Stress Induced Hotspot Seamount Formation at Ridge-Transform Intersections

Erin K Beutel

Department of Geology and Environmental Geosciences, College of Charleston, Charleston, SC 29424
beutele@cofc.edu

Contrary to our current understanding of ridge-hotspot interactions, several papers have noted small seamounts with hotspot geochemical signatures located at or near ridge-transform-intersections (RTIs) rather than at the center of ridge segments (Graham et al., 1996; Hekinian et al., 1999; Johnson et al., 2000; Klingelhofer et al., 2001).  Accounting for the presence of hotspot seamounts at ridge-transform intersections requires that 1) magma must be diverted or funneled to the ridge-transform intersection rather than the ridge center, and 2) this process must be governed by the lithosphere, as magma movement at the ridge appears to be governed by the lithosphere. 

The existence of seamounts with hotspot geochemical signatures (hereafter simply called hotspots with no implied inference to their origin) in the center of ridges is relatively well understood.  In the case of a plume origin for hotspots, ridges are believed to focus plumes: a) because hotspots are entrained in the mantle upwelling that feeds the ridges, b) because they are weak areas within the crust, and c) because there exists a physical and thermal gradient that effectively funnels the material to the ridge like a sink to a drain (Figure1) (Kincaid et al., 1996; Kincaid et al., 1995; Sleep, 1990). Once at the ridge, hotspots add magma to the central area of upwelling, i.e. the center of the ridge segment (Figure 1). In the case of a heterogeneous mantle origin for hotspots, ridges are believed to intersect areas of fertile mantle, which results in rapid decompression melting and seamount formation (Anderson, 1992).  Neither theory offers a clear explanation for the presence of hotpots at ridge-transform-intersections.

Based on the above physical constraints on magma movement at the ridge, excess magmatism at the ridge-transform-intersection can only occur if the physiographic and thermal gradients that funnel magma to the ridge center are either overcome or a local upwelling is developed.  Changing the stress state of the lithosphere and creating areas of local decompression melting due to large-scale extension may cause local upwellings and/or change the physiographic gradient along the ridge.  Previous work (Beutel & Okal, in press) suggests that large extensional stresses are concentrated at ridge-transform intersections during times of impeded slip along transforms.  To test this, and to examine the three-dimensional stresses associated with impeded transform slip, three-dimensional finite element models of a series of ridge segments were constructed.  

The elastic finite element program FElt by Gobat & Atkinson (1996) was used to construct a box model of 3 ridge segments 1200-1400 km long separated by transforms 600-800 km long.  The ridges are modeled as weak areas in a relatively strong 100 km thick lithosphere that is underlain by a 100 km weak low-velocity zone and then 400 km of mantle.  An exponentially decreasing, gravitationally driven, ridge-push force based on lithospheric age was applied from the each ridge segment to the edge of the model.  A variety of scenarios involving different transform strengths (i.e. ease of slip) and force ratios were run to determine what effect changing transform strength had on the stress field at the ridge-transform intersection.  In all cases, increasing the resistance to slip along the transform resulted in increased extensional stress at the RTI in the lithosphere, and a change in the stress-state in the mantle (Figure 2).

Mapview of X-Y stresses in the lithosphere

Figure 2: Mapview results, 33 km Depth: Results of ridge-push forces exerted on a 3D box with 3 ridges and 2 transforms. The model is fixed at the base of the box, 8 units (~800 km) below the surface. The background colors indicate the type and intensity of the maximum stress as indicated on the scale. The bars are maximum and minimum stress vectors, white indicates compression and black indicates extension. White arrows indicate zones of extension at ridge-transform-intersections.
A: Transforms in this model are the 3 orders of magnitude weaker than the surrounding lithosphere. Note the concentration of extensional stress at the ridge-transform intersection.
B: Transforms in this model are the same strength as the surrounding lithosphere. Note the very large concentrations of extensional stress at the ridge-transform intersections compared to the model with a weak transform.

While ridge propagation may result from the increased extensional stress at the ridge-transform intersection it is also feasible to develop a number of scenarios whereby excessive extensional stresses at an RTI would result in the formation of a hotspot volcano:

In the case of a heterogeneous mantle, decompression melting of fertile mantle would result in an excess of melt with a hotspot signature.  Decompression of the mantle and the formation of a melt would also a change the thermal and physical gradient at the ridge.  Crust at the transforms tends to be thinner than at the center of the ridge and the presence of melt would increase the positive buoyancy of the crust.  This change in the physical, and possibly thermal gradient, could draw plumes from the center of the ridge to the ridge-transform-intersection (Figure 3).  The formation of the hotspot volcano may then cause the transform to become unlocked, thereby preventing ridge-propagation.

Figure 3: Along ridge cross-section. Upper cartoon shows steady a state-ridge with a weak transform and the attendant melt directed towards the ridge center. Middle cartoon shows thinned lithosphere and extensional stress, resulting from a locked transform, over a fertile magma body which leads to the formation of a hotspot volcano. Bottom cartoon shows a change in the physiographic gradient resulting from decompression melting at a ridge-transform intersection due to impeded slip along the transform.

Conclusions

The presence of seamounts with hotspot geochemical signatures at ridge-transform-intersections may be caused by extensional stress concentrations.  Extensional stress is concentrated at ridge-transform intersections when slip along the transform in impeded. Therefore hotspot locations may be guided by lithospheric processes and stress-states.  If a changing lithospheric stress-state can affect the location of hotspot seamounts at the ridge, then it is important consider the effect of lithospheric stress on all hotspots.

References