Chicxulub crater

Given the massive black spot on the Yucatan, I'd guess it actually right after the impact. More on reddit.com

The Chicxulub impactor is not thought to have caused any change in the way plate tectonic processes were operating.

Pangea was fully assembled in the late Permian, about 270 million years ago. The breakup process started in the early Jurassic, about 200 million years ago, though this took tens of millions of years. By Late Jurassic the world looked something like this. Sometime in the Early Cretaceous, about 120 million years ago or so, reconstructions look like this. All of today's continents are clearly recognisable, though some shuffling about still needed to occur before we have today's map.

The Atlantic Ocean basin opened up in stages throughout the Cretaceous, so that by late Cretaceous times just before the asteroid hit we have something like this. The only things which have yet to occur are further widening of the Atlantic basin, India's collision with Asia, Australia's separation from Antarctica and a lowering of global sea levels. The forces responsible for moving tectonic plates around are linked to production of plates at mid-ocean ridges, recycling of plates at subduction zones, and to a lesser extent the convective drag on the underside of plates from currents in the solid mantle. It's not thought that the end-Cretaceous impact did anything to change any of these, definitely not to the extent that plates configured a certain way or anything. I'm sure there would have been the largest earthquakes the planet has ever experienced when it hit though.

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Prior to the impact hypothesis, there wasn't a single clear hypothesis for the cause of the extinction. First, to clarify, originally, an impact as the kill mechanism was argued for based on the discovery of an excess in iridium in the boundary layer between the Cretaceous (K) and Paleogene (Pg), i.e., the iridium anomaly , and while the Chixculub crater was known about at around the same time, it took ~10 years for the two (i.e., the iridium anomaly and the Chixculub crater) to be confidently related to each other . If you look at the paper that first tied the iridium anomaly to an impact and in turn the impact to the K-Pg extinction ( Alvarez et al., 1980 ), they briefly summarize some of the hypotheses that had been put forward previously. Specifically, based on the evidence of the time, there was clearly some sort of major change in ocean, atmosphere, or climatic conditions (e.g., Tappan, 1968 , Worsley, 1971 , and a variety of others), but no single mechanism could be settled on by everyone for what caused these changes (and the resulting extinction). Instead, what was largely argued for was a coincidental mix of several possible causes contributing to the extinction, including a reversal in Earth's magnetic field (e.g., Harrison & Prospero, 1974 ), a nearby supernova (e.g., Russell & Tucker, 1971 ), and/or a sudden massive release of freshwater into the ocean from a large arctic lake (e.g., Gartner & McGuirk, 1979 ). Variably different workers had argued that perhaps one of these was sufficient or that some combination of them was required (in Alvarez et al., 1980, they allude to a symposium in 1979 that largely failed to come to a consensus on a single causative mechanism which satisfied all of the evidence). Finally, it's worth noting that to this day, there remains disagreement about the causative mechanism. The arguments are summarized in greater detail in one of our FAQs , but in short, there are broadly two camps, 1) the impact was the main causative mechanism or 2) the eruption of the Deccan Traps was the main causative mechanism. Proponents of either tend to suggest that the other probably contributed, but was not the main reason for the extinction. There are also pretty firmly middle of the road takes, i.e., that on their own, neither the impact or the Deccan Traps eruption would have likely resulted in the K-Pg extinction, but the confluence of the two put too much strain on the global ecosystem, i.e., had the Deccan Traps erupted without the impact occurring in the middle or had the impact occurred without the Deccan Traps already having started to erupt, there would probably not have been a mass extinction. Amusingly, this has in some ways circled us back to the pre-Alvarez impact hypothesis view, i.e., that the extinction reflects contributions from more than one mechanism (though those mechanisms are different than the pre Alvarez impact hypothesis ones). More on reddit.com

The majority of it is offshore. It was seen first in seismic data shot by explorers for oil & gas. It's all buried under mud--no evidence of it on the floor of the sea.

Edit: This article is posted over in science today. The crater was buried pretty much immediately by debris flows and tsunami waves.

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