Hi my Core i7 6700 cpu is 37 deg. celcius when idle, and it reaches to about 76 deg. celcius when rendering photos using SweetHome3D portable version.5.4. Then I saw this:
As you can see there's nothing wrong with the motherboard.
The temperature of 76 C is a little bit to high as the max. temperature is 71 C @ the processor Integrated Heat Spreader (IHS).
Are the markings on the bottom of your CPU only in that one corner as show in the pictures?
If that is the case the mounted heat spreader might have been more firmly attached in that corner and could have left a little bit of room on the opposite corner and not have been cooling the CPU optimal.
Just bought it last year in December. I'm not sure if the heating was inadequate since the idle temp was normal. It raised to 76 c from the very first time I used it.
Seems like the markings are on the upper half of the CPU (seen how the picture is oriented).
I don't know what CPU cooler you are using, but that is the first thing I would check. Cleaning CPU and Cooler, re-applying the thermal paste and making sure all corners of the cooler have the same amount of force applied.
The idle temperature is also higher than what I am used to using a https://ark.intel.com/products/77779/Intel-Core-i7-4960X-Processor-Extreme-Edition-15M-Cache-up-to-4... Intel® Core™ i7-4960X Processor Extreme Edition (15M Cache, up to 4.00 GHz) Product Specifications it idles around 32 C and maximum I've seen is 59 C on a hot summer day.
The marks on the processor are the result of that corner of the processor being torqued down way, way too hard. This can result in an installation with higher temperatures because it can force the opposite corner of the processor to lift and the heatsink to not sit cleanly. The marks do not appear to have damaged the processor to the point of non-operation, but this could cause issues (oxidation, etc.) sometime down the road.
Bottom line, you need to be careful to install the heatsink-fan unit on the processor with equal force on all four corners and to not torque it down so hard.
Seeing temperatures as high as 76c when the system is busy is not too bad - provided it does not go much higher than this. You could run a significant stress test (like Prime95) and see how high the temperatures actually do go.
Hope this helps,