Abstract
There is growing interest in how geofluid emissions are released in the atmosphere by the planet’s geodynamic activity, and how much they contribute to the global budget of greenhouse gases. Many workers are addressing this issue with studies conducted at global scale, so as to get the required global-scale answers. The data available at the global scale on geofluids, faults, earthquakes and volcanoes, however, are generally too coarse to provide these answers. We investigate the relationships between geofluid emissions and tectonics at a more detailed scale. Building on over a century of data on geofluid emissions and on an extensive knowledge of the region’s tectonics and seismicity, we focused on Italy, one of the areas of the globe that experience the largest release of natural CO2and CH4. We systematically overlaid and compared data collected by a number of workers into 13 published countrywide databases concerning geofluid emissions, carbon-bearing deposits, seismogenic faults, historical and instrumentally documented earthquakes, and heat flow observations. Our results indicate that 1) thermal springs and CO2emissions dominate in areas of mantle upwelling and crustal stretching, but also that 2) some of them occur in the extending inner Apennines, generally along major lithospheric chain-perpendicular lineaments that bound the largest normal faults. Conversely, 3) CH4emissions and mud volcanoes dominate in areas undergoing active contraction, where no CO2emissions are observed; in particular, we find 4) that mud volcanoes concentrate where the crests of active anticlines intersect major lithospheric chain-perpendicular lineaments. An overarching conclusion is that, in Italy, the release of geofluids is primarily controlled by deep crustal discontinuities that developed over the course of 5–10 My, and is only mildly affected by ongoing crustal strains. Geofluid emissions bring information on processes that occur primarily in the lower crust, marking the surface projection of generally hidden discontinuities that control the geometry and modes of seismic release. As such they may also provide valuable insight for improving the assessment of seismic hazard in hard-to-investigate seismically active regions, such as Italy.
Highlights
HOW DO GEOFLUID EMISSIONS RELATE TO TECTONICS AND EARTHQUAKES?It is well established that pressure gradients existing in the Earth’s crust can expel geofluids from deep geological formations toward the atmosphere
Earthquakes causing significant shaking and permanents strains in a large rock volume allow the generation of new, episodic fluidflow regimes, which may become permanent when rocks having limited permeability are pervasively fractured by the upward propagation of seismogenic faults
Such geophysical conditions are attained near the edges of seismogenic faults, where transverse lineaments are known to occur from independent lines of evidence (e.g., Curewitz and karson, 1997)
Summary
It is well established that pressure gradients existing in the Earth’s crust can expel geofluids from deep geological formations toward the atmosphere. We postulate that these deeper faults may act as pathways for the rise of thermally anomalous fluids and CO2, even in areas of low heat flow Their potential role in controlling the distribution of geofluid emissions will be discussed in Correlations and Anticorrelations Section. A closer inspection shows that (1) thermal springs appear to be anticorrelated with the surface projection of major seismogenic sources, implying that the relevant crustal volumes are less permeable than adjacent areas This phenomenon has been inferred by Micklethwaite et al (2010) from the distribution of ore deposits, which occur mainly along transverse features located at fault tips, rather than along large, welldeveloped faults;.
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