Geology of North America
Geology of North America is discussed in this second of a series of papers. They describe the North American Craton structural geology, genesis and mineralisation from the 60 to 300 km depth using detailed seismic tomography.
These seismic tomographic papers will change the fundamental way in which the detailed geology of North America is viewed and explained.
The North American Craton occupies the core of the North American continent. The underlying cratonic Lithosphere extends unbroken from Alaska to Greenland to the Southern United States of Texas and Louisiana. This has dimensions of 5000 km NS and average 3000 km EW. It is about 2 times the area of the Lunar mares.
The basis of this paper on the North America Craton is the application of broad scale seismic tomography in plan at 60, 100, 150, 200, 250 and 300 km depth. This paper focusses on the -60 km depth in detail. The other depths have been more coarsely enhanced.
A set of sections, EW sections A, B and C from Alaska, Newfoundland and New York latitudes and NS sections on 110°W (Arizona), 95°W (Gulf of Mexico) and 80°W (Florida) longitudes (from Clouzet 2018) have been coarsely enhanced to focus only on the major ring and other structural geology features.
Basis of this Research
A search of seismic tomography publications in North America suggests that current research only defines structures of > 50 – 100 km smallest dimension. The author’s structural enhancement method gives high resolution (to < 5 km width) structures that relate to published, well researched, mineralised surface structures. The same structures correlate between plans and sections.
High resolution structural seismic tomography (5-10 km wide dimensions) has not yet been used to determine north America’s detailed deep geological structure. The paucity in references to research showing detailed seismic tomographic North American geology is because there are no papers, in the available database searched, that covered the topic.
The brown/grey colour to the left is the Pacific to Rockies series of subduction zones. Interestingly there are a series of giant ring structures observed in this area.
60-300 km plan insets. These plans show the structural geology below the crust. There is an enormous ring structure Saskatchewan (Sa) evident at all depths. The Belcher (Be) giant ring structure seen on the surface in Hudson Bay only extends down to 150 km depth.
The only other giant ring structure evident at all depths is the Nevada (Nv) on the Pacific plate side of the Rocky Mountain Front. How do ring structures form in mobile subduction zones? This is the ring structure accompanying the Carlin gold district, one of the most prolific in North America.
The other ring structures start and stop at different levels. This is consistent with the ring structures in the Australian Craton (Watchorn AC #2 2019) and lunar craters where younger craters overprint and often half destroy the older deeper craters (Hiesinger et al, 2000).
Geoscientists have researched extensively to discern these cratonic structures, with some near-surface local success but the overall Cratonic control had not been discovered. Have we been examining them too locally? It is only when the structures are viewed from afar, as a complete structural set, that their pattern becomes clearer. The controlling tectonic system is starting to be outlined in 2.5 D using my new detailed tomography.
Seismic tomographic images of EW A-C and NS D-F sections are the author’s enhancement which reveals the broad cratonic structures. The top set are uninterpreted and the bottom interpreted.
At this broad scale only the outlines of major structures are seen and highlighted. Only the bowl structures (downward facing arcs) are interpreted not the numerous linear (fault) structures.
The Pacific Plate oceanic crust and subduction zone are also interpreted (B, C and D). The lineation in the Pacific plate is in top 80 km and the down warp as it approaches the continental craton is evident. This subduction zone extends to near the Rocky Mountain Front. The oceanic crust beneath this linear zone has numerous steep east-dipping and flat west-dipping linear structures. These are most likely associated with the original rifting of the Pacific plate.
In the southern third of sections E and F there is a strong under-pushing (for want of a better term) of the Caribbean plate under the craton in the southern half of the US.
Neither of these subductions have affected the giant ring structures either in plan or section. This suggests the North American craton, once formed, has remained rigid even through this recent, and ancient, very intense geological activity.
These ring structures (yellow circles) have the morphology of impacts. This Archaean (most probably Hadean >3.8 Ga) impact zone encompasses the central core of North America.
Only the bowl structures (downward facing arcs) are interpreted not the numerous linear (fault) structures. The Pacific Plate, oceanic crust and subduction zone are seen on the left side of sections A, B and C.
The NW, Sa, Ma, Hb, Qu and Ne giant ring structures correlate in both plan and section.
The bowl structures terminate at various horizons suggesting their formation at different crustal levels. This is similar to the Lunar impact structures seen on the lunar surface where new impact craters form on top of impact created lava fields (mares).
The seismic tomographic B-B’ section (along black line) shows the vertical Archaean geological structure and the probable geochronology of the geological layers in the crust and Lithosphere. The starred ring structures are the major ones extending through more levels of the crust.
The sectional geochronology has been estimated as follows;
The North American craton surface Archaean crustal age ranges from 2.6 to ~ 4.3 Ga. The oldest rocks found on Earth are 3.8 to 4.3-Ga green-stone beds found along the eastern shore of Hudson Bay in northern Quebec (Belcher ring structure) and 4.03 Ga zircons from rocks in the NW Territories (Beaufort Sea ring structure).
The craton includes recent rocks to 1 Ga. The deeply glacially eroded areas in the north are the oldest. This suggests, as do conventional geology principles, that even older lithologies lie underneath. All morphological evidence points to their formation by LHB impacts.
The deep structures associated with the margins of the giant ring structures may be transferred to the surface as Trans-Lithospherical structures (Hronsky 2013). Many of them line up with surface large Proterozoic structures.
The Midcontinent Rift system – Keweenawan Rift is closely associated with the Nebraska ring structure (Ne) in the south, the Lake Superior ring structure (Ls) in the northern domal area.
The Trans Hudson Orogeny is closely associated with the Dakota ring structure (Dk) in the south, the Saskatchewan (Sa) and Manitoba (Ma) ring structures in the centre, The Hudson Bay (Hb) and Belcher (Be) giant ring structures are associated with the east-centre. The NE of the structure in Hudson Bay is associated with the Coates Island (Co) giant ring structure.
Foxe magmatic arc is associated with the Foxe Basin (Fo) ring structure.
In the north west of Canada the Taltson magmatic arc is associated with the Alberta (Al) ring structure in the south and the North West (NW) ring structure in the north.
The Wopmay magmatic arc is associated with the North West (NW) ring structure in the south and the Beaufort Sea (Bf) ring structure in the north.
Conclusions.
Geology of North America formed by giant impacts during the Hadean eon Late Heavy bombardment. These impacts also influenced the later geology as there is a nearly 100% spatial and structural association of the surface Proterozoic magmatic arcs and rifts with the giant ring structures at the 60 km depth. This is also true in the Australian craton.
| NA #1 North American craton tomography. 60 to 300 km depth 1.pdf | 4 MB |