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RESEARCH POSTER PRESENTATION DESIGN © 2015

www.PosterPresentations.com

Evidence for an Oceanic KT Boundary Impact

Geochemical evidence for oceanic crust/upper mantle in the KT

impact layer is inconsistent with excavation/ejection of

exclusively continental crust by the KT boundary impact

suggesting a crater in addition to Chicxulub.

• Chicxulub Crater is located on ~35 km thick continental crust!

• Geochemical evidence for ejection of oceanic crustal/upper mantle [1-5]

since publication of the Alvarez hypothesis in 1980 [6] resulted in the

search for an oceanic impact site until Chicxulub crater was identified with

the KT boundary impact in 1991 [7].

• KT boundary chromites are from Earth (not an asteroid or comet) and

are likely of ophiolite origin! Cr isotopes recently determined by the

Yin lab at UC Davis reiterate 1980s evidence for excavation/ejection of

a mafic/ultramafic target rock component by the KT boundary impact.

References: [1] DePaolo D. J. et al. 1983. EPSL 64:356-373. [2] Hildebrand A. R. and Boynton W. V.

1988, LPI Contributions 673:78-79. [3] Hildebrand A. R. and Boynton W. V.. 1990. Science 248:843-847.

[4] Montanari A. et al. 1983. Geology 11:668. [5] Bohor B. F. et al. 1989. Meteoritics 24:253. [6] Alvarez

L. W. et al. 1980 Science 208:1095-1108. [7] Hildebrand et al. 1991 Geology 19.9: 867-871. [8] Grieve

R.A.F. and Cintala M.J. 1992 Meteoritics 27: 526–538. [9] Pierazzo E. et al. 1997 Icarus 127/2:408-423.

[10] Ivanov B.A. et al. 2002 Asteroids III 89-101 [11] Iturralde-Vinent 1996. Ofiolitas y arcos volcánicos

de Cuba. No. 1. American Museum of Natural History. [12] Cobiella 2005. Geologica Acta 3.3:273. [13]

Tada et al. 2003. AAPG Memoir 79 :582-604. [14] Encarnacion et al. 1995 Journal of Petrology

38/6:1481-1503. [15] Yang, Robinson, and Dilek 2014, Elements, v. 10, no. 2, p. 127-130.

Acknowledgements: Thanks for stimulating discussions with Norm Sleep. Bob Coleman, Dave Walker,

Steve Graham and Manuel Iturralde-Vinent on potential testing of the Greater Antilles and Sulu Sea

hypotheses.

E. Peter Olds College of Alameda epolds@peralta.edu

Sulu Sea Basin - Spratly Islands multi-ring basin: Possible doublet crater from middle Miocene binary oblique impact?

Conventional wisdom is that Greater Antilles and Sulu Sea ophiolite belts are sutures/extinct

subduction zones. In the Greater Antilles case the Maestrichtian emplacement of the Moa-Baracoa

ophiolite does not correspond to the Eocene collision of Cuba with the Bahamas Platform.

In the Sulu Sea case, an Eocene age determined for a metamorphic sole of the SW Palawan ophiolite

does not fit the hypothetical middle Miocene impact event [14].

We speculate: 1. Some but not all ophiolite belt segments mark rims of large terrestrial

impact basins deformed to greater or lesser extent by, and serving as strain markers for, relative plate motions over geologic time.

2. Greater Antilles/Chicxulub and Sulu Sea Basin/Spratly Island occurrences constitute doublet craters of similar size ratio and separation distance.

3. Plate boundaries are formed and/or modified by such impacts.

Ophiolites mapped as purple.

A testable hypothesis is that oceanic crust and upper mantle

rock, exposed as ophiolite in the Greater Antilles island chain,

marks the rim of a roughly 700 km diameter impact basin

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 (Olds, Sanborn,

Teague, Yin submitted to Lithosphere). Relative velocity between

the North American and Caribbean plates is ~2 cm/year (DeMets et

al. 2010) or ~1000 km in 50 my (ignoring velocity variability).

Corollary: Greater Antilles is the parent of a KT boundary

doublet crater for which Chicxulub is the daughter.

Greater Antilles – Chicxulub KT Boundary Doublet Crater?

Chicxulub

Crater

Cuba

Yucatan Basin

Hispaniola

Puerto Rico

Suppose

• some large ophiolite rimmed basins are impact basins and in these cases

ophiolite emplacement is a cratering mechanics phenomenon related to

over-thrusting at the rim,

then

• ophiolite emplacement and/or ophiolitic melange formation plus

emplacement might be equivalent to emplacement of extremely course

ejecta and should reflect the age of impact.

Furthermore

• impacts of such size should generate large amounts of impact melt

resulting in

• spatial homogeneity of igneous basement age plus magnetization in

contrast to spreading origins, and

• petrological/geochemical signatures which differ from typical MORB

and BABB rocks (the latter generated by partial melting and

subsequent eruption at mid-ocean ridges and back-arc basins).

In the case of a doublet crater resulting from oblique impact

• both member craters should record essentially the same approach angle

and direction (azimuth).

Such considerations allow testing of the ophiolite rimmed impact basin

hypothesis for the KT boundary Greater Antilles case and suggest how to

look and test for additional large terrestrial impact basins.

Sulu Sea – Spratly Island

Middle Miocene Doublet Crater?

The ophiolite rimmed Sulu Sea basin and the Spratly Island

multi-ring structure have roughly the same aspect ratios and

long axis orientations, suggesting low angle oblique impact of a

large binary object approaching from SW to NE or from NE to

SW. Middle Miocene ophiolitic mélange formation in Sabah

suggests an impact age similar to Nordinger Ries crater and

the Middle Miocene Disruption (extinction) event of about 14.5

my.

Problems: • Impacts of Greater Antilles, Sulu Sea, etc. proportions should have produced global spherule beds several

tens of cm thick rather than a few mm thick based on conventional rock-vapor scaling models. Pierazzo, Vickery & Melosh 1997, Cintala & Grieve 1998.

• KT boundary and Middle Miocene extinction events should have been much more severe with only deep ocean life surviving. Sleep & Zahnle 1998.

• Impacts resulting in Greater Antilles/Sulu Sea crater sizes are not expected subsequent to the Archean eon (2.5 by) based on conventional size frequency distributions from NEO counting and rocky planet crater counting. Ivanov 2008

• Apparent discrepancy from 14.5 my in igneous basement age (18 or 19 my) at ODP site 768 from leg 124. Huang et al. 1991.

To confirm impact origin search for and identify: 1) A global impact layer in continuous Middle Miocene sedimentary sequences.

2) Middle Miocene course ejecta layer proximal to the structures in oil and exploratory

well cores in the South China and Celebes seas.

3) impactites in exploratory drill cores from the Spratlys.

4) high pressure and/or shocked mineral phases in Sulu Sea basin rim ophiolites: Can

ophiolitic diamonds be found in chromitites from Palawan and Sabah [15]?

5) Obtain spatially separated age and paleomag determinations on SE Sulu Sea igneous

basement (and Yucatan Basin basement in the Greater Antilles case).

Ophiolites mapped as purple.

Same scale as the Greater Antilles image to the

left. Again, ophiolites are mapped as purple.

Alternative hypotheses include: 1) excavation of an unknown/unmapped suture in Yucatan platform

basement (Kevin Burke personal communication), or 2) an additional small crater on oceanic crust

(Manuel Iturralde-Vinent, Norm Sleep personal communication).