Phanerozoic Climate and Vertical Tectonic Cycles

Evidence of kilometre scale uplift and subsidence at locations remote from any recognised plate boundaries, the existence of mega-sequences of post-rift marine sediments over widespread intra-cratonic areas, and the consideration that pulses of deposition display a clear periodicity and synchronicity over widely dispersed spatial domains, remain largely unresolved issues within current geological theory. While the exact timing of uplift and erosion associated with major unconformities are difficult to assess, the age of sediments immediately above provide vital temporal markers for the onset of subsidence and associated sea level rise. By reconsidering the much studied sedimentary sequences of the Grand and Bryce Canyon areas the following will show that the at least over the Phanerozoic eon the initiation of new pulses of deposition occur at times when earth climate is emerging from ice-house to hot-house conditions. Furthermore, the recorded periods in which global occurrences of epeirogeny have occurred will be shown to correlate closely with the end of hot-house periods and the onset of ice-house global climate conditions. Finally, some tentative thermo-geodynamic explanations for this apparent causal link between global climate and vertical tectonics will be suggested.


BACKGROUND
A recent meeting to celebrate 50 years of plate tectonic achievement 1 was challenged to provide plausible explanations for the largely unresolved observations of kilometre scale, cyclic, rises and falls of continental and oceanic crust 2 . Furthermore, this meeting was reminded that these kilometre scale cycles of burial and exhumation exhibit synchronicity over wide spatial domains and often occur in regions where plate tectonic models suggest passive tectonic activity 3 . That such motions occur and so often result in major unconformities in the sedimentary sequences were the cornerstones of James Hutton's awakening of the field of geology 4 in the closing decades of the 18 th C. And yet even with the undoubted advances in the field of geology over the past 50 years there still appears to be no satisfactory explanation for these processes. The following will reappraise the sedimentary records exposed within the Grand Canyon which, as Sloss 5,6,7 repeatedly pointed out, display clear cycles of deposition and non-deposition reflecting kilometre scale cycles of crustal rise and fall; these cycles of deposition and non-deposition and their associated vertical motions still explanation. This reappraisal will demonstrate that phasing of the cycles of subsidence and upheaval, burial and exhumation, have a strong correlation with the ice-and hot-house cycles of climate that have occurred at periodicities of circa 130 Ma over at least the Phanerozoic (-540 Ma to present) 8,9 . Because any sedimentary records within the Grand Canyon 10,11 have been lost from the early Mesozoic (-270 Ma to present) the evidence from the contiguous region of the Grand Staircase leading up to Bryce Canyon will be used to complete the analysis up to at least -40 Ma 12 . Figure 1 provides a commonly accepted summary of the sedimentary sequences exposed within the Grand Canyon 10,11 . As observed by Sloss 5,6,7 and others these sequences exhibit a number of unconformities at which substantial time gaps exist between adjacent sedimentary layers. For example, the youngest sediments remaining within the "Grand Canyon Supergroup" date from -740 Ma. At some time between -740 Ma and -525 Ma this Supergroup was so deeply buried beneath megasequences of sediments that a combination of extreme heat and associated horizontal stress caused the sediments to buckle and fracture into the angular distortions now evident in the exposures at the base of the Grand Canyon. What cannot be surmised from the extant rocks is whether other cycles of subsidence, sedimentation, uplift and erosion occurred between -740 Ma and -525 Ma. But what is certain is that at some time prior to -525 Ma this region experienced uplift and erosion back to an essentially horizontal, peneplain, surface before once again subsiding beneath sea level to have yet another sequence referred to as the Tonto Group laid down. Deposition of the Tonto Group commenced at -525 Ma and continued until at least -505 Ma and some unknown time prior to -385 Ma before another regional uplift and subaerial erosion resulted in the formation of a further peneplain with youngest exposed sediments of -505 Ma. At some time prior to -385 Ma the region underwent a further subsidence to beneath sea level to commence deposition of the Grand Stair Group which with continuing subsidence lasted until at least -270 Ma and possibly some unknown time prior to -265 Ma before again experiencing regional uplift and possibly erosion. These cycles of subsidence, sedimentation, uplift and erosion are summarised in Fig A.1 to A.3 of Appendix A.

Bryce Canyon Pulses of Sedimentation
Outcrops of sediments younger than -270 Ma have been largely eroded from the area of the Grand Canyon but fortunately are still abundantly exposed in the adjacent Grand Staircase leading up to   the close, and possibly causal 8,9 , relationships between the intensity of cosmic rays and climate cycles, and; the strong correlation between the onset of deposition as recorded by strata immediately above recorded major unconformities and their consistent phasing within the climate cycles. In each case, deposition is seen to commence shortly after earth climate emerges from an ice-house period, shown by one of the upper bars in Fig 3, and enters into a period of hot house. After a long period of glacial and inter-glacial cycles during the ice-house period, it might be anticipated that ice erosion will have reduced continental land surface elevations in the vicinity of ice sheets to near sea level. This means that moderate rises in average sea levels, due to the melting of ice sheet and permafrost accompanying the transition from average cold climate to hot-house, might be expected to inundate the low continental land surfaces -a clear precondition for the onset of marine sedimentation.

RELATING PHASES OF DEPOSITION TO CLIMATE CYCLES
A possible explanation as to how the transition from ice-house to hot-house conditions could trigger an extended period of sedimentation has focused on the adjustments to the geothermal flux occurring when low lying continental crust is inundated by rising sea levels as global ice sheets and extensive permafrost melts 13,14 . This is summarised in Fig B.1 of Appendix B. Increases in the geothermal heat flux caused by the greater heat transfer capacities of sea water, enhanced by mixing due to tides and currents, would over time result in steeper geothermal gradients and a concomitant decrease in crustal thickness brought about by phase change, re-magmafication, at the lower lithosphere-mantle boundary. This crustal thinning will result in an increase in average density within the lithosphere and upper mantle which in turn would be expected to result in regional subsidence of the crust. Such a model, involving as it does wasting of crust at the lower lithospheremantle boundary, starts to account for how km scale sedimentary sequences can be continuously added from above. As suggested, this requires maintenance of a consistent equilibrium thermal gradient even as sediments are added -a process much more likely given the relatively thin nature of oceanic crust.

RELATING EPEIROGENY TO CLIMATE CYCLES
Previous studies have noted the close link between periods of mountain building with climate cycles 14,15,16 . Recorded periods of mountain building 13 are summarised by the bar charts at the top of Fig 3. The 4 recorded periods of widespread mountain building during the Phanerozoic, often occurring synchronously over widely dispersed geographical domains, are shown to correspond closely with the periods of ice-house climate conditions. Tentative explanations for how ice-house conditions could result in spurts of uplift required for mountain building and subsequent erosion have again focused on how changes in surface disposition of ice and water could influence the rate of geothermal heat loss as expressed by the geothermal flux 13,14 . Reductions in the geothermal heat flux caused by the insulating effects of the development of deep surface ice sheets and permafrost would result in a lowering of the geothermal gradient and a concomitant increase in crustal thickness brought about by aggradation, caused by phase change, at the lower lithosphere-mantle boundary. Associated reductions in average density within the lithosphere and mantle could then be expected to result in regional uplift of the crust. This is summarised in Fig B.2 of Appendix B.

DISCUSSION
This brief note has concentrated on the sedimentary sequences of the Grand and Bryce Canyon areas simply because they are so clearly exposed and thoroughly researched. Furthermore, as the stratigraphic geometry shown in Fig 2 explicitly demonstrates, the Grand Canyon area in particular has recently, and certainly post breakup, been subject to major uplift and ice erosion. Earlier sequence stratigraphy suggests that this area has been subject to repeated cycles of ice-and hothouse climate cycles and consequently the direct correlation of tectonic activity and climate. But clearly the massive erosion occurring during the ice-house phases must produce in contiguous areas equally massive accumulations of ice erosion deposits, suggesting a counter-phasing of deposition in areas contiguous with those directly affected by the growth of ice-sheets -and of course vice versa during the erosion of possibly uplifted areas during hot-house climatic conditions.
From the direct evidence of the Grand Canyon, along with the reported synchronicity over cratonwide and even continent wide areas noted by Sloss and others, there appears to be strong evidence of a causal link between cycles of uplift and erosion during ice-house global climate, and subsidence and deposition during hot-house climatic conditions. It is therefore surprising that the thermogeodynamics underpinning these links have not been more thoroughly investigated. If as suggested there is a causal link between at least vertical tectonics and global climate cycles this could significantly add to our ability to explain other important aspects of global tectonics. While current mobilist concepts seek to explain evidence of changing climatic domains within the sedimentary record through latitudinal continental movements, to largely conform with contemporary global climate, it is possible that these same sedimentary records could equally well be explained by a fixist view of contemporary continental locations subject to the long term climate cycles discussed aboveor perhaps a bit of both.

CONCLUDING REMARKS
A reconstruction of the sedimentation records of the Grand Canyon region reveals a strong correlation between the timing of the recommencement of deposition cycles above major unconformities and the transition from global ice-house climates to hot-house. This has been explained in terms of sea level rises accompanying the melting of surface ice. Ocean currents and tidal flows provide a more effective heat transfer mechanism for the outflow of geothermal heat energy. An adjusted geothermal gradient resulting from the increased geothermal heat flux has the effect of thinning the crust, through a process of phase change, re-magmafication, at the lower crust boundary. Increased average density of the crustal column will then result in continuing subsidence as sediment is added from above and lower crust removed. This circa 130 Ma cycle of pulses in sedimentation in phase with the onset of hot-house climate conditions appear to be related to similarly timed pulses of uplift and mountain building observed to coincide with the onset of icehouse periods.
With hot-and ice-house conditions known to have had global reach starts to explain the often noted synchronicity of burial and exhumation events over widely dispersed geographic domains and often remote from any active tectonic zones. Furthermore, high levels of temperature change at a given crustal depth, associated with the periodic thickening and thinning of crust, will induce massive cycles of horizontal thermal strain and deformations. Restraint, of these thermally induced strains could in turn account for many of the observed fracture patterns during tension cycles, or folding and metamorphism during compression cycles.