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Shear-focusing of pre-existing melt as an origin for small-volume intraplate volcanism

Greg A. Valentine

Department of Geology, University at Buffalo, USA, gav4@buffalo.edu

 

This webpage is a summary of the following papers:

Valentine G.A., Hirano N., (2010), Mechanisms of low-flux intraplate volcanic fields – Basin and Range (North America) and Northwest Pacific Ocean, Geology, 38, 55-58, doi:10.1130/G30427.1

Valentine G.A., Perry F.V., (2007), Tectonically controlled, time-predictable basaltic volcanism from a lithospheric mantle source (central Basin and Range Province, USA), Earth and Planetary Science Letters, 261, 201-216, doi:10.1016/j.epsl.2007.06.029

Valentine G.A., Perry F.V., (2006), Decreasing magmatic footprints of individual volcanoes in a waning basaltic field, Geophysical Research Letters, 33, L14305, doi:10.1029/2006GL026743

 

Intraplate volcanic fields occur in both continental and seafloor settings, and examples can be found in all types of regional tectonic environments. While deep-seated mantle plumes, upwelling asthenosphere, and other traditional mechanisms make sense for some of these intraplate volcanic systems, there are many cases where these explanations really do not work. This is especially true for volcanic fields with low magma output rates (eruptive flux). Such systems are characterized by small volcanoes (~0.005-2 km3) that are commonly monogenetic, meaning that they have only a single eruptive phase, which might last days to perhaps a few decades, and then never erupt again, excepting the low probability event where a completely new volcano happens to intersect a previous one.

The volcanoes in these systems tend to be alkali basalts that reflect small melt fractions in their mantle sources. Volcanoes in low-flux fields typically do not follow simple spatial-temporal trends, for example where ages decrease in a certain direction. Rather, a new volcano might erupt anywhere in the volcanic field, seemingly independently of the location of the preceding event. Some of these volcanic fields have very low eruptive fluxes; in the Southwest Nevada Volcanic Field (SNVF, western USA), the eruptive flux is currently about 0.5 km3/Ma (Valentine & Perry, 2007). To put this in perspective, Kilauea volcano has a magma flux approximately six orders of magnitude greater.

Although there are few well-documented low-flux volcanic fields, many of them do have one thing in common–the presence of some sort of regional tectonic deformation. Valentine & Perry (2007) suggested that small monogenetic volcanoes in the SNVF are not the result of an active melting process in the upper mantle, but instead represent focusing of patches of preexisting partial melt. It is noteworthy that in the SNVF, geochemical and isotopic data suggest the magma sources reside in the lithospheric mantle, which reaches depths of ~100 km locally. These sources represent compositional heterogeneities such as local enrichments in hydrous phases, which in turn depress the solidus temperature such that at the ambient pressure and temperature (~60-80 km depth) they have small melt fractions. Regional deformation focuses melt within these sources until the melt interconnectivity is sufficient to develop a buoyant pressure head that can initiate upward dike propagation. This deformation-driven melt focusing is the mechanism explored by Holtzman & Kohlstedt (2007) in the context of mid-ocean-ridge spreading centers.

In the conceptual model proposed by Valentine & Perry (2007), there is a close relationship between volcano characteristics on the surface, and their magma sources at depth. For example, in the SNVF the eruptive fissure lengths are related to the volumes of individual volcanoes, as are lava effusion rates that are inferred from field data. Each volcano is sourced by a separate patch of partial melt, with its own geochemical signature, in the upper mantle. The volume of melt available to ascend and erupt is proportional to the volume of the partial melt patch. The strike-length of dikes, and ultimately eruptive fissures (the intersection of dikes with Earth’s surface), that ascend from a partial melt patch is related to the length scale or “magmatic footprint” of the patch (Valentine & Perry, 2006).

Larger-volume patches produce longer dikes, which are reflected on the surface as longer fissures and larger eruptive volumes. Effusion rate is proportional to dike width cubed, which is proportional to dike length. Thus, longer dikes erupt larger volumes at faster rates during individual monogenetic events. Each volcano can be considered as a sample of a heterogeneity in the upper mantle (Figure 1A). At the scale of the volcanic field, the eruptive volume flux exhibits time-predictable behavior, i.e., the repose interval is proportional to the volume of the preceding eruption. This is consistent with a close relationship between regional deformation and volcanism.

 

Figure 1: Cartoons showing cross sections through the crust (line pattern) and upper mantle (white) under regional deformation. Shaded area in the mantle represents a hypothetical patch containing partial melt. During regional deformation, the relatively weak, partially molten patch takes up relatively more deformation than the surroundings, and melt is focused until it is sufficiently interconnected to trigger ascending dikes. (A) In the Southwest Nevada Volcanic Field, the lithosphere, including the magma sources in the lithospheric mantle, is subject to horizontal s3 with an orientation that varies little with depth, facilitating direct dike ascent to the surface. (B) In the NPO, flexure causes extension perpendicular to the flexure axis in the lower lithosphere, and perhaps partly in the upper asthenosphere, which is not locally clearly defined, where magmas are sourced, but ascending dikes stall at mid-lithosphere levels because s3 has a different orientation in the upper lithosphere. Stalled magmas fractionate and eventually trigger new dikes with the proper orientation to travel through the remaining lithosphere and erupt on the sea floor.

 

Valentine & Perry (2007) use the SNVF as an example of the very-low-eruptive-flux end-member of volcanic field behavior, where magmatism can only occur in the presence of tectonic deformation (tectonically controlled). They compare this with the high flux end-member where the thermal structure of the upper mantle and high rate of dike injection into the crust is proposed to overwhelm regional tectonics (magmatically controlled volcanism, e.g.,  the mantle plume hypothesis). The spectrum of behavior may control other factors such as the likelihood of ascending dikes being “captured” by pre-existing structures vs. making their own pathways. Understanding this has the potential to contribute to hazard assessment.

Valentine & Hirano (2010) extended this concept to explain recently discovered, young fields of small-volume alkali basalt volcanoes on the floor of the Northwest Pacific Ocean (NPO), a setting that has long been thought to be volcanically quiescent, as it contains the oldest ocean lithosphere on the planet. Those small volcanoes form as the Pacific lithosphere is flexed upward prior to downwarping where it is subducted beneath Japan. Geochemical data suggest that melts feeding NPO volcanism are related to a heterogeneous upper mantle with locally depressed solidus temperatures. Valentine & Hirano (2010) compare SNVF and NPO volcanism for a broad suite of characteristics and propose that in the latter case the regional deformation associated with flexure is responsible for focusing preexisting melts sufficiently to trigger upward dike injection. In contrast to the SNVF, where dikes essentially ascend uninterrupted through the crust, in the NPO ascending dikes stall at mid-lithosphere depths in response to rotation of the principal stresses. There, at depth, s3 is perpendicular to the flexure axis, while in the upper lithosphere s1 is perpendicular to the axis (Figure 1). Stalling results in fractionation, assimilation and incorporation of xenolithic material, which partly overprints the source geochemistry.

This conceptual model for NPO volcanism is proposed as an alternative to the “petit spot” interpretation by Hirano et al. (2006), which assumes that magmas rise passively through flexure-induced fractures that penetrate the entire lithosphere. That model overlooks the effect of vertical rotation of principal stresses in flexure, as well as the difficulty of generating open fractures extending to depths of ~80-90 km. Also, perhaps unintentionally, the term “petit spot” seems to imply upwelling (i.e., small hot spots or microplumes) as well as implying that NPO volcanism is a unique phenomenon, when in reality it is very similar to volcanic fields on continents and likely on the sea floor.

Shear focusing for mobilizing and collecting small melt fractions, together with mantle that varies on a scale of kilometers in composition and solidus temperature, is a reasonable alternative to mechanisms that invoke active melting, for low-eruptive-flux volcanic fields.

References

last updated 1st October, 2011
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