Tectonic Control of Piercement Structures in Central Java, Indonesia

Abstract:

A recent field expedition in Central Java targeted the mapping and sampling of several piercements structures in central Java (Indonesia), most of which have never been documented before. Here, at least seven structures erupting mud water and gas are distributed along a NE-SW alignment that extends for about 10 kilometers. Some of the mapped structures (Bledug Kuwu, Bledug Cangkring Krabagan, Mendikil, Banjarsari, Krewek) have been named after the neighboring local village. None of these have obvious elevation despite the vigorous emission of gas and mud, suggesting that significant caldera collapse is ongoing. Among the most relevant:

Bledug Kuwu is certainly the most impressive structure with three main eruption sites in the crater area bursting more than 5 m large hot mud bubbles. Similar characteristics are present at the smaller (200 m in diameter) Bledug Cangkring Krabagan, that is also surrounded by numerous pools and gryphons seeping around the main crater. The smaller sized Mendikil is the only visited structure that, at the moment of the sampling, did not show seepage of hot fluids. Banjarsari and Krewek (up to 200 m wide) are characterized by scattered hot water-dominated pools where gas is vented vigorously. In particular the hot pools are systematically covered by travertine concretions.

Water and gas geochemisty confirms the seepage of CO₂ dominated gas and water with hydrothermal signature.

The investigated structures appear to follow an obvious NE-SW oriented lineament that most likely coincides with a tectonic structure (fault?) that controls their location. Indeed the field observations and the analyses suggest that likely scenario is that this fault (?) acts as a preferential pathway for the expulsion of hydrothermal fluids to the surface. Very little is known about this region, neither is known why several of these structures erupt hot mud despite their significant distance from the two closest volcanic structures (i.e. Mt. Muria 60 km to the NW and Mt. Lawu 50 km to the SE).

This system of piercement structures represents an ideal analogue for Lusi and other mud volcanoes hosted by the Watukosek fault in North-east Java.