Field trip B26

Bulgarian part

Blagoevgrad – Brezhani – Kresna
Mikrevo - Ilindentsi –
Melnik (about 160 km)

Leader: Ivan Zagorchev

Stop 4.1. Road to Brezhani
A panorama of the Kroupnik active normal fault is exhibited. It bounds at the South the Neogene asymmetric Simitli graben. The total vertical offset is estimated at about 3 km. The fault strikes NNE-SSW, and is crossing with the NNW-SSE Strouma fault in the northernmost part of the Kresna Gorge of the river Strouma. The two fault zones form a peculiar seismotectonic fault knot that was the site of several devastating earthquakes in historic time, the earthquak(4.04.1904) with the highest magnitude (7.83) on the Balkan Peninsula included. To the South, the narrow Kachovska horst divides the Simitli graben from the Palaeogene of the Brezhani graben. At the background,the faults of the West-Pirin fault zone are the western boundary of the Pirin horst. The uppermost planation surface in Pirin (at an altitude of about 2600 m) and some of the lower surfaces (at c. 2000-2200 and 1500-1600 m) are visible at the far background, too.The road cutting displays the active neotectonic Kroupnik fault as a slickenside with thin mylonites. It strikes about 25º (NNE) and dips 65-70ºWNW, and is traced well in the morphology of the terrain, with the faceted slopes of the footwall. Further North-East, the fault strike changes gradually to nearly 50º, and the dip angle, to 40ºNW. The striae on the slickensides point at a normal fault character, with a slight (about 10% of the vertical component) left-lateral strike-slip.The footwall is built of amphibolites of the Vucha Formation (Rupchos Group, Rhodopian Supergroup).The amphibolites are intensely fractured and mylonitised, and the older structure (foliation, lineation, folds)is hardly visible.The hanging wall is built of conglomerates of the Kalimantsi Formation (Pontian – Romanian).The conglomerates consist of well-rounded pebbles of equigranular granites of the Late Cretaceous North-Pirin (Dautov)pluton. Some pebbles situated near the slickenside of the fault display shear fractures with slight (one to several millimetre) displacements with the same sense as the master fault.The Kroupnik fault has been active since Sarmatian time, with a total neotectonic (Late Neogene and Quaternary) vertical offset of the order of 3500 m, and a mean velocity of 0.25 mm a-1. Most of this offset(more than 3000 m) refers to the Neogene times, with two major epochs of activity(Sarmatian - Maeotian and Pontian - Romanian). The recent activity is gauged with a station that measures the relative slow recent movements at the two sides of the fault. The fault was particularly active in the years 1901 - 1910. Fourteen earthquakes with epicentres along the fault trace had a magnitude exceeding 4.5, and the total number of tremors was more than 1700.The seismic foci migrated gradually from the NNE towards SSW,the first ones (1901 - 1903) being located mainly near and North of the Rila Monastery in the Rila Mountain. The strongest earthquake (M estimated at 7.83!) occurred on 4.04.1904 with an epicentre near Kroupnik.

Stop 4.7. Near Mikrevo:
Neotectonic panorama of the Sandanski graben and Pirin horst;metamorphic events in the Ograzhdenian Supergroup

The observation point is situated on the sinuous road towards the village of Tsaparevo, and exposes also the rocks of the upper parts of the Ograzhdenian Supergroup (Maleshevska "group") from the eastern margin of the Ograzhden unit. These are biotite and two-mica gneisses and migmatites interlayered with amphibolites and biotite schists. In the locality the biotite schists contain tiny tourmaline crystals. After folding, the schists had been intruded by pegmatites (with tourmaline) and aplites that underwent afterwards a new deformation event (again in amphibolite-facies conditions) together with the host rocks, with intense schistification (aplites and pegmatites transformed into quartzo-feldspathic gneisses) and folding in two episodes.

The Sandanski graben is an asymmetric structure, with maximum (3000-3500 m) vertical displacement along the eastern margin (West-Pirin fault zone), and a small (150 - 500 m) offset along the western margin (Ograzhden fault zone). The stop displays a panoramic view of the Pirin horst with the highest peak Vihren, the initial (early - middle Miocene) peneplain(orthoplain) at ca. 2600 m, and the peneplains at 1950 - 2200 m and 1500-1600 m, the tilted section of the Sandanski graben with the marble breccia of the Ilindentsi Member, and the post-Neogene surfaces (pediments) built over the tilted Neogene section (and partially, over the West-Pirin fault zone itself) at altitudes of
650 - 900, 500 - 600, 320 - 360, and 220 - 270 m.

Stop 4.13. Melnik: Neogene sediments of the Sandanski graben
The "Melnishki piramidi" (Melnik Pyramids) are erosion forms that are included in the list of natural sites protected by the Bulgarian state.They developed within the loosely cemented rocks of the Kalimantsi Formation(Pontian - Romanian). The whole section is gently dipping (5-20o)ENE, towards the West-Pirin fault zone.

The Sandanski Formation consists mainly of whitish or yellowish sandstones interbedded with some clays and coal clays (also a thin coal seam at the village of Hotovo), and fine-pebble conglomerate. It is covered by similar(whitish to yellowish) conglomerates with sandstone interbeds that belong to the Kalimantsi Formation. The principal diagnostic feature of this conglomerate is the abundance of well-rounded pebbles coming from the Palaeogene granitoids that have been unroofed in Pontian time. Graded bedding and cross bedding are often observed.The rocks of the Sandanski Formation are well exposed at the crossroad to Vinogradi, 2 km West of Melnik. Typical sandstones interbedded with clay and coal clay crop out, and the gentle dip towards East is well visible.

Correlation of Neogene sedimentary formations
and planation surfaces in SW Bulgaria

op - Early Miocene peneplain (orthoplain);
OI - OIII - Neogene oroplains;

pI - pIII - Quaternary mountain steps

© Geological Institute of the Bulgarian Academy of Sciences
Author: Ivan Zagorchev
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