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Structural evidence for hard linkage between the Predela and Haramibunar faults in Southwestern Bulgaria

Conference Paper

Structural evidence for hard linkage between the Predela and Haramibunar faults in Southwestern Bulgaria

81
БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2017“
BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2017”
Structural evidence for hard linkage between the Predela and Haramibunar
faults in Southwestern Bulgaria
Структурни данни за твърда връзка между Пределския разлом
и Харамибунарския разлом в Югозападна България
Alexander Radulov1, Marlena Yaneva1, Petra Štěpančíková2, Ianko Gerdjikov3,
Thomas Rockwell4, Yordanka Donkova1, Hamid Sana2, Jan Flašar2, Nikolay Nikolov1
Александър Радулов1, Марлена Янева1, Петра Степанчикова2, Янко Герджиков3,
Томас Рокуел4, Йорданка Донкова1, Хамид Сана2, Ян Флашар2, Николай Николов1
1 Geological Institute, Bulg. Acad. Sci., Acad. G. Bonchev str., bl. 24, 1113 Sofia; E-mail: radulov@geology.bas.bg
2 Institute of Rock Structure and Mechanics, Acad. Sci. Czech Republic, V. Holešovičkách 41, Praha 8, Czech Republic;
E-mail: stepancikova@irsm.cas.cz
3 Sofia University “St. Kliment Ohridski”, Dept. of Geology, Paleontology and Fossil Fuels; E-mail: janko@gea.uni-sofia.bg
4 San Diego State University, USA; E-mail: trockwell@mail.sdsu.edu
Keywords: relay ramps, normal faults, seismogenic faults, East Pirin fault, Southwestern Bulgaria.
Most of the studies in SW Bulgaria consider the
East Pirin fault system to be a neotectonic (post-
Oligocene) structure (e.g. Zagorčev, 1970). However,
the seismogenic potential of faults that strike at 330°
is usually rejected because of their non-optimal
orientation to modern N-S extension (e.g. Basili et al.,
2013; inset on Fig. 1a). In fact, traces of these faults
are associated with linearly orientated faceted slopes,
notches in transverse ridges, changes in valley depths
and widths, and the generation of deposits such as
colluvium, alluvial fans, and fluvio-glacial outwash
sediments that are sparsely but regularly arranged
along a narrow area in the hanging wall close to these
faults. The activity of non-optimally oriented faults
mainly depends on strain rates: a low current strain rate
favors reactivation along these faults. The morphology
and low strain rates suggests that faults from the East
Pirin system may accommodate strain regardless of
their orientation in the current stress field. Herein, we
present structural arguments supported by evidence for
post-Late Glacial Maximum surface rupture inferred
from a resistivity survey that the optimally oriented
Predela fault and non-optimally oriented Haramibunar
fault constitute a system that can rupture in a single
earthquake.
In order to test the hypothesis that the Haramibunar
fault has ruptured to the surface in the late Quaternary,
we collected a resistivity profile along an alluvial
fan in Desilitsa valley (23.5524° N, 41.7837° E;
Fig. 1a). Basement rocks at the site are granitoids from
Fig. 1. Bansko relay ramp links Predela, Dobrinishte and Haramibunar faults: a, map of fault traces. Active faults in inset map after Basili et al.
(2013); b, schematic perspective view of Bansko relay ramp with noted fault segments
82
the Spanchevo pluton. The profile is 400-m-long and
nearly perpendicular to an interpolated line between
a scarp and a notch on a nearby ridge. A Wenner-
Schlumberger array with 5 m electrode spacings was
applied to measure resistivity. Three layers of differ-
ent electrical properties are distinguished in the profile
(Fig. 2). We interpret the lower layer to be the granitic
basement rocks. The middle layer represents coarse
alluvium dominated by boulders. The upper layer cor-
responds to finer-grained overbank alluvium. The sub-
vertical zone of higher resistivity in the lower layer at
x=190 m along the profile hosts the main fault branch.
The fault displaces the middle layer, including the
base of upper layer, at x=145 m. The vertical offset
is estimated to be about 4 m. Another discontinuity in
the basement at x=278 m affects resistivity values of
the upper alluvial units, but does not displace the base
of the middle layer. The discontinuity likely coincides
with an antithetic fault branch where the offset is too
small to resolve. The age of alluvium is currently un-
known, although we infer that it was deposited after
the Last Glacial Maximum when fluvio-glacial out-
wash plains were formed in Rila and Pirin.
A map of tectonic landforms along the Predela,
Dobrinishte, and Haramibunar faults from the East
Pirin fault system indicates that they are linked
through a relay ramp in the Bansko area (Fig. 1). The
relay ramp is double breached. A front connecting
fault links the western tip of the Dobrinishte fault to
the rear Predela fault. A rear connecting fault has de-
veloped between the rear overlap and the northern tip
of the Haramibunar fault. The different orientations of
the Predela fault when compared to the Haramibunar
fault suggest that at least one of them is inherited as
Fig. 2. Resistivity profile in Desilitsa valley suggests post-Last Glacial
Maximum surface ruptures along the Haramibunar fault. Location
shown on Fig. 1.
a preexisting fault. Fault interaction has resulted in
a complex relay ramp shape where overlaps are not
parallel to each other. The relay zone aspect ratio of
overlap to separation is close to 2, which is a global
characteristic for large faults (e.g. Hopkins, Dawers,
2017). The presence of two breaching faults and low
aspect ratio indicate that the fault system is in mature
stage of evolution (extension >20%) when crustal
stretching is localized along large faults. An assump-
tion that only the Predela and Dobrinishte faults rup-
ture together, based on their similar morphologies
and orientations, seems to be inconsistent with global
and experimental observations on relay structures,
because the 5.5-km-long Dobrinishte fault scarp out
of the relay ramp is smaller than the overlap length.
More likely, the Bansko relay ramp evolution has also
been controlled by the Haramibunar fault. The front
overlap has been offset more than 500 m in the dip
direction providing additional evidence for activity
along the rear connecting fault. The combined length
of the Predela-Haramibunar system is about 37 km,
which is within the typical range for mature normal
fault systems.
Structural relationships, morphologic character-
istics and geophysical evidence for faulted post-Late
Glacial Maximum strata lead to a conclusion that the
Predela-Haramibunar normal fault system constitutes
a larger crustal seismogenic source than previously as-
sessed, and that its size is comparable to the nearby
Kroupnik fault and SE Rila fault which ruptured in
1904 AD.
Acknowledgements: This study is a part of Bul-
garian-Czech bilateral project “Active tectonics and
earthquake geology in Bulgaria and Czech Republic”.
The field survey is partially financed by the Geological
Institute at BAS in the frame of project “Earthquake
potential of faults in Bulgaria”.
References
Basili, R., V. Kastelic, M. B., Demircioglu, D. Garcia Moreno,
E. S. Nemser, P. Petricca, S. P. Sboras, G. M. Besana-
Ostman, J. Cabral, T. Camelbeeck, R. Caputo, L. Danciu,
H. Domac, J. Fonseca, J. Garćia-Mayordomo, D. Giardini,
B. Glavatović, L. Gulen, Y. Ince, S. Pavlides, K. Sesetyan,
G. Tarabusi, M. M. Tiberti, M. Utkucu, G. Valensise, K.
Vanneste, S. Vilanova, J. Wössner. 2013. The European
Database of Seismogenic Faults (EDSF) Compiled in the
Framework of the Project SHARE. Online Data Resource,
http://diss.rm.ingv.it/share-edsf/; doi: 10.6092/ INGV.
IT-SHARE-EDSF.
Hopkins, M. C., N. H. Dawers. 2017. The role of fault length,
overlap and spacing in controlling extensional relay ramp
fluvial system geometry. – Basin Research; doi:10.1111/
bre.12240.
Zagorčev, I. 1970. Certain features of young Alpine block
structure in a part of South-Western Bulgaria. – Bull. Geol.
Inst., Ser. Geotectonics, 20, 17–27 (in Bulgarian with an
English abstract).
... Независимо от различията, в тези работи се подчертават ясната морфоложка изразеност и белезите за съвременна активност. На базата на теренни, морфометрични и геофизични данни Radulov et al. (2017) характеризират системата от активни разломи в североизточното подножие на Пирин. Издигането на Пиринския хорст се свързва с транслациите по комплексна разседна зона -Пределско-Харамибунарска (фиг. ...
... Глазне. Според модела на Radulov et al. (2017) рампата се оформя от две групи разседни сегмента: тилна и фронтална. Движенията по тях оформят стъпаловидната морфология на североизточното Пиринско подножие в района на гр. ...
... Горна Раковица. В източна посока геометрията на активните разломявания е усложнена, оформя се свързващата рампа (Radulov et al., 2017) и това намира пряко отражение върху системата водосбори-алувиална седиментация в Разложкия грабен. Именно в участъка на рампата се формира най-големият алувиален конус в североизточното подножие на Пирин -този на р. ...
Article
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Relay ramps are integral components of normal fault systems that control sediment transport pathways in evolving rifts. We attribute differences in the geometry of fluvial systems that drain relay ramps to the scale of the ramp bounding fault segments, the spacing between segments and the amount of overlap between segments. Previous conceptual models for relay ramp geomorphological evolution have assumed that ramp fluvial catchments develop on the ramp surfaces and flow parallel to fault strike into the adjacent basin. Numerous examples exist in nature, however, that show that this is not ubiquitous. The fundamental question of what drives differences in fluvial geometry in these settings has, to date, not been fully addressed. We selected 27 relay ramps across the Basin and Range, western North America, and mapped, via GPS and remote sensing, the faults and ramp fluvial systems associated with each site. The sites represent a range of fault scales, which we define by the total outboard fault length, and a range of spacing and overlap values in order to better understand the structural controls on differences among ramp fluvial systems. Results show that the majority of a relay ramp surface drains parallel to fault strike when the outboard fault is less than about 15 km long. High overlap/spacing ratios are associated with relays along shorter (< 15 km long) outboard faults, whereas lower overlap/spacing ratios are associated with relays along longer faults. Relays with lower overlap/spacing values may be more common along longer outboard faults because they survive for longer periods of time in the landscape. Our geomorphological observations can be used to predict synrift depocenter locations along segmented faults, but these observations only apply if the faults are short (<15 km long) and in early rifting stages. At longer fault lengths, ramp fluvial system geometry has no discernable relationship with any specific structural parameter.
The European Database of Seismogenic Faults (EDSF) Compiled in the Framework of the Project SHARE
  • R Basili
  • V Kastelic
  • M B Demircioglu
  • D Garcia Moreno
  • E S Nemser
  • P Petricca
  • S P Sboras
  • G M Besana-Ostman
  • J Cabral
  • T Camelbeeck
  • R Caputo
  • L Danciu
  • H Domac
  • J Fonseca
  • J Garćia-Mayordomo
  • D Giardini
  • B Glavatović
  • L Gulen
  • Y Ince
  • S Pavlides
  • K Sesetyan
  • G Tarabusi
  • M M Tiberti
  • M Utkucu
  • G Valensise
  • K Vanneste
  • S Vilanova
  • J Wössner
Basili, R., V. Kastelic, M. B., Demircioglu, D. Garcia Moreno, E. S. Nemser, P. Petricca, S. P. Sboras, G. M. Besana-Ostman, J. Cabral, T. Camelbeeck, R. Caputo, L. Danciu, H. Domac, J. Fonseca, J. Garćia-Mayordomo, D. Giardini, B. Glavatović, L. Gulen, Y. Ince, S. Pavlides, K. Sesetyan, G. Tarabusi, M. M. Tiberti, M. Utkucu, G. Valensise, K. Vanneste, S. Vilanova, J. Wössner. 2013. The European Database of Seismogenic Faults (EDSF) Compiled in the Framework of the Project SHARE. Online Data Resource, http://diss.rm.ingv.it/share-edsf/; doi: 10.6092/ INGV. IT-SHARE-EDSF.
Certain features of young Alpine block structure in a part of South-Western Bulgaria
  • I Zagorčev
Zagorčev, I. 1970. Certain features of young Alpine block structure in a part of South-Western Bulgaria. -Bull. Geol. Inst., Ser. Geotectonics, 20, 17-27 (in Bulgarian with an English abstract).