Tectonic activity

Given a sufficient period of time, in the absence of tectonic processes, continents would presumably be eroded flat to about sea level. It is not surprising, therefore, that most tectonically quiet areas on continents tend to have low elevations and are often flat, whereas tectonically active areas, mostly mountain belts, have high elevations and steep slopes (Figs 9-2 and 9-4). 

Paolini, 1986 Stallard, 1980). Comparison with Fig. 9-7 shows a reasonable match between denudation rates and uplift rates for a particular type of terrain. The most concentrated water samples and highest denudation rates are observed in river basins in tectonically active areas. 

Contrary to popular concepts, sands do not always dominate arid and semi-arid zones. Aridisols occur on a wide variety of landforms, lithological formations and are of different ages. They are most common on stable land surfaces of Late Pleistocene or greater age in tectonically active deserts where they comprise alluvial fan, alluvial flats or stream terraces. Arid zones also include mountainous terrain with steep slopes (Nettleton and Peterson, 1983). Many arid or semi-arid zones include fluvial and aeolian materials of Pleistocene age. 

As shown in Table 11.1, hydrothermal emissions are a major source of soluble iron, manganese, and zinc and a minor source of aluminum, cobalt, copper, and lead. Other elements with significant hydrothermal inputs include lithium, rubidium, cesium, and potassium. Considerable uncertainty also surroimds these flux estimates because they are the result of extrapolations from measurements made at a small number of hydrothermal systems at single points in time. These fluxes appear to vary significantly over short time scales as tectonic activity abruptly opens and closes cracks in the oceanic crust. 

The ancient evaporites of the Phanerozoic eon were deposited at rates as fest as 100 m per lOOOy. These rapid rates are thought to have been caused by a lowering of sea level associated with tectonic activity and glaciation. Some of the largest of the salt giants are the Messinian evaporites that formed in the Mediterranean Sea during the late Miocene epoch, 5.5 to 6.5mybp. 

The Maritime Disturbance refers to tectonic activity that mainly occurred within the basin areas of the Late Paleozoic... 

CO2 is the second most abundant gas species in magmatic systems. In a survey of CO2 emanations from tectonically active areas worldwide, Barnes et al. (1978) attributed 8 C-values between -8 and -4%c to a mantle source. This is, however, problematic, because average crustal and mantle isotope compositions are more or less identical and surflcial processes that can modify the carbon isotope composition are numerous. A more promising approach may be to analyze the C-content of CO2 collected directly from magmas at high temperatures. 

The Sierra Nevada provide an example of the perils of relating erosion rates to relief. They are thought to have persisted since 60-80 Ma with high (1.5 km) relief over long wavelengths ( 50 km), but erosion rates over these timescales are estimated to have been only 0.03-0.08 mm/yr (House et al. 1997, 2001 Clark et al. 2005). Incision rates may have been higher ( 0.2 mm/yr) in the Pliocene (Stock et al. 2004), possibly aided by convective instability at depth (e.g., Ducea and Saleeby 1996), but this is still slow relative to erosion rates in many other tectonically active and inactive regions, and similar to some with much lower relief (e.g., Heffern et al. 2007). 

Thus for practical use in reconstructing paleotopography, estimated erosion rates in the past do not provide very good constraints on either mean local relief or surface elevation. This is both because of potential uncertainties in whether a landscape was tectonically active, and because of wide ranges in erosion rate at a given index of local relief. 

Montgomery DR, Brandon MT (2002) Topographic controls on erosion rates in tectonically active mountain ranges. Earth Planet Sci Lett 201 481-489... 

Numerous measurements of 222Rn exhalation have been reported. Global continental means of 15, 16 and 25 mBq m-2 s-1 have been calculated by Israel (1951), Wilkening et al. (1972) and Turekian et al. (1977), respectively. Higher rates of emission are found in regions of recent tectonic activity, and where the uranium content of the topmost rocks or soil is enhanced. 

Chernova, T.G., Rao, P.S., Pikovskii, Yu.I., Alekseeva, T.A., Nagender Nath, B., Ram-alingeswara Rao, B., Rao, Ch.M., 2001. The composition and the source of hydrocarbons in sediments taken from the tectonically active Andaman Backarc Basin, Indian Ocean. Mar. Chem. 75, 1-15. 

In many examples of hot springs, fumaroles, and the like, associated with tectonic activity, the water involved is meteoric but the noble gases are, in part, juvenile. Observations are described in Chapter 6. 

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