the term fence law is appropriate. There are lots.
there are also rabbinical mitzvot, but only 7 of them. (purim and chanukah are 2, washing hands for bread is another)
http://www.askmoses.com/article/411,429/What-are-the-seven-rabbinic-mitzvot.htmlmost rabbis would point to
pirkei avot 1:1
1. Moses received the Torah from Sinai and transmitted it to Joshua; Joshua to the elders; the elders to the prophets; and the prophets handed it down to the men of the Great Assembly. They said three things: Be deliberate in judgment, raise up many disciples, and make a fence around the Torah.
But you could then ask.. Where is fence really defined.. To be laws.. we have to obey. Where are they given permission to make such laws..
I am not sure..
There is of course the Sanhedrin,
and there is the authority of the beis din - jewish court.
The tenach says follow the judges that exist in your time. The talmud says it refers to the sanhedrin, and this is prob clear from the pshat too.. Rabbi binyamin kahane mentioned it.. that it refers to the sanhedrin. (other rabbis have at times pointed to this and looked at it as follow the sages, or follow the bet din or follow your local rabbi. But I don't think these interpretations have any evidence in tenach or gemara). The verse is
Deut 17:11 "Act according to the law they teach you and the decisions they give you. Do not turn aside from what they tell you, to the right or to the left."
Fact is though, even those that maintain this refers to sanhedrin..
Will agree to the authority of rabbis to create fence laws, or new mitzvot.
It must be in gemara..
The RAMBAM bases his decisions in his code very close to the gemara , and he I think talks about following the bet din, in hilchot mamri.. Everybody that accepts the tradition accepts rabbinical authority.. so no question it's there. Otherwise there would be dissent..
If it's not written in gemara, then I would that it is a living tradition, from around the time of hillel and shammai -and before. Our tradition came down to us (again, see mishna avot / pirkei avot , 1:1 ) , down the generations, from G-d to joshua to elders to prophets to MOGA, to 5 pairs of rabbis ending with hillel and shammai. At each generation being taught to the people. And they lived it.
There are of course examples in gemara, which makes it clear enough that the gemara teaches it.
You could look at that as a practical explanation and implementation of pirkei avot 1:1 , a living of that tradition.. lived for generations -before- hillel and shammai.. And I have mentioned how the generations go back to Moshe.