for ma tion of Tri - as sic strata hint at per sis tent tec tonic ac tiv ity of the zone.
RELIEF AND QUATERNARY GEOLOGY
The sub-Qua ter nary sur face was formed by gla cial and melt wa ter ero sional pro cesses and by tec tonic ver ti cal move - ments that took place dur ing the Qua ter nary pe riod (Šliaupa et al., 1995a, b; Dzier¿ek, 1997; Kurzawa, 2002, 2003; Šliaupa and Šliaupa, 2011; Krotova-Putintseva and Verbitskiy, 2012).
The subglacial re lief had sig nif i cant in flu ence on the ice sheet
dy nam ics, re sult ing in lat eral vari a tions in the com po si tion of the Qua ter nary suc ces sion (Šliaupa and Bitinas, 1986;
Niewiarowski and Pasierbski, 1999). Con versely, the cy clic gla - cial load ing-un load ing may have re ac ti vated pre-Qua ter nary tec tonic struc tures, such as lo cal- and re gional-scale faults and frac ture zones, as in ferred from the align ment of the mar ginal mo raine chains, thick ness gra di ents of Qua ter nary de pos its, and lin ea ments de fined in the drain age net work (Liszkowski, 1993; Thorson, 2000; Morawski, 2009). There are no di rect ob - ser va tions of faults cut ting Qua ter nary strata in Lith u a nia and other parts of the Bal tic sed i men tary ba sin, that can be ac - counted for by the low tec tonic ac tiv ity of the re gion and a lack of pre cise geo phys i cal doc u men ta tion. Yet, con clu sive ev i dence of such fault ac tiv ity has been re ported from ad ja cent (more tec - toni cally ac tive) re gions. A num ber of postglacial faults of pre - dom i nant compressional type have been doc u mented in the Fennoscandian shield bor der ing the Bal tic sed i men tary ba sin to the north-west; the fault ac tiv ity was re lated to NNW–SSE hor i - zon tal com pres sion (Rob erts et al., 1997). Most of ter ri tory of Den mark is sit u ated within the north west ern cor ner of the East Eu ro pean Craton. Some faults pen e trat ing the Qua ter nary suc - ces sion there have been de fined by high-res o lu tion seis mic sur vey (Gregersen et al., 1996). Some of these are re lated to re cent seis mic ac tiv ity. Frac ture val leys dis sect ing the re lief have been stud ied in de tail in cen tral Jylland (Jakobsen and Pedersen, 2009).
A close cor re la tion be tween the Pre cam brian crys tal line base ment and the tec tonic evo lu tion of the pre-Qua ter nary sed - i men tary cover has been iden ti fied in Pol ish ter ri tory (Ber and Ryka, 1998) that also in flu enced the Qua ter nary suc ces sion (Ber, 2000; Karnkowski, 2008; Dzier¿ek, 2009).
The sub-Qua ter nary sur face of Lith u a nia is char ac ter ized by com plex re lief (Šliaupa et al., 1995b; Šliaupa, 1997). It is dis - sected by abun dant palaeoincisions, var i ously termed overdeepened or tun nel val leys (Šliaupa, 1997; Bitinas, 1999).
Fig. 2. Map of 1st order residuals of thicknesses of Cambrian deposits (without the oldest Baltic Group)
Contours of the Mid-Lithuanian Suture Zone are shown for comparison (after Šliaupa et al., 2012)
Yet, some sub-re gional trends can be de fined us ing vari a tions of al ti tudes of the sub-Qua ter nary sur face that can be best un - rav elled by ap ply ing the sum mit al ti tudes method, which is based on in ter po la tion of the depths of the pos i tive forms of the sub-Qua ter nary re lief (Fig. 3). Two dis tinct el e vated ar eas are de fined in the west and the east that are re ferred to as, re spec - tively, the Žemaièiai el e va tion and the Aukštaièiai el e va tion.
The al ti tude of the Žemaièiai el e va tion reaches +90 m close to Šiauliai. Sim i lar +90 max i mum al ti tudes are re ported from the Aukštaièiai el e va tion.
The Mid dle Lith u a nian trough, co in ci dent with the MLSZ, sep a rates these two ma jor el e va tions (Fig. 3). It trends NNE–SSW. The sum mit al ti tudes range from +25 in the north to –30 in the south in the ax ial part of the trough. The axis of the Mid dle Lith u a nian trough is con fined to the west ern part of MLSZ. The dif fer ence be tween av er age depths of the trough and the flank ing el e va tions reach 70 m. The width is ~50 km in the north and wid ens to 80 km in the south.
The in cli na tion of the trough axis changes from south ward to north ward in the north; the hinge zone is sit u ated close to Panevežys Town (Fig. 3). The west ern slope of the trough is steeper than that in the east. This is sim i lar to the bed rock trough asym me try re ported from Es to nia and Lat via. Karukäpp (1996, 2004) re lated this asym me try to the Coriolis force ef fect.
This is a rather am big u ous sug ges tion as the troughs were formed by ice sheet ero sion rather than melt wa ter ac tiv ity and the Coriolis force is too small to ex ert any discernable ef fect on ice sheet dy nam ics. Most likely such reg u lar ity in trough asym - me try is ev i dence of the re gional-scale west ward tilt ing of the ice sheet sub strate that re sulted in more in tense sculp tur ing of the west ern trough slopes.
The sub-Qua ter nary re lief con sid er ably af fected the gla cial sed i men ta tion pro cesses. The map of thick ness of the Qua ter - nary de pos its re flects the sub-Qua ter nary Žemaièiai and Aukštaièiai el e va tions and mid dle Lith u a nian de pres sion (Fig. 4). In gen eral, the el e vated ar eas are marked by more in - tense sed i men tary ac cu mu la tion com pared to the de pres sions.
This can be re lated to the more in tense ero sion pro cesses af - fect ing the sub-Qua ter nary sub strate and pre vi ously de pos ited Qua ter nary sed i ments within the sub-re gional scale de pres - sions that is ac counted for by faster ice sheet flow, while el e - vated ar eas fa voured pres er va tion of the sed i ments due to a slower dy nam ics of the over rid ing ice sheets.
The mod ern to pog ra phy also closely cor re lates with the sub-Qua ter nary sur face re lief (Fig. 5). More over, the pres ent to pog ra phy shows more dis tinct vari a tions in al ti tude that is re - lated to the in flu ence of the sub-Qua ter nary sur face on the thick ness of the Qua ter nary suc ces sion noted above. The Žemaièiai and Aukštaièiai sub-Qua ter nary el e va tions are con - fined to the Žemaitija and Baltija up lands. The sur face al ti tude of the Žemaitija Up land reaches +230 m, while ex ceed ing +290 m on the Baltija Up land.
The sur face al ti tudes of the ax ial part of the Mid dle Lith u a - nian trough are about +45–60 m. The Mid dle Lith u a nian trough com prises the NevÅûis River ba sin in the north and the eëupÅ River ba sin in the south-west. The north ern seg ment of the Mid - dle Lith u a nian trough, cov ered mainly by basal mo raine, is punc tu ated by long (up to 40 km) subglacial bedforms (mega-scale gla cial lineations; Fig. 6). They likely re cord fast-flow ing ice (Stokes and Clark, 2002). Mega-scale gla cial lineations are long in the ax ial part of the trough, sug gest ing max i mum flow rates in this part of the de pres sion. Those forms are not re corded in the south west ern seg ment of the trough as they are cov ered by glaciolacustrine de pos its. More over, they are not discernable in the ex posed mo raine ar eas ei ther. This sug gests slower flow of the gla cier due to wid en ing of the trough to the south.
The re lief of the Mid dle Lith u a nian trough was clearly ac tive dur ing the last Nemunas (Weichselian) Gla ci ation and deglaciation. The south ern mar gin of the trough is marked by the Baltija Mar ginal up lands (Fig. 7). The most dis tinct in flu ence of the de pres sion on the ice sheet dy nam ics is de fined in the north - ern part of the trough that ac com mo dated the Mid dle Lith u a nian gla cial lobe which marks the MLSZ. The lobe prop a gated for Fig. 3. Top depths (m) of the sub-Quaternary surface
Fig. 4. Approximate thickness of the Quaternary deposits Line of cross-section is indicated (see Fig. 12)
Fig. 5. Topography of Lithuania
Major features of the MLSZ are shown for comparison
~220 km south wards, which strongly sug gests that the pre-ex ist - ing top o graphic de pres sion fa voured fast ice sheet flow. A mar - ginal ridge is prom i nent on the sur face, clearly out lin ing an ice-lobe ad vance in the trough. It is re ferred to as the Mid dle Lith - u a nian Ridge (Fig. 7). Re mark able fluted mo raine fields and sev - eral ra dial eskers are re lated to the till plain out lined by the ridge.
The ridge is com monly in ter sected by subglacial chan nel val leys and proglacial spill ways. The Mid dle Lith u a nian (VL) phase limit is drawn along the dis tal slope of the ridge.
A de tailed study of frag ments of glaciolacustrine kame ter - races on the dis tal slope of the Mid dle Lith u a nian Ridge re - vealed them to have orig i nated be tween the ac tive ice lobe and blocks of dead ice. It is likely that there was no dis tinct bound ary of the fron tal Mid dle Lith u a nian ice sheet. More over, ar eal rather than fron tal deglaciation dom i nated the area (Bitinas et al., 2004), and this could ex plain the wide spread of boul der ex - po sure ages be tween 11.0 and 15.410Be ka (Rinterknecht et al., 2008) ob tained from seven dated boul ders in the two gen u ine boul der fields in north west ern Lith u a nia. An age of 13.5 ± 0.610Be ka, ob tained from ten dated boul ders, is sug - gested to rep re sent the fi nal deglaciation from the Mid dle Lith u - a nian ice mar gin (Rinterknecht et al., 2008).
TOPO-LINEAMENT PATTERN
Top o graphic lin ea ments are a char ac ter is tic fea ture of gla cial re lief. They are de fined us ing dif fer ent meth ods, such as re mote sens ing, anal y sis of the drain age net work, or morphometric tech - niques. Lin ea ments are de fined as lin ear, curved or cir cu lar fea - tures of lo cal and re gional ex tent. Some lin ea ments might re flect the pat tern of frac tured zones in the bed rock (Oakey, 1994;
Šliaupa et al., 1995b; Šliaupa and Popov, 1998; Kuivamäki, 2000, 2007; Jakobsen and Pedersen, 2009; Rychel et al., 2015).
Yet, most of the lin ea ments iden ti fied are most likely re lated to dy nam ics of the ice sheet that leads to its frac tur ing punc tu ated
by sed i men ta tion fea tures. Some might be con fined to pre-ex ist - ing tec tonic struc tures, while other lin ea ments are seem ingly not con trolled by any sub strate struc tures.
An in tri cate pat tern of ma jor lin ea ments is de fined in Lith u a - nia (Fig. 8; Šliaupa, 2001). West ern Lith u a nia is dom i nated by NE–SW trending lin ea ments su per im posed by or thogo nal and rare NW–SE strik ing lin ea ments. In the east, lin ea ments ori - ented NW–SE pre dom i nate with abun dant W–E strik ing fea - tures and scarce N–S and NE–SW trending lin ea ments. The MLSZ rep re sents the core of this fish bone-shaped lin ea ment pat tern. The west ern part of the MLSZ is dom i nated by NNE–SSW strik ing lin ea ments, while NNW–SSW lin ea ments pre dom i nate in the east ern part of the zone. Also, N–S strik ing lin ea ments are abun dant. Those dif fer ences are ex plained in terms of (1) the dif fer ent struc tural grain of west ern and east ern Lith u a nia and the MLSZ; and (2) dif fer ent ice sheet dy nam ics re flected by a spe cific gla cial frac tur ing pat tern.
RECENT TECTONIC ACTIVITY OF THE MLSZ
There is a mod er ately dense net work of pre cise lev el ling bench mark lines of Class 1 in Lith u a nia (Paršeliñnas et al., 2000). The re peated lev el ling cam paigns pro vide the ba sic in - for ma tion on re cent ver ti cal ground mo tions. The de tailed cor - re la tion of the geo detic and geo log i cal data un rav elled a close re la tion ship be tween an cient tec tonic struc tures and re cent ver - ti cal move ments, im ply ing in her i tance of tec tonic ac tiv ity in Lith - u a nia (Zakarevièius et al., 2008).
The Vilnius–Jonava–Šiauliai geo detic lev el ling line, 290 km long, crosses the MLSZ from the north-west to the south-east (Figs. 9 and 10). Data from mea sure ment cam paigns of 1970 and 1998 were used in the pres ent study to ana lyse trends of ver ti cal ground mo tions across the MLSZ. The zone is dis tinct, with rel a tive ground sub si dence at tain ing 0.8 mm/y in its ax ial part (Fig. 10).
This pat tern of ver ti cal ground mo tions closely cor re lates with the to pog ra phy and the sub-Qua ter nary re lief (Fig. 11).
More over, some short-wave length sub-Qua ter nary sur face un - du la tions are re flected in ver ti cal mo tions, such as the lo cal up - lift in the ax ial part of the Mid dle Lith u a nian trough (geo detic bench mark N317). The cor re la tion of ver ti cal mo tions is as high as +0.55 with the sub-Qua ter nary re lief and +0.64 with the mod - ern re lief.
DISCUSSION
The Mid dle Lith u a nian top o graphic low land is con fined to the Paleoproterozoic Mid-Lith u a nian Su ture Zone (MLSZ) that also shows per sis tent tec tonic ac tiv ity through out the Phanerozoic, mainly characterized by rel a tive sub si dence. The per sis tent tec tonic ac tiv ity can be ex plained in terms of the tec - tonic strain ac cu mu la tion along the crustal-scale tec tonic bound ary, characterized by con sid er able change in the crustal me chan i cal prop er ties.
The MLSZ is re flected on the sub-Qua ter nary and pres ent sur face, rep re sent ing a dis tinct trough. The trough can be of ei - ther tec tonic (sub si dence) or ero sional na ture.
The pre vi ous stud ies of the pre-Qua ter nary lay ers un rav - elled the per sis tent tec tonic ac tiv ity of the MLSZ, dom i nated by a rel a tive sub si dence trend. This in her ited ten dency was also reg is tered in the high-pre ci sion geo detic lev el ling net work show ing re cent mm-scale rel a tive sub si dence within the zone.
Tec tonic ac tiv ity of the MLSZ can there fore be rea son ably sug - gested dur ing the Qua ter nary Pe riod.
Fig. 6. LIDAR image of a fragment of the Middle Lithuanian topographic low
Please note the difference of mega-scale glacial lineations in the west and the east
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Fig. 8. Topographic lineaments defined by morphometric methods (after Šliaupa, 2001) Major features of MLSZ are shown for comparison
(please note the fishbone-shaped pattern of lineaments along the MLSZ)
Fig. 9. Topography (grey scale) and line of precise geodetic levelling Vilnius-Šliauliai (benchmark identification numbers)
Major features of MLSZ are shown for comparison
Fig. 10. Recent ground vertical motions along the Vilnius–Šiauliai geodetic levelling line Benchmark identification numbers are indicated;
shaded area shows location of the MLSZ
Fig. 11. Topography and sub-Quaternary relief, Vilnius–Šiauliai geodetic levelling line
Hatched lines show limits of MLSZ
Fig.12. Geological cross-section NW–SE across Lithuania
gIIdz – gla cial de pos its of Dzñkija Stage; gIIdn – gla cial de pos its of Dainava Stage; gIIžm – gla cial de pos its of Žemaitija Stage; gIImd – gla - cial de pos its of Medininkai Stage; gIInm3 – gla cial de pos its of Up per Medininkai For ma tion; ggIInm3 – gla cial de pos its of Up per Nemunas For ma tion Stage; lgIIdz – limnoglacial de pos its of Dzñkija Stage; fIIdz – flu vio gla cial de pos its of Dzñkija Stage; fIIdn – flu vio gla cial de pos its of Dainava Stage; lgIIdn – limnoglacial de pos its of Dainava Stage; lgIIžm – limnoglacial de pos its of Žemaitija Stage; fIIžm – flu vio gla cial de - pos its of Žemaitija Stage; lgIImd – limnoglacial de pos its of Medininiai Stage; fIImd – flu vio gla cial de pos its of Medininaki Stage; f3IImd;
lgIIInm3 – limnoglacial de pos its of Up per Nemunas Stage; fIIInm3 – flu vio gla cial de pos its of Up per Nemunas For ma tion; bIV – Ho lo cene biogenic de pos its
The pre-ex ist ing sub strate to pog ra phy strongly con trols the ice sheet flow dy nam ics (Durand et al., 2011; De Rydt et al., 2012). This is well-il lus trated by the Mid dle Lith u a nian ice lobe that prop a gated to the south for >100 km to the south along the trough dur ing the last gla ci ation. Such fast ice flow can re sult in more in tense ero sion of the sub strate lithologies. Fur ther more, the per sis tent ac tiv ity of the MLSZ in ev i ta bly led to for ma tion of the wide zone of in creased frac tur ing in the over ly ing sed i men - tary cover. The ev i dence pro vided from dif fer ent re gions im plies in creased rates of ero sion of frac tured lithologies (Dühnforth et al., 2010) that is also con sis tent with mod el ling re sults (Iverson, 2012). The spac ing of frac tures also con trol the pre vail ing mech - a nism (e.g. abra sion vs. pluck ing) of ero sion (Krabbendam and Glas ser, 2011). This can po ten tially ex plain the more in tense ex - ca va tion of the MLSZ sur face by the ad vanc ing ice sheet, and the for ma tion of the Mid dle Lith u a nian trough.
How ever, these ero sional fac tors can hardly ac count for the 70 m dif fer ence in sub-Qua ter nary sur face depths (al ti tudes) be tween the Mid dle Lith u a nian trough and the flank ing Žemaièiai and Aukštaièiai el e va tions, as the old est Qua ter nary de pos its (Dzñkija Stage) are mapped both on el e va tions and in the trough. This sug gests only mi nor (if any) dif fer ence in the ero sion rate of the sub-Qua ter nary sub strate (Fig. 12). It is there fore con cluded that a tec tonic mech a nism should ac count for the for ma tion of the Mid dle Lith u a nian through.
It is no ta ble that the un du la tions of the sub-Qua ter nary sur - face are well-re flected in the vari a tions in the thick ness of the Qua ter nary suc ces sion. This can likely be re lated to dif fer en tial ero sion and sed i ment pres er va tion rates within these forms.
The pre-ex ist ing top o graphic low fa vours faster ice sheet flow that leads to a higher ero sion rate of the sub strate (in clud ing un - der ly ing Qua ter nary sed i ments) that re sults in re duced thick - ness of the Qua ter nary lay ers, es sen tially of the melt wa ter sed i - ments most af fected by an ad vanc ing ice sheet.
Al ter na tively, the vary ing thick ness of the Qua ter nary de - pos its re flects ac tive tec tonic struc tures. The Neo gene lac us -
trine and flu vial sed i ments are dis trib uted in the east ern part of Lith u a nia cov er ing the Aukštaièiai el e va tion, while ab sent from the Mid dle Lith u a nian trough (Fig. 13). Fur ther more, the Daumantai stage sed i ments com pris ing the old est Qua ter nary de pos its known in the East ern Bal tic re gion (Kondratiene et al., 1993) are also con fined to this area. This Qua ter nary/Neo gene bound ary was es tab lished by pol len and lithological cri te ria.
The stage is rep re sented by lac us trine de pos its, dis trib uted as the patches up to a few doz ens of kilo metres wide. This hints at a rel a tive sub si dence of the east ern part of Lith u a nia dur ing the Neo gene–Prepleistocene, with rel a tive up lift of MLSZ at his time. Ac cord ing to this sce nario, in ver sion of the tec tonic move - ments should have taken place lead ing to for ma tion of the Mid - dle Lith u a nian trough dur ing the Late Qua ter nary. These pro - cesses can scarcely be re lated to a gla cial isos tasy as they are much larger-scale phe nom e non. This rather sug gests re - gional-scale changes in the geodynamic sit u a tion in the re gion.
CONCLUSIONS
The Mid-Lith u a nian Su ture Zone (MLSZ) of the Paleoproterozoic age is marked by the Mid dle Lith u a nian top o - graphic low land that is one of the ma jor mor pho log i cal fea tures cross ing the whole Lith u a nian ter ri tory from north to south.
The pres ent to pog ra phy di rectly cor re lates with the to pog ra - phy of the un der ly ing pre-Quternary de pos its. The Mid dle Lith u - a nian top o graphic low is con fined to the sub-Qua ter nary sur - face de pres sion. It sep a rates the Žemaièiai and Aukštaièiai el e - va tions lo cated, re spec tively, to the west and the east. The depth (al ti tude) dif fer ence be tween the av er age depths of the trough and flank ing el e va tions reaches 70 m. Ero sion can not ac count for such a large dif fer ence, as the old est gla cial de pos - its are pres ent within both the trough and flank ing el e va tions.
This im plies that a tec tonic mech a nism is the main fac tor that formed the Mid dle Lith u a nian de pres sion. The MLSZ showed Fig. 13. Distribution of continental Neogene deposits (black polygons)
Hatched line indicates generalised limit of distribution of Neogene deposits
per sis tent tec tonic ac tiv ity through out the Phanerozoic, mainly of rel a tive sub si dence. This sug gests in her i tance of the tec tonic move ments dur ing the Qua ter nary, and the morphotectonic na - ture of the Mid dle Lith u a nian low land.
In creased tec tonic ac tiv ity of MLSZ is also re flected in the pat tern of the top o graphic lin ea ments show ing a fish bone pat - tern as so ci ated with the zone.
The Mid dle Lith u a nian Low land is char ac ter ized by a re - duced thick ness of the Qua ter nary suc ces sion. This can be a re - sult of in creased ice sheet and melt wa ter flow within the de pres - sion lead ing to a higher rate ero sion of the sub-Qua ter nary sub - strate and of Qua ter nary sed i ments de pos ited dur ing pre ced ing gla cial cy cles. Al ter na tively, it might be ex plained in terms of tec -
tonic in ver sion of the MLSZ dur ing the lat ter part of the Qua ter - nary, as sug gested by the oc cur rence of Neo gene de pos its within the Aukštaièiai Up land, while ab sent in the trough.
The Mid dle Lith u a nian low land is also well re flected in the re cent ver ti cal ground mo tion pat tern as de fined by pre cise geo - detic lev el ling, point ing to re cent tec tonic ac tiv ity of the MLSZ.
This cor rob o rates the ac tive tec tonic na ture of the Mid dle Lith u - a nian de pres sion.
Ac knowl edg ments. The study was sup ported by the Lith u - a nian Sci ence Coun cil (pro ject No. MIP 11319). We ap pre ci ate help ful re views by J. Dzier¿ek and an anon y mous re viewer.
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