In the last few decades there have been several massive programmes aimed at imaging the lithospheric structure beneath continents, often linked with a re-assessment of the various tectonic provinces thought to be present. One of the first was a joint Indian-Soviet project managed by the National Geophysical research Institute in Hyderabad to investigate the crust of South India in the 1970s, which still graces my office wall as a memento of my own contribution to unravelling the underpinnings of this ravishing area. This was followed-up by one from the Himalaya southwards, and others have focused on Britain, the Baltic Shield and the USA by the Consortium for Continental Reflection Profiling (COCORP); the last revealing in detail large-scale, low-angle thrust faulting in the Appalachians and crustal-scale detachment faults in the eastern Basin and Range. These experiments must have been great fun, as they involved detonating large amounts of high explosive to produce sufficient energy to get returns from 100 r more km below, with all the planning needed to avoid fear and loathing among the populace, let alone frightening the horses. Nowadays, most seismic profiling onshore is done using Vibroseis, best imagined as large trucks jacked up on pads on which they bounce up and down, in manner of an LA ‘lowrider’. By comparison, marine surveys are far easier, although marine mammals have seemingly had major setbacks as a result of endless closely spaced seismic lines needed for 3-D subsurface analysis. Onshore, you only get one chance and need to pick your route with great care. Now a Canadian consortium has gone one better by using state-of-the-art seismic refraction and reflection techniques (Hammer, P.T.C. et al. 2011. The big picture: A lithospheric cross section of the North American continent. GSA Today, v. 21 (June 2011 issue), p. 4-9). Uniquely, the Canadian Lithoprobe project coordinated a full spectrum of geological, geochemical, and geophysical research, covering 20 years of deep-crustal research by hundreds of contributors.
A large-format profile in a supplement to the paper shows the deep relationships in the Mesozoic Cordilleran Orogen in the west, through the plexus of Precambrian Provinces of the Canadian Shield to the Palaeozoic Orogen in the east: a tract some 6000 km from west to east. The general picture is repeated stacking of orogens, with a remarkable repetition of very similar gross tectonic styles. Clearly, large-scale compressional processes have remained largely unchanged since the middle of the Archaean, and several upper parts of long-dead subduction zones and accretionary duplexes spring from the profile. The surface picture of much of the crust crossed by the stitched-together traverses gives the impression of both complex tectonics and many plutons of different ages, yet on the grand scale of the crust and lithosphere it is the tectonics that dominates: the passage of voluminous melts towards the surface has left the plethora of gently dipping deep shear zones and faults largely unmodified. Indeed, the seismic data reveal astonishingly well-preserved subducted or delaminated crust associated with collisions that occurred 2-3 billion years ago. Despite repeated accretionary tectonics spanning 3 Ga, and the Phanerozoic erosion of the Shield to reveal its innermost and deepest secrets, the crust-mantle boundary, the Moho, is astonishingly flat, ranging from 33-43 km deep. Nor is there much sign of ‘roots’ beneath orogens in the underlying lithospheric mantle; a long standing concept that appears not to be generally supportable over this stretch of the North American continent. The synthesis raises questions as to whether the Moho has always been that shallow or whether it can, in some situations, be a dynamic ‘boundary’. For that to be the case requires that the geologic crust-mantle boundary may not always correspond to the seismic discontinuity with which the Moho has previously been correlated.
PDFs of the profile can be downloaded from ftp://rock.geosociety.org/pub/GSAToday/1106insert-hammer/