Most cruising keel boats are designed to have weather helm by, among other thing, placing the mast forward of the keel. So is the Hylas 70. A keel stepped mast is somewhat of a misnomer in the sense that the mast is not on the top (read centerline) of the keel, at best it may be at the very forward end of the keel. In every Hylas I am familiar with, the mast sits on a very beefy SS beam that runs along the bottom of the hull and connects to the keel bolts. So in case of a collision between the keel and a solid surface, the keel is pushed back an rotated backwards, around the keel bolts, with the aft of the keel being pushed into the hull and the forward keel bolts being subjected to tension (not compression). So with the mast sitting at the leading edge of the keel or even further forward, there is no upward compression into the mast.
Also, when a carbon fiber tube is compressed above its limits, the crushing motion splinters the carbon. Looking at the picture of the mast stub, there is too little splintering to indicate excessive compression.
I believe that once the boat hit the granite wall and came to an abrupt and almost immediate stop, the momentum of the mast overloaded the back stay(s). Speaking of back stays, most Hylas have split back stays about half way up, where they combine into a single stay, which is probably the one that failed.
Just my two cents..
That's a very interesting point that I hadn't really considered... One of the reasons I changed from the original 'shark fin' keel on my boat, was the extreme leverage moment that would be created during a hard grounding, the simultaneous downwards force on the leading edge, and the upwards force at the trailing edge...
So, perhaps indeed instead of a compressive force being placed on the mast, the hull was deformed sufficiently upon impact - it wouldn't take much, after all - that a sudden slackening
of the standing rigging occurred, sufficient to weaken the shrouds' support of the mast?
Or, perhaps some combination
of the extreme forces of slackening and compression in rapid succession? The few instances of a hard grounding that I've either experienced, or witnessed, seem to have involved such a combination... Initially, the forward momentum stopped abruptly, accompanied by a degree of lifting of the stern as the hull 'rotates' somewhat around the point of impact... This 'lifting' can even be assisted to some degree, by the boat's quarter wave continuing to move forward beneath the boat as it's stopped, allowing the boat to in effect 'bounce' over the initial obstruction... Then, there can be a secondary impact, the force of which would have a more vertical component, which might still exert a considerable compressive force on the rig...
Impossible to know given how little we know, of course... But my hunch would be that the rig failure may not have been simultaneous with the moment of initial impact, but rather shortly afterwards, during a second or third "bounce" off the ledge...