Journal of Earth Science  2019, Vol. 30 Issue (3): 451-459   PDF    
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Hypervelocity Impacts and Exposed Lithospheric Mantle: A Way to Recognize Large Terrestrial Impact Basins?
Peter Olds     
Chemistry Faculty, College of Alameda, Alameda, California 94501, USA
ABSTRACT: On the Moon and Mars olivine of probable mantle origin is detected at rims of large Late Heavy Bombardment (LHB) age impact basins for which excavation depth estimates exceed crustal thickness estimates. But lunar Crisium size impact basins are not recognized on Earth nor expected in the Phanerozoic from conventional interpretations of crater size frequency distributions. In this study several large circular to elliptical basin structures on Earth, for which hypothesized impact excavation depth would greatly exceed crustal thickness, are examined for the presence of exposed lithospheric mantle, expressed as ophiolite, at the rims. Three Phanerozoic impact basins, modified by plate tectonics and tentatively correlated with "ophiolite obduction" plus global extinction events, are proposed here. These tentatively suggested Phanerozoic impact basins are:(1) Yucatan Basin/Puerto Rico Trench with a Greater Antilles ophiolite rim. Cretaceous-Paleogene Boundary global extinction may correlate with Maastrichtian ophiolite obduction in Southeast Cuba. (2) Loyalty Basin with a New Caledonia ophiolite plus d'Entrecasteaux Ridge rim. Late Eocene global extinction may correlate with obduction of the New Caledonia Peridotite Nappe. (3) Sulu Sea Basin with a Palawan, Sabah etc. ophiolite rim. The Middle Miocene Disruption Event may correlate with ophiolite obduction plus ophiolitic mélange emplacement in Sabah and in Palawan. These originally circular to elliptical belts of exposed lithospheric mantle may serve as strain markers for relative plate motions in the vicinity of plate boundaries during post-impact geologic times. It is further speculated that plate boundaries may be initiated and/or modified by such impacts.
KEY WORDS: olivine    mantle    impact basin    ophiolite obduction    mass extinction    
0 INTRODUCTION

Evidence for mantle ejecta on surfaces of rocky planetary bodies might be expected near crater rims for which estimated impact excavation depths exceed estimated crustal thicknesses. To this end, Yamamoto et al. (2010) spectrally detected olivine (Kaguya NIR reflectance) near the rims of lunar impact basins Imbrium and Crisium but not Orientale. Koeppen and Hamilton (2008) similarly detected Mg rich olivine (Mars Global Surveyor TES) near the rims of Martian impact basins Argyre, Hellas and Isidis. Excavation depth estimates are seen to exceed local crustal thickness estimates for each of the above impact basins except for Orientale where olivine has not been detected near the rim (Tables 1 and 2).

Table 1 Estimated impact excavation depths are compared with local crustal thicknesses on the Moon and Mars
Table 2 Estimated impact excavation depths are compared with local crustal thicknesses on the Moon and Mars

Earth's largest known craters: Vredefort, Sudbury and Chicxulub, of original diameters~300, 250, and 180 km respectively, are all on continental crust of thickness far exceeding the maximum expected excavation depth of ~15 to 20 km for the originally ~300 km diameter Vredefort Structure. That mantle material has not been identified in impactites drilled near the Chicxulub rim (Urrutia-Fucugauchi et al., 2004; Ward et al., 1995) or within or above the Chicxulub peak ring (Morgan et al., 2016) seems consistent with these simple considerations.

The modest thickness of oceanic crust on the Earth should result in significant mantle excavation by large impacts: Earth's oceanic crust is from about 5 to 10 km thick while continental crust is from about 30 to 50 km thick. It follows that impacts resulting in craters similar in scale to the lunar and Martian basins of Table 1, say larger in diameter than 500 km or so, should excavate and eject substantial amounts of mantle from all oceanic targets and from some continental targets. However, the large lunar and Martian impact basins of Table 1 are thought to be of Late Heavy Bombardment (LHB) age at 4.0±0.3 billion years (Fassett and Minton, 2013). Any LHB age impact basins on the Earth would have been erased by plate tectonics over the course of perhaps five subsequent supercontinent cycles and complementary Wilson cycles that have occurred since the LHB. Furthermore, Phanerozoic impact basins of sizes considered here are not expected from conventional interpretations of size frequency distributions derived from crater counting on the rocky planets (Ivanov et al., 2002). Setting aside these conventional expectations, several elliptical and circular features of Phanerozoic age, rimmed with mantle material, are identified as impact basin candidates. Such large circular and elliptical craters may be dismembered and/or deformed by relative plate motions, serving as finite strain markers for deformation in the vicinity of plate boundaries. Indeed, we suggest new plate boundaries may be formed and existing plate boundaries modified by impact basin forming events of magnitudes considered here.

1 MANTLE EXCAVATION BY IMPACTS ON MOON AND MARS AND EARTH 1.1 General Features of Impact Basins

Impact basins are very large impact craters and are classified as either "two-ringed" basins or "multi-ringed" basins where on the Moon multi-ring basins tend to be larger than two-ring basins, though the transition diameter between these lunar basin types overlaps and is neither well defined nor well understood (Spudis, 2005, Fig. 2.8 therein). The classic example of a multi-ring basin is the Moon's Orientale Basin (Fig. 1). In multi-ring basins, outer rings are typically inward dipping normal fault scarps while inner rings may be structurally uplifted "peak rings". The inner ring or rings of a large impact basin typically surround an impact melt sheet consisting of solidified, approximately homogeneous mixture of, melted target rock. An approximately continuous blanket of ejected, but mostly not melted, mélange of fragmented target rock is of maximum thickness near, and thins radially away from, the "principle" crater rim according to empirically determined power laws (McGetchin et al., 1973).

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Figure 1. LRO Photomosaic of Orientale Basin. Image credit: NASA/GSFC/ Arizona State Univ./Lunar Reconnaissance Orbiter. Image mosaic width is 1 350 km. Note the pooled impact melt or "mare" bounded by the innermost ring (inner Montes Rook range).
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Figure 2. Magellan spacecraft SAR (Radar) image of Mead Crater, the largest crater on Venus at 280 km diameter. Image credit: NASA-Jet Propulsion Laboratory and http://www.espacial.org/planetarias/crateres/venus_crater1.htm.
1.2 Transient Crater Concept

Final basin diameter, as determined by a so-called "principle" ring, is much larger than the diameter of the original impact excavation cavity which roughly corresponds to the diameter of an inferred "transient crater" resulting from both excavation and shock initiated movement of non-ejected rock down and radially away from ground zero (Spudis, 2005; Melosh, 1989). Transient crater diameter is inferred with scaling models (Johnson et al., 2016; Holsapple, 1993) and cannot be observed. Transient crater diameter estimates typically range from 1/2 to 2/3 of final basin diameter (Tables 1 and 2). Many properties of interest, including maximum excavation depth, scale with transient crater diameter. Even "observed" crater diameter itself is often ambiguous: For example Chicxulub diameter estimates ranging from 150 to 295 km depending on what structural features are used for measurement (Turtle et al., 2005). So for any given basin, there may be disagreement among experts on both observed final crater diameter (Spudis, 2005) and how transient crater diameter scales with the final crater diameter (Johnson et al., 2016) leading to significant uncertainty in inferred maximum excavation depth, generally thought to scale as approximately 1/10 the transient crater diameter (Spudis, 2005; Melosh, 1989).

1.3 Impact Basins on Venus and Earth

Mead Crater (280 km) on Venus is a two-ringed crater which Spudis (2005) would classify as a multi-ring basin rather than peak ring crater (Fig. 2) since both rings are normal fault scarps with probable pooled impact melt in the central depression bounded by the inner scarp ring. Presence of a structurally elevated "peak ring" is absent, possibly having disappeared by way of viscous relaxation (Karimi and Dombard, 2017).

Conventionally Chicxulub, Sudbury and Vredefort are considered terrestrial multi-ring basins notwithstanding their relatively small diameters (Grieve et al., 2008). So-called simple craters and central uplift craters are smaller than peak ring or multi-ring basins and not are considered here. For a discussion of these small crater types and threshold transition diameters between crater types in general on various planetary bodies see Melosh (1989) and Head (2010).

1.4 Proposed Late Phanerozoic Impact Basins on Earth

In this work, three terrestrial Late Phanerozoic circular to elliptical two-ring or multi-ring basin size impact structures in the 500 to 1 000 km diameter range are proposed. Such surviving terrestrial impact basins in this size range might manifest: circular to elliptical rims (or rim segments), with exposed lithospheric mantle, serving as strain markers for plate boundary motion; thick ejecta near rim expressed as mantle peridotite and/or "ophiolitic melange"; power law decay of ejecta thickness with radial distance from rim (McGetchin et al., 1973) and systematic azimuthal thickness variation for low angle impacts (Schultz, 1999); weathering resistant shocked mantle minerals such as chromite in the ejecta (Bohor et al., 1990) and possibly ophiolitic diamonds (Yang et al., 2014); global spherule layers with PGE anomalies (Alvarez et al., 1980); rim structures consistent with cratering mechanics (Kenkmann et al., 2014; Melosh, 1989); impact melt basement (Pierazzo et al., 1997; Grieve and Cintala, 1992) recording uniform cooling age and Earth's magnetic polarity of the time. Global impact induced seismic effects (Sleep and Lowe, 2014).

1.5 Yucatan Basin/Puerto Rico Trench: A Possible Dissected K-Pg Boundary Impact Basin?

The Yucatan Basin is a bathymetric low bounded in the north by Cuba, in the west by the Yucatan Platform and in the south by the Cayman Ridge which in turn rises from the Cayman Trough, a pull-apart basin occurring along the Caribbean- North American left-lateral transform plate boundary (Fig. 3) between Hispaniola and Guatemala. The Puerto Rico Trench is a bathymetric low bounded in the north by the steep scarp of an eastern segment of the transform and in the south by Hispaniola, Puerto Rico and the Virgen Islands (Fig. 3) and their associated carbonate platform. So together the Yucatan Basin and the Puerto Rico Trench can be considered bathymetric lows dissected and displaced from each other by the left-lateral transform, and bounded in the north and south respectively by the western and eastern Greater Antilles ophiolite belts. Conventional interpretations have the Yucatan Basin as a back-arc basin (Rosencrantz, 1990) resulting from south dipping subduction of the "proto-Caribbean" along Cuba before Cuba's collision with the Bahamas Platform in the Middle Eocene.

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Figure 3. Bathymetric, image of the Yucatan Basin, Puerto Rico Trench and North American/Caribbean transform plate boundary. Image Credit: Google Earth Pro 7.3.2.5495 (Jan. 30, 2019) Caribbean, 19°30'12.48" N, 75°01'00.07" W, eye alt 2 600 km Image Landsat/Compernicus, Data SIO, US Navy, NGA, GEBCO. USGS Plate Boundaries: https://earthquake.usgs.gov/learn/plate-boundaries.kmz.

Lithospheric mantle exposed as Greater Antilles ophiolite forms the northern rim of the Yucatan Basin (Cuba) and the southern rim of the Puerto Rico Trench (Hispaniola and Puerto Rico), suggesting the following hypothesis.

Hypothesis: The Greater Antilles Island chain ophiolite belt marks the rim of an approximately 700 to 800 km diameter impact basin of K-Pg boundary age, deformed and dismembered from an originally circular form by at least 50 million years of left lateral shear along the North American-Caribbean transform plate boundary (Fig. 4).

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Figure 4. Map of the Greater Antilles Island chain ophiolites straddling the North American Caribbean plate boundary. Ophiolites (primarily lithospheric mantle) are coded red and exaggerated for visibility. Cayman Trough magnetic anomaly extent corresponds to about 1 000 km minimum left-lateral offset between the northwest and southeast Greater Antilles Island ophiolite belt segments (and corresponding North American and Caribbean plates). The Puerto Rico Trench has essentially identical strike, and relative strike-slip plate velocity. as Cayman Trough bounding transform faults, i.e., the Oriente and Swan "fracture zones". Yellow arrows indicate sense of present day relative plate velocity of ~2 cm/yr (DeMets et al., 2010) and displacement sense since at least the Early Eocene. Image credit: Same Google Earth attributes as Fig. 3 but (May 2, 2019) with eye alt 2 800 km.

Exposed lithospheric mantle is ubiquitous in Cuba, the largest island of the Greater Antilles ophiolite belt. Lesser exposures also occur in the Dominican Republic and Puerto Rico. Cuban ophiolites were apparently obducted in Latest Cretaceous/Earliest Danian times, inconsistent with the documented Eocene collision of Cuba with the Bahamas Platform (García-Casco et al., 2008; Iturralde-Vinent et al., 2006) but possibly consistent with the timing of the K-Pg boundary impact. Cuba also hosts the world's thickest known K-Pg boundary deposits (Goto et al., 2008; Tada et al., 2003; Iturralde-Vinent, 1992). These and simple geometric considerations (Fig. 4) suggest oceanic crust and upper mantle rock, exposed as ophiolite in the Greater Antilles Island chain, marks the rim of a roughly 700 to 800 km diameter impact basin deformed and dismembered from an originally circular form by at least 50 Ma of left-lateral shear along the North American- Caribbean transform plate boundary. Current plate boundary motion is ~2 cm/yr left-lateral (DeMets et al., 2010) which, if assumed approximately constant for the last 50 Ma, would result in roughly 1 000 km of left-lateral displacement, a distance consistent with the extent of spreading recorded by magnetic anomalies in the Cayman Trough, a left-lateral transform pull-apart basin thought to have opened about 50 Ma ago (Leroy et al., 2000; Rosencrantz and Mann, 1991; Rosencrantz, 1990). Restoring the southeastern Greater Antilles (Hispaniola, Puerto Rico, Virgin Islands) to a co-radial position with the northwestern Greater Antilles (Cuba) requires that today's total left-lateral displacement be roughly 1 600 km. The discrepancy might be accounted for by pre-Cayman Trough left-lateral distributed shear (shear not localized to a plate boundary) corresponding to unknown, and possibly time variable, relative plate velocity preceding Cayman Trough magnetic anomaly record. It should be noted that the full extent of the Cayman Trough is roughly 1 600 km when including its continuation to the east and west of recognized magnetic anomalies, a distance in agreement with extent of left-lateral displacement implied in the above "rim" restoration.

Corollary hypothesis: The Greater Antilles/Chicxulub system constitutes a doublet crater with separation distance of ~700 km (center to center) roughly equal to the Greater Antilles "crater" diameter of ~700 to 800 km.

1.6 Loyalty Basin: A Possible End-Eocene Impact Basin?

The Loyalty Basin is bathymetric low, in the shape of a truncated ellipse, bounded in the southwest by New Caledonia and in the north by the d'Entrecasteaux Ridge, features which are continuous with each other and together constitute the elliptical New Caledonia-d'Entrecasteaux orocline (Fig. 5). To the east, Loyalty Basin is bounded by, and being actively subducted into, the New Hebrides (or Vanuatu) Trench. Conventional interpretation has the North Loyalty Basin originating with mid-ocean ridge spreading between 28 and 44 Ma (Mortimer et al., 2014). Lithospheric mantle is exposed in and along New Caledonia as the Peridotite Nappe. The New Caledonia-d'Entrecasteaux orocline forms the rim of a nearly perfect ellipse partially erased by east dipping subduction into New Hebrides Trench (Fig. 6), observations which suggest the following hypothesis.

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Figure 5. Bathymetric image of the Loyalty Basin, the dʼEntrecasteaux Ridge, New Caledonia and the New Hebrides Trench. Image Credit: Google Earth Pro 7.3.2.5495 (Jan. 30, 2019) Loyalty Basin, 18°45'28.69" S, 167°35'49.84" E, Eye alt 2 600 km. Image Landsat/Compernicus, Data SIO, US Navy, NGA, GEBCO. USGS Plate Boundaries: https://earthquake.usgs.gov/learn/plate-boundaries.kmz
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Figure 6. Google Earth image of Loyalty Basin "elliptical crater" (my truncated ellipse) partially subducted beneath the Vanuatu Trench for which the present day convergence rate is ~12 cm/yr (or 120 km/Myr). So ellipse will be gone, i.e. completely subducted, in less than 5 my. Exposed lithospheric mantle (Peridotite Nappe) is color coded red. Image credit: Same Google Earth attributes as Fig. 2 but (Nov. 12, 2018) with eye alt 2 800 km.

Hypothesis: The Peridotite Nappe bearing New Caledonia- d'Entrecasteaux orocline forms the rim of an approximately 900 by 700 km NW to SE trending elliptical impact basin of Late Eocene age now being subducted beneath the New Hebrides.

The Peridotite Nappe of New Caledonia is famously the example of exposed lithospheric mantle for which the term "obduction" was first coined (Coleman, 1971). Its contact with underlying rocks is sub-horizontal and in southern New Caledonia, the nappe thickness is at least 1.5 km and possibly reaching 3.5 km (Guillon, 1975). Here, ages of uppermost sediments buried under the nappe and oldest intrusions into the nappe bracket obduction timing (at this locality) to between 35 and 27 Ma (Gautier et al., 2016; Paquette and Cluzel, 2007; Cluzel et al., 2001). While these authors currently interpret obduction timing as diachronous (progressing from north to south with models of an oblique subduction zone collision with the Norfolk Ridge continental ribbon) it is tempting to interpret structural evidence for southwest thrust vergence combined with right lateral shear observed in New Caledonia (see Gautier et al., 2016) as resulting from down range momentum transferred to target rock from an impacting bolide approaching at a low angle from the NW heading SE, consistent with the major axis azimuth of the Loyalty Basin ellipse. The continuity of the d'Entrecasteaux double ridge with the New Caledonia double ridge suggests a two-ring or peak ring crater (Collins et al., 2018). The age of presumed stratovolcanoes making up the "inner ring" of the d'Entrecasteaux Ridge may constrain the timing of its formation, hence impact, to between 33 and 37 Ma (Quinn, 1994). The "inner ring" Loyalty Ridge segment east of New Caledonia has not been drilled.

1.7 Sulu Sea Basin: A Possible Middle Miocene Impact Basin?

The Sulu Sea Basin is an elliptical bathymetric low bounded in the northwest by Palawan, in the northeast by the western Visayas Islands, in the southeast by Mindanao and the Sulu Archipelago (all belonging to the Philippines), and in the southwest by Sabah of northeastern Borneo belonging to Malaysia (Fig. 7). The Sulu Sea ellipse is actually divided into two basins by the mostly submarine Cagayan Ridge striking SW-NE parallel to Palawan and to the ellipse long axis. Conventional interpretations have the Sulu Sea originating with intra-arc rifting or back-arc spreading in the Miocene (Hutchison, 1992). Lithospheric mantle exposed in Sabah (Malaysia) and the islands of Palawan, Panay, and Mindanao defines an elliptical rim of the Sulu Sea Basin (Fig. 8) leading to the following hypothesis.

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Figure 7. Bathymetric image of the Sulu Sea Basin, Palawan and the Spratly Island ring structure. Image Credit: Google Earth Pro 7.3.2.5495 (Jan. 30, 2019) Sulu Sea Basin, 8°22'20.41" N, 120°43'48.25" E, Eye alt 2600 km. Image Landsat/Compernicus, Data SIO, US Navy, NGA, GEBCO. USGS Plate Boundaries: https://earthquake.usgs.gov/learn/plate-boundaries.kmz.
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Figure 8. Sulu Sea Basin ellipse is 900 by 550 km for an ellipticity of ~1.6. Suggests an oblique (low angle) impact approaching from the SW heading NE. Ophiolitic mélange formation age in Sabah (Clennell, 1991) and obduction in Palawan (Keenan et al., 2016) suggests Middle Miocene (~15 Ma) impact. Spratly Island multi-ring structure suggests a 400 by 260 km impact basin with similar ellipticity and major axis direction (SW to NE). So Spratly multi-ring basom plus Sulu Sea Basin suggests doublet crater of Middle Miocene age resulting from oblique impact of a binary object approaching from the SW. Exposed lithospheric mantle is color coded red. Image Credit: same Google Earth attributes as Fig. 3 but (Nov. 12, 2018) with eye alt 2 800 km.

Hypothesis: The Sabah, Palawan, Panay and Mindanao ophiolites mark the rim of an approximately 900 by 550 km SW to NE trending elliptical impact basin of possible Middle Miocene age.

The basin is apparently sufficiently young to be yet relatively un-deformed by plate tectonics compared to the previous two examples. Nevertheless, the ellipse is undergoing detectible deformation due to left-lateral shear within the Philippine mobile belt plus incipient subduction within the basin at the Negros and Sulu trenches (see Fig. 1 of Aurelio et al., 2013).

Corollary Hypothesis: The Spratly Island 400 by 260 km SW to NE trending elliptical multi-ring basin has similar aspect ratio and long axis orientation to the Sulu Sea Basin (Fig. 8) and may be the smaller member of a doublet crater resulting from low angle impact of a binary object approaching from the SW. The Reed Bank resembles a rampart debris flow directed NE from the Spratly Structure, roughly consistent with expected down-range momentum imparted to the target. Doublet crater separation distance (center to center) is about 700 km, similar in scale to the hypothetical Greater Antilles-Yucatan Basin/Chicxulub doublet separation.

Problems: Ophiolite obduction in Palawan is interpreted as a middle Miocene event (Keenan et al., 2016) while ophiolite obduction in Darvel Bay, Sabah is interpreted as Eocene or earlier (Omang and Barber, 1996). Also, Keenan et al. (2016) describe an Eocene (34 Ma) metamorphic sole in Palawan. Ophiolitic mélange emplacement in Sabah is considered diachronous (Clennell, 1991). And Sulu Sea basement at ODP hole #768 is dated at 18.8 Ma (Nichols et al., 1990).

2 DISCUSSION

The above three examples of proposed Phanerozoic impact basins have in common that: circular or elliptical structures are systematically rimmed with exposures of lithospheric mantle; excavation depths inferred from basin diameters are easily in excess of any oceanic crustal thickness, suggesting mantle would be ejected; apparent deformation of the structures appears consistent with known plate motions; present day plate boundary configurations in these regions are complex; consensus on plate tectonic reconstructions in these regions is lacking.

Problems include: (1) Homogeneous magnetic polarity (in EMAG 2) is not observed within the Loyalty Basin nor within the Sulu Sea Basin (Maus et al., 2009) such that in these cases magnetic anomalies are now interpreted as reversals recorded during seafloor spreading (see Fig. 2 of Mortimer et al., 2014). (2) Observed extinction intensities are much weaker than expected for basin forming impacts of magnitudes considered here (Sleep and Zahnle, 1998). (3) Observed K-Pg and Late Eocene spherule bed thicknesses are too low, being of millimeter scale rather than expected decimeter scale for impact basins of this size (Sleep and Lowe, 2014; Johnson and Melosh, 2012). (4) Conventional interpretations of size frequency distributions based on crater counting for the rocky planets don't predict Phanerozoic impact basins of this size (Ivanov et al., 2002). (5) While global K-Pg and Late Eocene impact horizons have been identified and studied, a global Middle Miocene impact horizon has neither been recognized nor searched for.

The Phanerozoic impact basin hypothesis can be tested vis a vis the possible manifestations given in the introduction. The weathering resistant mantle mineral chromite is apparently present in K-Pg boundary ejecta (Bohor et al., 1990), but has not been looked for in Late Eocene impact layers. A global Middle Miocene impact layer can be looked for in California's Monterey Formation and continuous Miocene sediments elsewhere. Multiple IODP drill holes can be located inside each basin to confirm or rule out predicted homogeneity in impact melt sheet crystallization/cooling age and recorded magnetic polarity. Preserved ejecta thicknesses in uninterrupted sedimentary sequences can be checked for reasonable scaling and power law thinning with distance from the rims. And rim structures can be checked for consistency with cratering mechanics.

Initiation of plate boundaries might be imagined to result from impact induced: thermal weakening of the lithosphere facilitating basin scale distributed shear strain at relatively low stress (Greater Antilles between 66 and 50 Ma?); triggering of basin- ward subduction of gravitationally unstable negatively buoyant "old-cold" oceanic lithosphere by outward over-thrusting at the rim (New Caledonia-dʼEntrecasteaux orocline?). Thermal anomaly in the mantle resulting in convection cell initiation or perturbation (O'Neill et al., 2017).

The outward (from the basin) dipping Sulu and Negros trenches of the south and east Sulu Sea Basin, however, are young incipient subduction zones resulting in Pliocene and younger volcanic arcs (Yumul et al., 2008). In the Sulu Sea, subduction initiation within the basin would have postdated the Middle Miocene impact by about 10 Ma. The Cagayan Ridge is currently interpreted as a volcanic arc corresponding to basin-ward dipping subduction initiating in the Palawan Trench at 34 Ma and lasting up until the Middle Miocene obduction event (Keenan et al., 2016). Sorting out the complexities of tectonic history may be facilitated, and relative event ages calibrated, if it can be established that a basin forming impact took place.

3 CONCLUSION

It is proposed here that several large basins rimmed with circular to elliptical belts of exposed lithospheric mantle may be Phanerozoic impact basins. These originally circular to elliptical belts of exposed lithospheric mantle may serve as strain markers for relative plate motions in the vicinity of plate boundaries during post-impact geologic times. Plate boundaries may be initiated and/or modified by such impacts. The lithospheric mantle rimmed impact basin hypothesis can be confirmed or ruled out by checking multiple phenomena characteristic of large crater forming impacts.

ACKNOWLEDGMENTS

Bob Coleman, Norm Sleep, Jingsui Yang, Manuel Iturralde- Vinent, Angelica Isabel Llanes Castro, Dominique Cluzel, Jonathan Aitcheson, Dave Walker, Sarah Stewart, Jay Melosh, Michael Manga, Bruce Bohor, Bruce Buffett, Walter Alvarez, Alicia Cowart, Jaime Urrutia, Mark Richards, Steve Self, Max Rudolph, Brook Peterson, John Wakabayashi, Raymond Jeanloz, Roland Burgmann, Inez Fung, Tim Teague, Bob Grill, Diane Tompkins, Mark Greenside, Qingzhu Yin, Matt Sanborn, Al Verstuyf, Paul Henshaw, Linda Swift, Rodney Yee, Siri Brown, Wise Allen, Maurice Jones, Rochelle Olive, Char Perlas, Tim Karas, and my students are all acknowledged. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1225-x.


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