Episodic kilometre-scale burial and exhumation and the importance of missing section

The incompleteness of the stratigraphical record is well documented, and the concept of missing time has attracted renewed attention in recent years. However, the importance of missing section, comprising rock sequences that were deposited and removed within time intervals for which no rock is now preserved, is not so widely appreciated. We focus here on studies showing that in diverse regions, many of which are regarded as free of tectonic influences over intervals of 10s to 100 s of millions of years, kilometre-scale thicknesses of rock have been deposited and subsequently removed, typically within a few tens of Myr, leaving little or no trace of their former presence in the preserved rock record. Much of the evidence for these episodes comes from low tem-perature thermochronology (LTT) studies of basement regions including passive continental margins, ancient orogens and cratons. Such studies have commonly been carried out within a framework of continuous slow cooling, on the assumption that denudation of these supposedly tectonically stable regions is mainly a response to changes in climate and isostatic response to erosion. In many regions, integration of LTT results with geological evidence, principally the presence of remnants of sedimentary cover and erosion surfaces, shows that histories involving long-term slow cooling/denudation are not appropriate. Instead, histories involving repeated episodes of burial and exhumation/ heating and cooling, appear to be common in these supposedly tectonically inactive regions. During these episodes, thicknesses of several kilometres may be deposited and subsequently removed, returning the underlying rock to the surface or close to it. These events affect areas of many 1000s of km2, and may affect neighbouring regions on a continental scale more or less synchronously. They may affect both basement regions and adjacent sedimentary basins. The resulting unconformities, including erosion surfaces in basement, are often very low angle in nature, reflecting the spatial scale of the affected regions, and are often mistakenly regarded as representing periods of stability. Apparent long-term preservation of elevated topography and preservation of exposed ancient landscapes have been regarded by many as enigmatic. However, both can be understood in terms of vertical motions similar to those revealed from LTT. A number of studies point to the conclusions that regions of supposedly long-lived elevated topography have been uplifted relatively recently and ancient surfaces have been preserved by burial and uncovered as a result of relatively recent exhumation. The concept of km-scale missing section has been regarded with scepticism by many geologists, on the basis that there is no evidence that a significant cover was ever present. But the evidence exists in the form of data from a variety of paleo-thermal and paleo-burial techniques. Some of the earliest geologists recognised the importance of former geological sequences that are no longer present. We suggest that serious consideration should be given to explaining this behaviour, rather than dismissing it as unrealistic. Intra-plate stresses from far-field plate boundary interactions and/or processes related to mantle circulation may explain the processes involved in producing episodic kilometre-scale burial and exhumation.

Automated summary

The incompleteness of the stratigraphical record has been well studied, and it has been found that significant sections of rock have been deposited and subsequently removed within relatively short time intervals. These episodes have been observed in various regions that are considered free of tectonic influences, and their importance in shaping the geological history and landscape cannot be ignored.