Compressional Pulses and Present Stresses

Compression in Non-orogenic Areas
It became apparent by 1997 that the present SHmax Direction (SHD) in Northwest Europe was perpendicular to basin-forming faults, indicating compression. These faults often had normal displacement and were particularly active during the Jurassic. Plate tectonics (PT) considers forces only in the horizontal plane and is therefore 2-Dimensional, that is, PT attributes the present North Sea ‘push’ to the Alps or North Atlantic Ridge compression. Both forces are northwesterly directed and, in particular, would have placed the North Sea Central Graben under extension thereby failing to account for the northeasterly directed compression today.



Similarly in Northwest Australia, measured SHD varies from east-west to southeast but also perpendicular to Permo-Jurassic crust cutting faults. The nearest plate tectonic forces are the Indian Ocean Ridge 4000km to the south and collision with Timor to the north. Both would generate north-south SHDs. None of these forces afforded an explanation for non-orogenic Recent compression in these and most basins.


The Earth is an oblate spheroid with the rotationally induced equatorial bulge some 21km greater than the polar radii of 6357km. In moving some 40° of latitude northwards during the last 50Ma, Australia has experienced this vertical force resulting in some 7-8km of flattening or curvature reduction concentrated at crust cutting faults.  These act as compressional hinges in the otherwise rigid crust. Simply due to the shape of the earth, this vertical force adds a third dimension to plate tectonics which in the southern hemisphere can in part account for today’s compression across crust cutting faults.


Northwest Europe, however, moving with a northwards component from the Equator would be expected to experience an increased curvature, probably with Recent extension across crust cutting faults. This is not the case, indicating another source for the non-orogenic compression.



In 1974-5 John K. Davidson conducted Exxon’s structural geology courses in North America from the Mexican border to southern Canadian Rockies. Since 1983 he has been conducting field-based structural geology courses, in-house and industry-wide, based in Bristol and centred on the excellent exposures of the former type sections of the Mesozoic to Recent, Bristol and English channel sections. The photograph on the course brochure shows north-south Early Jurassic folding followed by Early Jurassic normal faulting which indicates the Recent compression across Northwest European crust cutting faults may have had precursors.



Over the years it became apparent the exposures in Lulworth Cove and nearby seismic showed the famous Lulworth Crumple on the northern edge of the English Channel was more than a single ‘Alpine’ compression, but had experienced up to 9 north-directed compressions with intervening subsidence. The earlier eroded section in the Channel, south of the east-west crust cutting fault at the edge of the Channel (‘Purbeck Disturbance’), had a further 6 compressions since the Early Triassic.

Compressional Pulses with PT+
Concurrent, onlapping events on anticlinal features and inversions plus compressive features against normal faults were also documented in the area of the Carnarvon Sub-basins of Northwest Australia and elsewhere. In 1997 the evidence for 12 compressions in the Lulworth area was published in ‘Marine and Petroleum Geology’ and globally synchronous events postulated. Unfortunately the proposed mechanism of earth expansion, certainly on the scale proposed, was in error, however, there is increasing evidence for a slight up and down pulsed vertical force in sedimentary basins which appears to be synchronous. This led to the notion of compressional pulses and Plate Tectonics Plus (PT+), 2-Dimensional plate tectonics with the 3D pulsed and predictable vertical force.



Compressional Pulse Chart
The Compressional pulses described in the Lulworth Cove area are plotted on the above chart under ‘Europe, UK’. The compressional events of some of the other basins and areas displayed appear on the PSI website. Most of the seismic evidence for the onlaps and their ages are derived from the literature, indicating the synchronous distributions are highly unlikely to be due to chance. The observation of onlapping seismic sequences was developed by Pete Vail’s group at Exxon during the 1970s and the first order patterns (Vail, 1977) are also shown opposite. Vail claimed global synchroneity of the ‘eustatic cycles’ and attributed their occurrence primarily to glacial control. PSI has observed the same onlaps occur on compressional structures, and there may be other factors such as ridge spreading rates, both of which could have pulsed controls over the established eustasy. The observation of compressional pulses adds the fourth dimension,Time, to the pulsed vertical force in PT+.





Orogenic Pulses
In orogenic areas and particularly at their boundaries, post Mid Eocene compressional pulses (above) can be seismically identified as components of the orogen which themselves form cyclic or pulsed events of much longer duration.

The Eocene to Recent Alpine orogen (after Stocklin) is shown coloured yellow. Note the black shapes within the narrow Indus Suture which represent Triassic-Cretaceous, pre northward Indian continental drift, Tethyan ocean floor relics. This ocean was consumed under Tibet by Cretacous drift, leaving a new Tethys, the present Indian Ocean. The map of Southeast Asia exhibits two major horizontal forces, one to the north-northeast from the Indian Ocean and the other to the west-northwest from the Pacific Ocean, also coloured yellow over the West Pacific arcs. This confluence of stresses produces a large orogenic ‘T’ intersection.


To the north are pink areas of the Triassic Indosinian orogen containing pre-Triassic Tethyan ophiolite bearing sutures. Likewise the extensive, dominantly Devonian ophiolites are embedded in the Hercynian Mid to Late Carboniferous, grey orogenic areas. An earlier phase of closing early Paleozoic oceanic basins is evidenced in the green Caledonian Silurian orogenic areas. These opening and closing of oceans can be interpreted to be caused by the pulsed, relatively north movement of successive Gondwanan continental blocks with a trailing new Tethys which become younger to the south. The present Tethys is the Indian Ocean which is showing evidence of Pliocene to Recent compression at the Kormarine Ridges south of Sri Lanka.

The pulse chart also shows three of the orogenic cycles, Hercynian, Indosinian and Alpine adjacent to the PSI compressional pulses to the Carboniferous and magnetic reversals to the Triassic. Vine and Mathews (1962) used magnetic reversals across the East Pacific Ridge to ‘prove’ continental drift, 50 years after Wegener proposed it. Magnetic reversals have a period of 1-25Ma and average 1.76Ma (34 in 65 Ma) during the Tertiary and 2.76 (90 in 250Ma) since the Early Triassic. Basin compressional pulses have a period of 5-25Ma and average 10Ma (32 in 350Ma) from the Carboniferous to Recent. Orogenic cycles have a period 100-200Ma and average 110Ma for the Silurian-Recent. Physicists can induce the magnetism and reversals by modeling differential rotation at the core/mantle boundary. If magnetic reversals have such an origin, friction-increased temperature may create the slight, present Earth radius expansion of 0.2mm per annum (Shen et al.) which would amount to 1.2km of flattening or curvature reduction over the Pliocene-Recent (5.8Ma) compressional pulse. This is a 0.02% increase over the 6357km Earth polar radius, preceded in the Late Miocene (and to be followed by) reversal or contraction by the same amount, to the mean radius.


As shown above, the rigid crust of Australia has experienced the vertical force resulting in some 7-8km flattening or curvature reduction over the past 50Ma. This amounts to an average or 1.1-1.3km per 6Ma, approximately that of the Pliocene to Recent compressional pulse. This extra compression has been mapped at crust cutting faults in several basins in Australia using PSI’s stress from seismic technique.


Stress Application
In sedimentary basins, SHD is perpendicular to crustcutting faults and therefore constant during each compressional pulse provided the strike of the fault remains unchanged. As the earth is experiencing a compressional pulse derived during the Pliocene to Recent, SHD is the same as derived from stress isochores of earlier compressional pulses. If the magnitude of each of the compressional pulse stresses has remained similar, SH Magnitude (SHM=SH/SV) is likewise the same today as the distance from the stress isochore from the rigid basement has not changed. In terms of stress from refection seismic, the past is the key to the present. (The method and the software covering it are patented in several countries).




The green, grey, pink and yellow colours extending southeastwards from China to present West Pacific arcs is recording similar orogenic pulsing in the Pacific. This means the orogenic ‘T’ junction has had a pulsed history over the last half a billion years and the current SHDs have been essentially repeated four times in the Tethys-Pacific areas; a primary repeated feature of the Earth.









Pulsed Stress Mechanism vertical_link


Davidson, John K.(1997) Synchronous compressional pulses. Marine and Petroleum Geology, Vol. 14, No. 5,pp. 513-549.

Reynolds, S. D., Coblentz, D. D. and Hillis, R. R. (2003) Influences of plate tectonic boundary forces on the regional intraplate stress field of contienetal Australia, Geol. Soc. Australia Spec. Pub. 22 and Geol. Soc. America, Spec. Pap. 372, pp. 59-70.

Shen, W.-B., Sun, R., Chen W., Zhang, Z, Li, J., Han, J. and Ding, H. (2011) The expanding earth at present: evidence from temporal gravity field and space geodetic data. Annals of Geophysics, Vol 54, 4, pp 436-453.

Stocklin, J. (1983) Himalayan orogeny and Earth expansion. In Expanding Earth symposium, Sydney, University of Tasmania.

Vail, P. R. et al. (1977) Seismic stratigraphy and global changes of sea level. In Seismic stratigraphy-applicationsto hydrocarbon exploration: AAPG Memoir 26, pp. 49-212.