Mantle plumes, triple junctions and transforms: A reinterpretation of Pacific
Cretaceous – Tertiary LIPs and the Laramide connection
-
Abstract
The Shatsky and Hess Rises, the Mid-Pacific Mountains and the Line Islands large igneous provinces (LIPs) present
different challenges to conventional plume models. Resolving the genesis of these LIPs is important not only for a
more complete understanding of mantle plumes and plume-generated magmatism, but also for establishing the
role of subducted LIP conjugates in the evolution of the Laramide orogeny and other circum-Pacific orogenic
events, which are related to the development of large porphyry systems. Given past difficulties in developing
consistent geodynamic models for these LIPs, it is useful to consider whether viable alternative geodynamic
scenarios may be provided by recent concepts such as melt channel networks and channel-associated lineaments,
along with the “two mode” model of melt generation, where a deeply-sourced channel network is superimposed
on the plume, evolving and adapting over millions of years. A plume may also interact with transform faults in
close proximity to a mid ocean ridge, with the resultant bathymetric character strongly affected by the relative age
difference of lithosphere across the fault.
Our results suggest that the new two-mode melt models resolve key persistent issues associated with the
Shatsky Rise and other LIPs and provide evidence for the existence of a conduit system within plumes that feed
deeply-sourced material to the plume head, with flow maintained over considerable distances. The conduit system
eventually breaks down during plume – ridge separation and may do so prior to the plume head being freed from
the triple junction or spreading ridge. There is evidence for not only plume head capture by a triple junction but
also for substantial deformation of the plume stem as the distance between the stem and anchored plume head
increases. The evidence suggests that young transforms can serve as pathways for plume material migration, at
least in certain plume head – transform configurations. A fortuitous similarity between the path of the Shatsky and
Sio plumes, with respect to young spreading ridges and transforms, helps to clarify previously problematic
bathymetric features that were not readily ascribed to fixed plumes alone. The Line Island Chain, which has been
the subject of a vast number of models, is related mainly to several plumes that passed beneath the same region of
oceanic crust, a relatively rare event that has resulted in LIP formation rather than a regular seamount track.
Our findings have important implications for the timing and mechanism for the Laramide Orogeny in North
America, demonstrating that the Hess Rise conjugate may be much smaller than traditionally thought. The Mid
Pacific Mountains conjugate may not exist at all, given large parts of these LIPs were formed at an ‘off-ridge’ site.
This needs to be taken into account while considering the effects of conjugate collision on mineralization and
orogenic events.
-
-