It Cracked in the Middle Miocene

And it was quite a rumble …

The Earth back then – some 12 million years ago – was different, but uncannily similar. Arabian peninsula and Zagros mountains just crashed into Eurasia proper, severing the Tethys from the Indian ocean, and Africa collided with Iberia, closing the Gibraltar straits that lead to the Messinian salinity crisis when the whole Mediterranean sea dried up.

In a rather nondescript part of Europe between the Alps and the Paratethys sea to the east a river flowed from its spring in the Dinaric mountains to the north and east, towards a bay in the Western Paratethys. And then suddenly, with a tremendous bang, something cracked.

Jakovica hill and Planina polje

That something was more or less the whole area. A huge, over 100 km long crack appeared and went roughly southeast to northwest in the general Dinaric direction. The part to the east slid down for hundreds of meters – the cumulative drop is around 480 m – creating something only a geologist would call normal: a normal fault.

And the hill on the photo above is a witness of its birth: it was there when the whole shit went down.

Cindy delta kite above Jakovica hill

Idrija fault as it is called now (the mercury mine in Idrija was like a honeytrap for scientists in the 18th and 19th century, and they soon discovered the big crack) is clearly visible on satellite photos and LIDAR maps. It could be responsible for the devastating earthquake in 1511 that destroyed most of the castles in the land. It is still seismically active, though not much. Sometime in the middle Pliocene – when the Zanclean floods filled the Mediterranean again – it transitioned from a normal fault to a dextral strike-slip fault; the western part is moving north at about 0,1 mm per year.

Idrija fault from the Alps to the Croatian border

The sudden appearance of the Idrija fault really changed the geography of this part of the Earth. The limestone highlands went full Karst, and along the fault a series of karst fields – polje, to use a terminus technicus – formed.

One of these is Planina polje, a superb example of this Karst phenomenon. A flat basin flanked by limestone hills and plateaus, with river Unica meandering across it. The river emerges from a cave on the southern end of polje, and disappears into sinkholes on the northern end.

Unica is one of the surface flows of the unsuspecting pliocene river mentioned before. Idrija fault wrought havoc with it – its direction changed to follow the fault, but the largest impact were the limestone karstic highlands. The river – Palaeoljubljanica – was forced to go underground a couple of times. From its spring, now called Trbuhovica, it flows on a series of poljes and sinks under the highlands repeatedly. One of its apparitions is Unica on Planina polje, the penultimate above-ground flow. When the river emerges for the final time in a series of springs at the southern end of Ljubljana Marshes, it is called Ljubljanica.

The hill – known as Jakovški grič, with the village of Jakovica under the wings of Archangel Michael – is a bit bizarre. Sitting in the middle of Planina karst field (polje) it looks seriously out of place, even for a solitary outcrop. The southeastern (right) flank is made of crushed, crumbly dolomite, while the northwestern is of limestone. Idrija fault goes right over it: when the whole thing went down this poor little hill remained, standing stunned in the middle of the newly formed polje, never really getting what has just happened.

Jakovica is not the first settlement of the hill. There may be an Iron Age (or even a Neolithic) village on it, and some Roman stuff were found, a cemetery and some buildings, and a putative claustra. Jakovica the village is first mentioned in 1324; the church of Archangel Michael guards it, and there is a pilgrim chapel of St. Mary in the Shrubs built above a sacred spring.

Jakovica and Planina polje don’t really show their tumultuous past. The fault is more or less dormant, the crushed bedrock is hidden under the forest and the sediments; the village, the river, the polje are all serene and peaceful. Yet subtle clues do tell a story, if one cares to listen.

Or flies a kite over it 🙂

All kite aerial photos shot with Insta360 on The Rokker kite.

3 thoughts on “It Cracked in the Middle Miocene”

  1. Superb photos as always. Living in Christchurch, NZ, I can appreciate the power of earth movements. The devastating earthquakes we had here in 2010 and 2011 (over 7,000 houses destroyed or made uninhabitable and many thousands more damaged) shifted large areas up, down, and sideways. My side of the city ended up a metre lower than it had been, and big stopbanks were needed to prevent flooding from the river every high tide. We live with the knowledge that a much bigger event, on what’s called the Alpine Fault, will come along sometime, no doubt without warning. Nature is all-powerful.

    • Thanks, Jim! 🙂

      Yeah, the Earth is a dynamic planet … Idrija fault is now supposedly ‘dormant’, but the Alps (the real ones 😉 …) are seismically active, and we are all waiting for the big event (the last serious quake shook Ljubljana in 1895).


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