Thank you for your question about marigold calyx splitting.  There is not a lot of literature on this phenomenon in marigolds.  Several studies of calyx splitting on carnations (which are not related to marigolds, but have similar calyx structure and formation) point to the effects of lower temperatures  Here are the links (1962) and (1953) if you're interested in reading them.  I don't know where you're located, but if you've had a recent cold spell, that may be the problem.

There are also a couple of references to the amount of fertilizer applied in the soils in which marigolds are growing.  So, in future years, you might want to do a nutrient test on your soil, and follow the advice in this article  Note, especially, the recommendation of avoiding aluminum fertilizers when the temperature falls below 65 F.
Good luck!

Hi

We are located South west part of the India.
The current temperature varies between  70 to 88 F.

Thank you! It's obviously not low temperature, so must be the fertilizer/nutrient issue. Note that all the studies have been conducted on commercial nursery plants, not home gardening settings. I don't have any other suggestions. Sorry! Good luck to you!