Karst morphology and cave sediments as indicators of the uplift history in the Alpi Apuane (Tuscany, Italy)

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Abstract

In the Alpi Apuane (Tuscany, Italy), Late Pliocene to Pleistocene karst landforms are preserved as relict phreatic caves, which were formed in a geomorphic setting very different from that of the present day. The largest karst drainage basin in the region, the Frigido, hosts cave systems with a vertical development totalling 1600 m. Abandoned phreatic cave passages preserved within this and neighbouring basins indicate that former base-levels were situated at up to ∼1000 m above the modern valley floors. The passages constitute morphostratigraphic markers that can be used to reconstruct the uplift history of the Apuane. Their vertical distribution suggests two major phases of base-level standstill—one at 1000–1200 m a.s.l. and one at 600–700 m a.s.l. Some of the passages situated at the latter level contain >5 m thick flowstones whose top-beds have an age exceeding the limits of U/Th alpha spectrometric dating (>350 ka). Cave morphology and chronological constraints obtained from speleothems suggest that an important uplift event occurred during the Middle Pleistocene following a period of tectonic standstill of probable latest Early Pleistocene age. Active spring caves close to present-day valley floors contain speleothems whose ages exceed 100 ka, implying that no significant downcutting of the seaward valleys, and consequently no tectonic uplift, has occurred during Late Pleistocene.

Introduction

The Alpi Apuane are situated in northern Tuscany (Italy), where they rise abruptly from the Tyrrhenian coastal plain (Fig. 1). The region is one of the most spectacular high-relief areas in Europe, with several peaks exceeding 1800 m in elevation. The timing of both the uplift of the Apuane and the exhumation of its metamorphic core has stimulated much research over the last few decades (Carmignani and Kligfield, 1990; Abbate et al., 1994), and has major implications for regional tectonic history and landscape evolution (Bartolini et al., 1982). However, the uplift history is not well understood. The geomorphology and chronology of karst caves is one source of evidence that can help resolve issues of uplift history and landscape development (Palmer, 1991). In this paper, we use elevation and age data from dewatered phreatic cave passages to constrain periods of tectonic standstill, rates of incision and minimum ages of former valley-floor elevations.

Section snippets

Regional geomorphology and geology

The Alpi Apuane massif consists principally of a NW–SE trending ridge extending for 32 km approximately parallel to the Northern Appennines, which is part of the main mountain chain that runs the length of the Italian peninsula. The massif consists of three tectonic units that were overthrusted, one upon the other, during the Upper Oligocene (Carmignani and Kligfield, 1990). Two of these units make up a single metamorphic complex of carbonate to terrigenous formations, which have been overlapped

Apuane karst, cave levels and landscape evolution

Surface karst landforms are not well developed in the Apuane because the high relief and regional climatic conditions (i.e. high temperature excursions and rainfall up to 3500 mm/year) enhance mechanical-denudation processes. In particular, mechanical erosion must have been very active during the last glacial and postglacial phases, destroying most of the pre-Würm surface karst landforms. Only in some restricted low-relief areas are small to medium-scale karst landforms (e.g. karren) found (

The use of speleothem age dating as a chronostratigraphic constraint in the apuane

It is widely accepted that phreatic passages form below the local piezometric surface and that they evolve to large tubes close to the karst water table (e.g. Ford and Williams, 1989; Palmer (1987), Palmer (1991), and references therein). If the local base level remains stable, cave systems develop into a complex and hierarchical network of epiphreatic tubes just above the altitude of springs (Palmer, 1991). During a tectonic standstill or a stage of low uplift rates, tubiform passages can

Conclusion

The morphological features of the upper cave-levels (above 1400 m a.s.l.), which are certainly older than the main uplift stage of the Alpi Apuane, probably developed during a stage of tectonic standstill or moderate uplift-rate when the prevailing landscape was of low relief. Remnants of this landscape are currently preserved as horizontal ridges and small summit plains along the secondary crests of Alpi Apuane (Piccini, 1994a). The structural setting of the Alpi Apuane suggests that the

Acknowledgments

This work was supported by a grant (99/067) from the Australian Institute of Nuclear Science and Engineering and from Ministero dell’Università e della Ricerca Scientifica (MURST Cofin 1999), Project resp. C. Bartolini. The authors are grateful to C. Bartolini, G. Hancock and an anonymous referee for useful comments.

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