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Decompressional Melting During Extension of Continental Lithosphere

Jolante van Wijk

Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0225, USA.

jvanwijk@ucsd.edu

Lithosphere Architecture as a Controlling Factor of the Rifting Process

It is known that continental rifting tends to follow weak zones in the lithosphere, such as orogenic belts, while stronger cratonic regions are usually not significantly deformed. For example, in the northern North Atlantic, rifting occurred along the Caledonian belt, and in eastern Africa, the East African Rift Zone is located within a Late Proterozoic belt. My colleagues and I suggest that these old structures not only influence the location of rifting, but that the inherited lithosphere architecture plays a role in the rifting process itself, the thermal evolution of the rift, and decompressional melting during rifting. The amount, timing and distribution of decompressional melting for example, could be controlled by factors such as pre-rift crustal or lithosphere thickness, orientation of inherited structures, and lithosphere composition. A mantle plume is not a pre-requisite to form a volcanic province. Dynamic processes related to lithosphere extension can explain the sometimes enigmatic amounts of melt observed in intra-continental rifts and volcanic passive margins.

However, when studying continental rifts, it is difficult (or impossible) to establish the pre-rift situation. It is not the aim of our studies to “reconstruct” the pre-rift situation. Instead, we investigate how small variations in lithosphere structure that may be present at the onset of rifting influence the system behavior.

To study the role played by lithosphere architecture in the rifting process, we use two-dimensional and three-dimensional numerical models. The models are designed to study lithosphere deformation, and detailed descriptions can be found in Van Wijk & Cloetingh (2002) (2-D model) and Van Wijk & Blackman (2005) (3-D model). The models differ from previous models that were used to study decompressional partial melting during rifting (e.g., Pedersen & Ro, 1992) and as a consequence, the results predicted differ. One of the most interesting results that are different is that elevated mantle temperatures, e.g., as are required by mantle plumes, are not needed to explain volcanic margin formation in our models.

2-D Modeling Studies: Focusing on Passive Rifted Margins

In these studies the model domain represents a vertical section through the lithosphere.

When the lithosphere is extended, the crust starts to thin and warm mantle material wells up (Figure 1). In the head of the upwelling mantle material decompressional melting occurs. Most melt is generated just before continental breakup, in a ~175 km wide zone, typically between about 20 and 50 km deep.

Figure 1. A) This figure (Van Wijk et al., 2001) shows the evolution of tensile stresses (upper panels) and temperature (lower panels) in the lithosphere during extension. Times are in My after stretching started. B) This figure (Van Wijk et al., 2001) shows the evolution of crustal and mantle thinning during passive margin formation, the timing of melt generation (indicated by the yellow dashed lines), and amount of melt predicted. Models predict that volcanic margins can form without elevated mantle temperatures.

One of the factors tested is the thickness of the crust and the topography of the Moho at the onset of rifting. We find (Figure 2) that for a fixed extension rate, the pre-rift lithosphere configuration influences rift duration until continental breakup occurs, melt production, and width and symmetry or asymmetry of the continental margin pair. Numerical experiments show that pre-rift lithosphere structure alone can determine whether a margin pair will be volcanic or non-volcanic (Table 1).

Figure 2. This figure (Corti et al., 2003) shows how the thermal evolution of a rift basin may vary, depending on Moho topography and thickness of the crust at the onset of rifting. A) Initial Moho configurations for tests A-F. The Moho tested varied in width of depression, depth, and steepness of Moho topography. B) Thermal evolution of lithosphere during extension. The black line in each panel shows the Moho. C) Final stage: Moho depths of conjugate rifted margins predicted by the model.

 

Table 1. This table shows which of the rifted margins resulting from the tests shown in Figure 2 are volcanic and which are non-volcanic. The only parameter varying in the experiments is the thickness of the crust at the onset of extension.

Experiment

Volcanic or non-volcanic margin?

A

Volcanic

B

Volcanic

C

Non-Volcanic

D

Volcanic

E

Non-volcanic

F

Volcanic

3-D Modeling Studies: Focusing on Intra-continental Igneous Provinces

Recently we have begun to study continental lithosphere extension with a 3-D numerical model (Van Wijk & Blackman, 2005). Initial modeling efforts show the importance of lithosphere architecture in the rifting process. The location and rift-axis orientation of individual continental rift basins for example (Figure 3, and Van Wijk, 2005) are controlled by inherited lithosphere structure. We find also that the thermal evolution and structure of the basin (alternating asymmetric for example), how rift zones and loci of melt production propagate (Van Wijk & Blackman, 2005), and mantle flow beneath rifts (Van Wijk & Blackman, 2005) are influenced by lithosphere architecture. Next we will apply such models to study intra-continental hotspots (such as the Yellowstone hotspot) and volcanic provinces.

A focus of these future studies will be whether regional extensional tectonics might explain the formation of, for example, the Yellowstone hotspot. Continental extension is a 3-D process, and we have only just begun to study lithosphere extension from a 3-D point of view.

Figure 3. This figure (Van Wijk, 2005) shows the location and orientation of individual rift basins that form where a weak structure in the lithosphere is oriented oblique with respect to far-field plate forces (panels labeled M2 and M3). The figure shows crustal thinning factors, and boundaries of the weak structure are super-positioned (white dashed lines). Crustal thinnings and upwellings of warm mantle material are localized in regions that follow the inherited lithosphere structure as a whole, but cross it individually.

Formation of the Vøring Margin (a Volcanic Passive Margin)

We suggest that volcanic rifted margins, such as for example the mid-Norwegian Vøring margin, are a consequence of plate tectonics, formed without the influence of deep mantle plumes, and that characteristic features of these margins such as underplated or lower crustal bodies and pre-breakup reduced subsidence (or uplift), can be explained in an alternative way, i.e., not in terms of mantle-plume influences. The vicinity of the Iceland hotspot (Figure 4) has led to many discussions on its role in the formation of the Vøring margin, but a closer look at seismic data (Gernigon et al., 2003; 2004), and recent modeling results (Van Wijk et al., 2004), provide an alternative explanation for the typical volcanic margin features (see also pages on Iceland and the Norwegian volcanic margin).

Gernigon et al. (2003; 2004; and VM_Norway.html) suggest that the continental part of the lower crustal body is not of magmatic origin (as has often been stated previously) and was not formed during or just prior to the breakup event as a consequence of the influence of the Iceland hotspot. The melt volumes observed near the continent-ocean boundary can be explained by plate tectonic processes alone, and do not require elevated mantle temperatures (Van Wijk et al., 2004). Uplift of the Marginal High was previously explained by arrival of the Iceland hotspot. However, modeling results (Van Wijk et al., 2004) show that uplift of the Marginal High can be explained by dynamic processes in the lithosphere/upper mantle, as a direct result of fossil imprints in the lithosphere from prior geological events.

Figure 4. Locations of the mid-Norwegian volcanic margin and the Iceland hotspot. Many characteristic features of the Vøring margin have been previously explained in terms of hotspot influences.

Summary

Initial 2-D and 3-D model experiments show that continental rifting and decompressional melting are strongly influenced by inherited architecture of the lithosphere. For example, pre-rift structure alone can determine whether passive rifted margins will be volcanic or non-volcanic, where rift basins localize, what their orientation is, whether and how they propagate (and how melt loci propagate), the direction of mantle flow below the rift basin and the amount and distribution of decompressional melting etc. Next we aim to use these insights to study intra-continental hotspot tracks and magmatic provinces to test whether those tectonic features can also be explained in terms of plate tectonics.

References

last updated 29th March, 2005
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