﻿Ancient impact-scars control regional geology worldwide

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Ancient impact-scars control regional geology worldwide

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Bombardment of the Earth at an early date established initial conditions that have affected all later regional geology. No part of the Earth's crust escaped. Activity at the brittle-ductile boundary has caused large-diameter scars to be sporadically regenerated upward throughout the four-billion years of our planet's history. Although these three-dimensional 'craterform' scars have evolved in many different manners, and many are covered over at any given time, many retain observable two-dimensional map-outlines with circular curvature. These inherited scar-features have been regenerated 'cold' from below and are fundamentally different from 'astroblemes', as presently defined, whose constituent rocks had been directly subjected to the high temperatures and pressures that accompany extra-terrestrial impacts. The varied present-day manifestations of these scars need to be described and categorized as has been done for faults, folds, rocks, and minerals by earlier workers in the earth sciences.

Geology

Planetary Geology

Late Heavy Bombardment

cratons

circular scars

inherited faults and fractures

Snowball Earth

The initial conditions of our planet were twice reset, once during the formation of the Moon and later during the prolonged bombardment that scarred the Moon and all the inner planets. Although this second episode, which affected the entire Earth, is labelled the 'Late Heavy Bombardment' (LHB), it can perhaps be better understood as the tail end of our planet's accretionary period [1],[2]. In either case, the LHB reset the Earth's initial geological conditions.

By the time of the LHB, which ended approximately 3850 − 3800 Ma, the Earth already possessed a solid crust. The existence of this early crust, which is presumed to have been thin and hot, is attested by still older detrital zircons derived from granites that had crystallized at considerable depth [3],[4],[5].

LHB impact craters

Most LHB impact scars were too shallow to have penetrated the Earth's early crust and would have been rapidly eroded out of existence. But the fate of deeper impacts is less evident. After a deep impact, melted materials gradually solidify. In a text-book case uncomplicated by nearby circumstances or later tectonic events, this would produce an unfractured 3–D craterform-mass of solidified melt, bounded by rocks that had not quite melted, i.e., that had been maximally shocked.

Yet although large LHB impacts produced great long-lasting 3–D craterform fracture-patterns, these patterns could not have persisted below the brittle-ductile boundary. Below this boundary, rocks flow. Flow involves shear, and shear obliterates original features. Ductile materials have no memory. In consequence, vestiges of large impact craters (above some presently unknown size) do not continue at depth throughout geological time. Instead, they would be truncated at the brittle-ductile boundary, leaving a circular fracture-pattern just above the depth at which rocks flow.

The resultant form would be a frozen core surrounded by maximally shocked rocks, with the core and its surrounding envelope of shocked rock both truncated at depth.

Persistence in time

Many rock-fractures heal, the two sides welded together or reconnected by quartz, carbonate minerals, direct annealing, or otherwise. It is not clear, however, how fractures at the brittle-to-ductile transition might ever entirely heal on a planet as active as ours. Flow, convection, outgassing, circulating fluids, changes in crystal structure and of volume, earthquakes, and the twice-daily earth-tide provide constant, intermittent, periodic, and sporadic movements. (The magnitude of the earth-tide, which can be as great as 55 cm at the Equator these days and was greater in the past, is not affected by the rigidity of rocks.) In consequence, fractures that had attained the brittle-ductile boundary would be activated and repropagated upward ever after.

With time, the surfaces of LHB impact-craters would become eroded and covered by younger geological formations of various sorts. In some instances, overlying formations might acquire circular fracture-patterns regenerated from below. In other cases, the ancient fracture patterns would not attain the Earth's surface as when, for instance, sedimentation was more rapid than the rate at which the fractures were rejuvenated, or where the fractures were covered by recent lava flows.

A debate closed

The Earth was heavily bombarded during the LHB, c.4100 − 3800 Ma, and when the first generations of satellite imagery became available, many geologists expected to see a Moon-like pattern of terrestrial impact scars. But the expected scars were not seen. Doubts then arose, which caused NASA’s Paul D. Lowman to warn that 'arguing that the Earth had somehow escaped' the bombardment that battered the Moon was tantamount to 'invoking magic or divine intervention' [6].

Impact craters were in fact spotted on Landsat and other imagery, but most were already known or small, and in time a consensus of sorts was reached that the action of water on the Earth's surface had erased our planet's ancient Moon-like scars. A few scientists and non-scientists nevertheless still thought they could discern large impact-like patterns or scars with circular curvature on certain images and maps [7],[8],[9], with Hudson Bay, the coastline south of Prince Edward Island (Canada), and the bulge on the east coast of Sweden as favoured candidate-structures.

But in 1971 or thereabouts, the theory of plate tectonics began to acquire its modern form and thereafter it was assumed that, if not eroded, the traces of the LHB had been subducted. This was just an assumption but it was seemingly sealed in a definitive manner when certain criteria were recognized at younger (post-LHB) meteorite-impact sites as characteristic of hypervelocity impact-shock. These 'diagnostic shock metamorphic features' included shatter cones, planar deformation features, toasted quartz, ballen quartz, shocked zircons, and the mineral coesite. In the absence of at least one such feature, a proposed site would not be included in the list of 'confirmed impact structures' in the Earth Impact Database [10], which had 190 structures as confirmed astroblemes by April 15, 2022.

Absent from the list of diagnostic features was circularity itself. Yet circularity was, and remains, the prime characteristic by which lunar and other extra-terrestrial impact sites are recognized.

Reopening the discussion

Several arguments suggest that on Earth, too, near-perfect circular patterns with large diameters may be sufficient for the recognition of vestiges of the LHB.

First of all, the features used to identify shock metamorphism only form in solid rocks. They do not form in fluids. But large impacts produce disproportionately great quantities of melt-fluids [11],[12],[13],[14],[15]. In consequence, diagnostic shock metamorphic features may have been immediately swamped by melt or eventually annealed by the heat retained in the molten mass, thermal metamorphism overwhelming shock metamorphism. Alternatively, they may have never been produced at large impact sites.

In addition, all of the recognized post-LHB impact-sites include rocks that had actually been subjected to the high pressures and temperatures that accompany crater-forming meteorite impacts and, hence, are by definition potential host rocks for shock-metamorphic features. But the far more numerous vestigial fracture-patterns derived from LHB impacts [16],[17],[18] were formed 'cold' in younger rocks, gradually regenerated from below in circumstances unsuitable for the formation of shock features, not violently produced from above as at the 190 astroblemes listed in the Database.

The oldest site in the Database was formed approximately one and a half billion years after the end of the LHB.

No site in the Database has a diameter greater than 160 km, which is far smaller than many lunar impact-scars.

Images of the Earth that show large patterns with circular curvature were collected from sources as varied as satellite imagery, geological maps (both old and recent), newspaper clippings, flight screens, and snapshots taken at lectures. These images are my data, comparable to the materials with which scientists in other fields are obliged to work, broadly comparable, for instance, to indistinct photos of animals taken by camera traps or fossils in various states of preservation. In general, LHB patterns are not neatly defined or complete, nor should we expect that they would be, one specialist referring to 'the embarrassing fact that the original rim of very large [lunar] basins such as Orientale cannot be found (or agreed upon)' [19].

In the following sections, selected images of large features with circular curvature are presented and emphasis is placed on the diversity of their geological settings and effects.

Wisconsin: snow and ice

A circular feature in Wisconsin (Fig. 1) with a diameter of approximately 90 km has been known from before the advent of aerial photography, presumably because half of its perimeter corresponds to the limit of glaciation, with other sections bounded by the Mississippi and other rivers. A search by the Wisconsin Geological and Natural History Survey for shock-metamorphic features diagnostic of shock metamorphism was not successful.

The Congo and Australia: internal drainage

A NASA site describes the Congo Basin (Fig. 2a), as 'a vast, shallow depression' that 'rises to form an almost circular rim of highlands' [20]. The basin has been compared to 'the circular lunar maria of impact origin', with the rocks underlying the basin characterized as possessing 'unusual stability and rigidity [21].

A similar pattern (Fig. 2b), with a diameter of approximately 1260 km, was recognized on a 'continental drainage map' of Australia [22].

Sweden: mineralization and geological contacts

The northwest quadrant of the ~ 230-km-diameter circular region in eastern Sweden (Fig. 3) hosts numerous mineral deposits on its perimeter and along a short secant oriented NNE-SSW. Several deposits are themselves similarly oriented. In the southeast quadrant, some islands in the Stockholm Archipelago have geological contacts tangent to the circle.

The 'Arizona Transition Zone': mineralization and comparison with the Moon and Mars

The thin-crusted Arizona Transition Zone is marked by 19 or more circular features; Fig. 4a [16]. Twenty-four economic deposits were registered in the district (Fig. 4b), most of which were polymetallic, with production of Cu, Ag, Au, Mo, V, W, Zn, and Pb [16], [23]. Most of the deposits are located within the 9% of the total area covered by the rim-zones of the 19 circles, with all others categorized as 'very close' to a rim [16]. The deposits include major mines and at least four large porphyry deposits (Globe-Miami-Old Dominion, Magma-Silver King-Superior, Ray-Kelvin, Christmas-Troy-Dripping Springs-Hayden). The blank areas in Fig. 4b are indeed blank, i.e., devoid of known economic mineral occurrences as of 1973 [23]. Many of the mineralized rim-zones are intensely fractured or brecciated, and several of the deposits are known to have undergone more than one period of mineralization.

Figure 4a is a photograph taken of a relief map with twofold vertical exaggeration, illuminated at a low angle. Using the same method to detect large circular patterns throughout much of North America Byler [17], produced a diameter-distribution plot (Fig. 4c) that showed scaled similarity with diameter-distribution plots for impact craters on the Moon and Mars. Byler took this as evidence for a common origin. He attributed the bend in his data at diameters of ~ 400 km to the removal of smaller terrestrial scars by erosional processes [17],[18].

The correlation of mineralization with circular features in the Arizona Transition Zone stands on its own and does not need to be linked to any particular theory, and similarly for the circle in Sweden in Fig. 3.

Continents and Cratons

As recently reconfirmed, established scaling laws 'fail to estimate the melt volume' for 'giant' impacts [15]. This failure would be even more severe in estimating the amount of melt produced by large impacts that pierced the thin hot crust of the early Earth [15]. The disproportionately great amount of molten material produced by large impacts [11],[12],[13],[14],[15] includes melts produced during long-lived post-impact decompression-melting [24].

Sufficiently large craters may fill past the point of overflowing, which, by one published calculation, would occur on today's Earth in cases where the transient crater (before rebound) had had a diameter of a thousand kilometres or somewhat more [13]. In consequence, terrestrial craters of sufficient size 'may not produce an obvious crater form, as the melt volume … would actually exceed the cavity volume' [11],[25]. In times long after a giant impact, (i) solidified crater-fill and (ii) solidified melt-overflow would have appeared much the same, whether viewed from space or as geologically mapped. Figure 5 shows the approximate extent of cratonic rocks in North America. Figure 6 shows a ~ 3700-km-diameter circular scar within these rocks.

The circular outline of the area within North America (Fig. 6) – source of the melt-spillover (Fig. 5) – would not have become visible until the combination of net erosion from above and net fracture-regeneration from below had allowed fractures along the circumference to reach the Earth's surface.

Figure 6 shows the inherent circularity of the North American craton. Materials within this and other large craton-forming impact-sites remained fluid far longer than at small impact sites. Later-arriving LHB bodies that fell into these great expanses of solidifying melt might have left circular scars – circles within circles (Fig. 6c, top inset, for one instance) – or they might have physically disappeared like raindrops in a puddle. But according to the Nice Model for the dynamic evolution of the solar system, the impacting bodies would have originated at various distances from the Sun. In consequence, they would have been chemically variable. Thus, whether they left physical scars or not, the later-arriving impactors would have introduced local variations in the chemistry of the great craton-forming melt-puddles into which they fell. Reheating and re-melting would have subsequently occurred as a consequence of the decay of radioactive isotopes, far more important for the early Earth than now. In two dimensions, the resultant craton would be composed of rock-units of diverse chemical compositions, circular (as in the top inset in Fig. 6c) and not, welded together by heat, and subsequently clamped by thermal contraction.

The circular feature within the North American Craton in the top inset in Fig. 6c hosts both the Acasta Gneiss and a concentration of rich diamond-bearing kimberlites; its diameter is approximately 480 km.

Cratons are coherent and long-lived due to the relative lack of deep fractures in their interiors. Plate collisions barely impinge in their interiors. Instead, collisions produce fold-belts, mountain ranges, and orogens along their perimeters. In North America, parts of the craton-circle are abutted on the southeast by the Appalachian Mountains (formed during the Acadian Orogeny, 375 to 325 million years ago) and on the west by the Canadian Rocky Mountains (formed 70 to 40 million years ago during the Laramide Orogeny).

With no reference to the Late Heavy Bombardment or to later impacts, Keith Bloomfield, who worked for years as a geologist in Brazil, observed that 'when information from drill holes is included, and the younger rocks of the Amazon Basin are ignored, the Guyanan and Brazilian Shields together form a nearly perfect circle' (personal communication, 1980). See Fig. 7. This was independently reported by Burgener who attributed his slightly different version to a post-LHB impact [26].

Much of Europe is also defined by a craton-circle framed by orogenic belts crumpled against its perimeter: the mountains of western Norway (formed during the Caledonian Orogeny 490 to 390 million years ago), the Urals (formed approximately 318 to 252 million years ago during the Uralian Orogeny), and the Alps–Carpathians–Dinaric Alps–Greek Mountains (formed approximately 55 million to 5 million years ago during the Alpine Orogeny); Fig. 8.

Each craton has had its own history. The North American craton has been scoured by glaciers; much of the South American craton is covered by the sediments of the Amazon Basin and may be cracked along the Amazon valley; and much of the craton that defines Europe is overlain by younger geological formations. Their inherent circularity may be difficult to see or to recognize, but so are many other features of our planet [16],[28].

Craton rims

Despite four billion years of healing and annealing, the rim zones of craton-forming impact-scars would remain brecciated and would provide enduring zones of relative weakness. During collisions, rim-zone materials might buckle and fold to produce orogens. In addition, places along cratonic rims might facilitate the tectonic uplift of coherent rock-units. As proposed here, these coherent rock-units would include plugs of solidified impact-melt, also produced during the LHB, but later than the much larger craton-forming impacts themselves.

In North America, two such cases are the uplifted Adirondack Mountains, which itself has a circular core ~ 160 km in diameter (Fig. 6c, middle inset), and the uplifted Black Hills, depicted as circular in Newton and Jenney's Bird's eye view of the Black Hills to illustrate the Geological Structure (1880) [29]; Fig. 6c, bottom inset.

In Europe, the craton-circle is marked by the uplifted ~ 270-km-diameter Bohemian Massif in the west, and in the east by the partly uplifted > 400 km 'Middle-Urals Ring Structure – MURS' [30],[31], notable for its major commercial deposits of Fe, Mn, Cr, Ni, Cu, Ti, Pb, Au, Pt and transparent coloured gemstones [31]; Fig. 8. According to Burba [30], the MURS has 'a sharp expression in the basement topography'.

Figure 9: Map showing the Romashkino Dome, the Middle-Urals Ring Structure (MURS), oil and gas fields, occurrences of transparent coloured gemstones, part of the Central Urals, and selected faults. (Some of the gemstone occurrences may be alluvial.) The base map is a 1:2,000,000 oil-and-gas map of the Volga-Urals region, infolded in Trofimov (2014) [32]. For a proposed association of petroleum fluids with a circular structure in the Gulf region, see [33]; for circular scars and occurrences of transparent gemstones in eastern East Africa, see [34].

In Asia, the Himalayan arc forms a quarter-circle that appears to end, or to be blocked, in the west by the Pamir Mountains and the Hindu Kush, both of which are uplifted and both of which have circular cores themselves with respective diameters of ~ 640 km and ~ 290 km; Fig. 10. In common with the Central Urals (parts of which lie within the Middle-Urals Ring Structure; Fig. 9), the Pamirs and the Hindu Kush are rich in deposits of transparent gemstones, crystallized during the extremely high frictional heating when and where one continental plate collided with another [34],[35].

Missed opportunities

A great advance in understanding the Earth's geological structure might have been achieved c.1896, and again a century later, two occasions when maps were made on which was visible the great circular feature that encompass much of Asia. The older of the two maps was drawn by the geographer T. Ruddiman Johnston for a proposed 84-foot hollow globe to be constructed in Madras (Fig. 11a). The second map was shown in August 1984 in Moscow during the 27th International Geological Congress and captured by a snapshot from the back of the auditorium (Fig. 11b).

This 'Asian Circle' – which is not the same feature as the Himalayan arc – has a diameter of approximately 5350 km, a figure that should be compared to the 'approximately 5000 kilometres', twice independently calculated from the lunar cratering record for the diameter of the largest expected terrestrial LHB scar, once by Thomas Gold (personal communication, 1986), and once by Ryder et al. (2000) [14].

There are two points where the Asian Circle is intersected by the Himalayan arc which, if extended, would form a circle with a diameter of ~ 3390 km [37]; Fig. 11. These crossing points (syntaxes), east and west, are places where the Earth is deeply and doubly weakened (or triply so as by the Pamirs and the Hindu Kush in the west; Fig. 10). Portions of the 'Himalayan Circle' other than the familiar arc itself are indistinctly discernible in Fig. 11b and 11c.

Secondary rims

Novaya Zemlya is situated where an outer secondary rim to the Asian Circle would be located (Fig. 11), and portions of the Italian peninsular may furnish sections of an outer rim of the European scar, with the Adriatic Sea as an intervening moat (Fig. 8).

In northern Russia, the Asian Circle is manifested by the Polar Urals with part of an outer secondary rim indicated by Novaya Zemlya. See Fig. 12, which also shows part of the European Circle, as indicated by the ridges on the Kanin Peninsula, with Pai-Khoi in the position of a secondary rim.

'Drift units' and rebalancing the Earth's spin-axis

The European and Asian scars abut at the Middle Urals (Figs. 8, 9 and 11), with an initial contact arguably occurring at the Middle-Urals Ring Structure (Figs. 8 and 9). The geographical relationship between these deep 3–D continent-scale features does not date to the LHB: their present positions are the result of drift and of a collision during the Uralian Orogeny (~ 318 − 252 Ma) that produced the straight section of the Middle Urals where the two rim-zones coalesced.

Another feature of our planet that could not have been in existence since the LHB is the alignment along the Earth's equator that is formed by the South American circle (Fig. 7); the Congo Basin (Fig. 2a); the 'nearly circular' [20] Lake Victoria basin [28] and the part-circle formed by the arc of the Pangani Rift, Mount Kilimanjaro, Ol Donyo Sabuk volcano, and Mount Kenya (famed for snow the Equator) [28],[34].

This equatorial alignment may be a consequence of gyroscopic rebalancing of the Earth's spin-axis (by moving dense materials to the equator), an effect that minimizes wobble [40] and may be the prime driving force for drift; also see [28] and [41].

Deep subduction

LHB impacts, whatever their diameters, produced crater walls that initially intersected the Earth's surface at steep angles. Erosion subsequently exposed the walls at lower levels. At the newly exposed levels, the ancient impact-fractures intersected the Earth's surface at various less-steep angles. At some stage during the erosional process, some of the extremely numerous arcuate crater-wall fracture-zones would have necessarily dipped into the Earth at angles geometrically suitable for deep subduction [42]. See Fig. 13.

Erosion has been most extreme following Snowball Earth episodes, in particular after c.750 Ma, during the 'protracted period of widespread continental denudation' [43] that produced the Great Unconformity (and greatly troubled Darwin). Average global erosion during this episode is estimated to have been 3 to 5 vertical kilometres(!) [44]. This suggests a causal link between Snowball Earth episodes and the initiation of subduction, or of deep modern-style subduction.

A second requirement for the initiation of subduction is the existence of relatively dense material that overlies material that is less dense, a condition fulfilled by the long-term cooling, hence densification, of the lithosphere [46],[47],[48]. A likely third requirement is the lubrication that is furnished by water and soft sediment in the down-going slab [49].

Subduction arcs and mid-oceanic ridges

The entire Earth was affected by the LHB, necessarily so, but deep scars in areas of oceanic crust are now covered by young basalt, much it unfractured. Subduction arcs, where the basalt cover has been fractured from below, are an exception.

Another exception is provided by the mid-oceanic ridge system, composed of fractures along local paths of least resistance, which, as proposed here, were furnished by LHB rim-sectors and tangents, and jumps from one to another.

Ancient impact-diamonds, impacts, and plate tectonics

Arguing from the evidence of 'carbonado, ballas and other impact-diamond varieties in placers' in 'zones of erosion of early Precambrian rocks', Polosukhin (1981) concluded that 'the “indestructibility” and active evolution, over a period of over four billion years, of faults formed by explosion-impact processes in structural zones of very different ages and types, and commonly penetrating into the mantle, raise questions about present tectonic concepts, particularly that of Recent global tectonics' [50]. Such questions were echoed at the Snowbird Conference seven years later in Marvin's suggestion that 'new research programs may prove even more subversive to classical geology if causal links can be found between impacts and plate tectonics' [51].

Theory and observation indicate that the entire Earth was, and remains, affected by the Late Heavy Bombardment from the surface down to the brittle-ductile boundary. These scars provide a damaged canvass on which later geology has been painted. Effects of the LHB endure and should be considered in all regional geological studies. This work provides a first step toward establishing a typology of vestigial LHB impact features.

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Ancient impact-scars control regional geology worldwide

Status:

Version 1

Abstract

Figures

Introduction: Earth As Target

LHB impact craters

Persistence in time

A debate closed

Reopening the discussion

Materials And Methods

Data And Discussion

Wisconsin: snow and ice

The Congo and Australia: internal drainage

Sweden: mineralization and geological contacts

The 'Arizona Transition Zone': mineralization and comparison with the Moon and Mars

Continents and Cratons

Craton rims

Missed opportunities

Secondary rims

Deep subduction

Subduction arcs and mid-oceanic ridges

Results

References

Status:

Version 1

﻿Ancient impact-scars control regional geology worldwide

Status:

Version 1

Abstract

Figures

Introduction: Earth As Target

LHB impact craters

Persistence in time

A debate closed

Reopening the discussion

Materials And Methods

Data And Discussion

Wisconsin: snow and ice

The Congo and Australia: internal drainage

Sweden: mineralization and geological contacts

The 'Arizona Transition Zone': mineralization and comparison with the Moon and Mars

Continents and Cratons

Craton rims

Missed opportunities

Secondary rims

Deep subduction

Subduction arcs and mid-oceanic ridges

Results

References

Status:

Version 1

Ancient impact-scars control regional geology worldwide