Abstract
We propose to test if gravimetry can prove useful in discriminating different models of long-term deep crustal processes in the case of the Taiwan mountain belt. We discuss two existing tectonic models that differ in the deep processes proposed to sustain the long-term growth of the orogen. One model assumes underplating of the uppermost Eurasian crust with subduction of the deeper part of the crust into the mantle. The other one suggests the accretion of the whole Eurasian crust above crustal-scale ramps, the lower crust being accreted into the collisional orogen. We compute the temporal gravity changes caused only by long-term rock mass transfers at depth for each of them. We show that the underplating model implies a rate of gravity change of −6 × 10−2 μGal yr−1, a value that increases to 2 × 10−2 μGal yr−1 if crustal subduction is neglected. If the accretion of the whole Eurasian crust occurs, a rate of 7 × 10−2 μGal yr−1 is obtained. The two models tested differ both in signal amplitude and spatial distribution. The yearly gravity changes expected by long-term deep crustal mass processes in Taiwan are two orders of magnitude below the present-day uncertainty of land-based gravity measurements. Assuming that these annually averaged long-term gravity changes will linearly accumulate with ongoing mountain building, multidecadal time-series are needed to identify comparable rates of gravity change. However, as gravity is sensitive to any mass redistribution, effects of short-term processes such as seismicity and surface mass transfers (erosion, sedimentation, ground-water) may prevent from detecting any long-term deep signal. This study indicates that temporal gravity is not appropriate for deciphering the long-term deep crustal processes involved in the Taiwan mountain belt.
Highlights
Temporal gravimetry is an efficient tool to monitor mass transfers, such as those implied by volcanic reservoir charge (e.g. Battaglia et al 2008), groundwater movements (e.g. Jacob et al 2010), or crustal thickening (e.g. Sun et al 2009)
We discuss the possibility of detecting such small signals since (i) short-term processes other than deep crustal mass transfers are likely to occur at much higher rates and (ii) deep crustal processes sustaining long-term mountain building are not expected to be linearly downscaled to short-term timescales, because they are discontinuous over time
These models of mountain building in Taiwan will theoretically create distinguishable gravity signals related to their deep crustal kinematics
Summary
Temporal gravimetry is an efficient tool to monitor mass transfers, such as those implied by volcanic reservoir charge (e.g. Battaglia et al 2008), groundwater movements (e.g. Jacob et al 2010), or crustal thickening (e.g. Sun et al 2009). We compute the gravity change rates expected from deep mass transfers considering two models of the long-term evolution of the Taiwan wedge We find that these rates have opposite signs, suggesting that the two models tested could in theory be discriminated from gravity. We discuss the possibility of detecting such small signals since (i) short-term processes (earthquakes, landslides, hydrological variations) other than deep crustal mass transfers are likely to occur at much higher rates and (ii) deep crustal processes sustaining long-term mountain building are not expected to be linearly downscaled to short-term timescales, because they are discontinuous over time These results critically lower the practical interest of using repeated land-based gravity measurements to unravel the long-term deep crustal processes in the Taiwan mountain belt
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